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Carbenoid Complexes of Electron-Deficient Transition MetalsЧSyntheses of and with Short-Lived Building Blocks.

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Carbenoid Complexes of Electron-Deficient Transition MetalsSyntheses of and with Short-Lived Building Blocks
Riidiger Beckhaus”
Dedicated to Professor Karl-Heinz Thiele
The relation of thermodynamic stability and kinetic lability of cs-organometallic compounds of transition
metals, together with an improved understanding of the subtle interactions
between central metal, ligands, and substrates, has increased the chemist’s ability to plan organometallic syntheses.
This article presents new results on intermediary and isolable synthetic building blocks incorporating metal -1igand
multiple bonds of electron-deficient
transition metals; the main emphasis
will be placed on compounds with titanium-carbon double bonds. This particular class of compounds is mainly generated by H-transfer reactions starting
from readily accessible alkyl and alkenyl
derivatives. The preparative use of
[L,Ti(CHR,)R] derivatives as sources
for [L,Ti=CR,] intermediates will be
discussed, as well as the nature of these
intermediates. Application of the same
approach to vinyltitanium compounds
[L,Ti(CH=CH,)R] opens up an access
to a short-lived metallaallene derivative
[L,Ti=C=CH,] of an electron-deficient
transition metal. The reactivity of these
synthetic building blocks is mainly characterized by the nucleophilic properties
of the a-C atoms as well as by the spatial
orientation of the n-bonding planes.
Numerous cycloaddition products with
unsaturated substrates could be isolated
and characterized for the first time by
using [L,Ti=C=CH,] intermediates.
Hence it is possible to compare the properties of a multitude of metallacyclic
ring systems with those obtained from
“Tebbe-Grubbs chemistry”, and in this
context, the dependence of the properties of metallacyclic four-membered
rings on the substitution pattern is discussed. This class of compounds in-
1. Introduction
Organometallic chemistry has shown to be a principal source
of innovations in the search for new synthetic principles, new
procedures for stoichiometric or catalytic reactions, or reaction
pathways that simultaneously guarantee high selectivity and
high reactivity. While the “formative years” of organometallic
chemistry (1955- 3 975“’) were characterized by the search for
stable, isolable compounds, as well as for useful ligands and
structural types,[’
today’s research is concerned with the
multitude of reactions and applications in this field. Initially it
was considered essential to avoid thermal transformations of
Priv.-Doz. Dr. R. Beckhaus
I n h t u t fur Anorganische Chemie der Technischen Hochschule Aachen
Professor-Pirlet-Strasse 1
D-52056 Aachen (Germany)
Fax: Int. code +(241)8888-288
Angew. Chem. I n f . Ed. Entl. 1997, 36, 686-713
cludes the metallaoxetanes, which have
been obtained for the first time by the
cycloaddition of the [CpTTi=C=CH,]
intermediate with cumulenes and metal
carbonyls. The differing cycloreversion
behavior of these metallaoxetanes enables the differentiation of species exhibiting classical and nonclassical reactivity. The number and position of the
exocyclic double bonds are the determining factors of the reactivity of the
formed metallacycles. The discussion of
the products obtained from titanium
methylene and vinylidene building
blocks is an up-to-date report on the formation and applications of carbene
complexes and carbene intermediates of
group 4 metals.
Keywords: carbene complexes * cycloadditions
titanium *
vinylidene complexes
organometallic compounds, which had been given the negative
label decomposition pathways. Symptomatic for this conception
is the inclusion of decomposition reactions only as an aside at
the end of relevant review articles.[’. 91 Today, however, such
reactions~are increasingly employed for preparative applications.
Transition organometallic chemistry is so exciting todayevery new issue of our favorite journal brings news, from
Novosibirsk or Miilheim or College Station. o f a dozen new
structures, twice as many reactions. Not only can we make
molecules, and often see their structures, but we begin to understand the intricate, beautiful patterns in which they move, what
is easy and what is dfficult for them to do.
R. Hoffmann, S. D. Wijeyesekera, S.-S. Sung‘’’]
Thermolysis reactions resulting in the complete degradation
of organometallic compounds-reactions, which in materials
VCH Verlugsgrsellschrrfi mhH, 0-69451 Weinhrini, 1997
0570-0X33/97/3607-OSX7$17.50+ 5 0 0
R. Beckhaus
science are used, for example, for coating surfaces or preparing
novel species or alloys-will not be covered here.[" 18] This
article focuses on recent research on alkyl and alkenyl derivatives of electron-deficient transition metals, in particular with
respect to their carbenoid properties. Particular attention is paid
to isolable complexes with metal-carbon multiple bonds as well
as to novel reaction products of short-lived carbene complexes
of titanium group metals, with the aim of extending our knowledge of this area.r19-'71
The term carbenoidwill be used in the sense originally formulated by Kobrich, based on work on a-halogen-substituted
organolithium compounds.['* -331 Hence, the group of carbenoid compounds of transition metals will be understood to
include those species with reactivities characterized by the primary formation of a carbene complex, whether they be isolable
species or short-lived intermediates. The variety of reactions
available for the formation of carbene complexes of electron-deficient transition metals indicates a high synthetic potential
(Scheme 1). Preparative applications of carbene complexes generated by path a, as opposed to those obtained by path b, arc
frequently connected-when used in stoichiometric and catalytic reactions-with higher yields and selectivities, more choice of
reaction media and substituents on the carbene ligand as well as
more tolerant functional groups.
Scheme 1. Possible synthetic pathways a-f for complexes 1 with M-C double
Even though a-H eliminations have been well-established
since the discovery of Schrock c a r b e n e ~ , [3 5~1 ~this
. method (a)
for the generation of 1[36-531
has only recently been successfully
applied to electron-deficient transition metal complexes. This is
in contrast to the extensive use of dimetallic species,[54-I' of
which, for example, the Tebbe reagent (L,M'X = Me,AlCl;
path b)[24.61-691 h as assumed a prominent role in organic
chemistry. Metallacyclobutanes derived from the Tebbe
reagent-which have been extensively rescarched by Grubbs'
groupL7' 721-have also enhanced our knowledge about carbenoid transition metal compounds (path c) . In addition,
reagents are employed that exhibit active species that can only
be derived by examining thc reaction products, such as the Takai
reagent (path d).[21,73-751
The reaction with geminal dinuclear
complexes (path e) leads to the formation of intermediates with
Ti-C double b ~ n d s . [ ~ ~New
* ' ~pathways
have also been obtained from the direct complexation of stable carbene ligands
(path f) . [ 7 8 , 791 The carbene complexes of electron-deficient
transition metals will be discussed according to the pathways of
2. Reactivity of Alkyl and Alkenyl Derivatives
of Titanium Group Metals
Since the discovery of stable homoleptic and heteroleptic
alkyl compounds of the titanium group metals, their preferred
decomposition reactions by H transfer have also been established.[x0-s61For compounds incorporating no /I-H-carrying
ligands, cc-hydride eliminations predominate.[87,"I The thermal
reaction behavior of o-organometallic compounds is demonstrated particularly well by the chemistry of I-alkenyl species of
electron-deficient transition metals, in particular by o-vinyl
compounds. No other class of compounds allows the realization
of all typical degradation pathways of M-C o-bond~,[~']
provided that the metal and ligand are well
For example,
the divinyl compounds of unsubstituted zirconocene and
hafnocene, as well as the [(CpR),Zr(CH=CH,),] derivatives
(R = Me, Et, iPr, PhCH,)[91-931 form diene complexes by
reductive elimination (Scheme 2 A) .[94-961 In contrast, substituted I-alkenylzirconium compounds,[971 permethylated
Riidiger Beckhaus was born in 1955 in BoizenburgIElbe. He studied chemistry at the Technische Hochschule Leuna-Merseburg, obtaining his Diploni in 1980 with a study on
organocopper compounds and his doctorate in 1984 ,for work on low-valent titanium and
zirconium complexes under the supervision Prof. Dr. K.-H. Thiele. His habilitation thesis
"The Chemistry of Vinyl C0mpound.y of Electron-Deficient Transition Metals", submitted
in 1991, laid the,foundation,for his current work on carbenoid compounds of the titanium
group metals. These studies were extended during a period of research at the Technische
Universitat Miinchen as an Alexander-von-Humboldt fellow (Proj: Dr. P. Hofman,
1990191). During 1991192 he served as substitute C3 professor at the RheinischWestfulischen Technischen Hochschule ( RW T H ) Aachen and in 1994, he spent time working with Prof: Dr. R. H. Grubbs at the California Institute of Technology. The author is a
Heisenberg fellow of the Deutsche Forsckungsgemeinschaft and since 1992 has been a
Privatdozent at the Institut ,fur Anorganische Chemie der R W T H Aachen. His main area of research covers the coordination chemistry and catalytic aspects of the organometallic chemistry of electron-deficient transition metals.
Angew Chem Int Ed Engl 1997,36, 686 713
Carbenoid Complexes
reductive elimination
i3-H elimination
C-H central
C-H lateral
Scheme 4. Rotamers of 1-alkenyl groups on pseudooctahedral ICplM] fragments.
M = Ti, Zr, Hf.
Scheme 2. Reactions of I-alkenyl metal compounds.
[Cp:Zr(CH=CH,),] complexes (Cp* = C,Me,),198-'001 and
sterically bulky substituted [Cp,#Zr(CH=CH,),] derivatives
(Cp" = tBuC,H,, tBu,C,H,, iPr,C,H,19'.921) react by primary 8-H elimination (Scheme 2 B) to yield metallacyclopentenes.
Probably the most interesting reaction in this chemistry is the
transformation of [Cp:Ti(CH=CH,),] to methylenetitanacyclobutane [CpTTiC(=CH,)CH,CH,J,~lo'l
which proceeds
quantitatively in situ even below room temperature and is characterized by the appearance of a vinylidene intermediate
(Scheme 2C). This reaction is one of the few in which an a-H
elimination is preferred over a P-H
reactions always proceed with high selectivity. Thermodynamic
considerations suggest that the reductive elimination of vinyl
groups should be preferred, as is shown, for example, by ab
initio calculations (Scheme 3) . [ ' 0 5 ] Kinetic factors are able,
Scheme 3 Results of ab initlo calculations for [CI,M(CH=CH,),] transformation
products M = Ti. Zr. values for titanium compounds given in parentheses (pseudo
potential basis set according to Stevens, Basch, and Krauss).
however, to force the reaction to go in different directions, especially when the metal center is not readily reduced, when hydrocarbon ligands with acidic hydrogen atoms are employed, or
when only limited rotation of the I-alkenyl groups is possible.
The latter factor in particular provides insight into the reactivity
of M -C o-bonds of electron-deficient transition metals. A detailed analysis is required to understand why the divinyl species
[Cp,Zr(CH =CH& (Cp = C,H,) reacts quantitatively in situ
to yield the diene complex [Cp,Zr(q4-C,H,)], while substitutions on the alkenyl or the cyclopentadienyl moiety lead to 8-H
elimination products.[971For the comparable phenylzirconium
derivatives [Cp,ZrPh,] (Ph = C,Hs), thermal conditions lead to
aryne products (B-H elimination),['06- l o 9 ] while photochemlAngelic
~ / 7 1 W l Inr.
Ed. Engl 1997, 36, 686-713
caI conditions yield diphenyl, the product of reductive elimination.""' Why is it that the question ofwhether an alkenyl group
can freely rotate determines the nature of the reaction products?
The preferred rotamers of 1 -alkenyl groups on pseudooctahedral [Cp,M] complexes enable the central metal to reduce its
electron deficiency through p,-d, interactions with the vinyl
group. The rigid orientation of the frontier orbitals in the mirror
plane of the bent metallocene fragment," ' however, only allows effective interactions for the rotamer I1 (Scheme 4). The
properties of rotamers I ( C - H lateral) and 111 ( C - H central)
will be discussed in Section 3.1. The orientation of the C-H
bond under consideration is similar to that of acyl groups in
[Cp,Zr] complexes.[' '21 The coordination of the rotated alkenyl
group in I1 to a second transition metal center allows the expected shortening of the M-C bond to be observed!1131 In divinyl
compounds, an interaction of type I1 results in an asymmetric
charge distribution between the a-C atoms,['051forming nucleophilic and electrophilic a-C atoms, which favors the spontaneous thermal reductive elimination. When these rotations are
hindered by substituents, the central metal interacts with neighboring C - H bonds instead of the described d,---p,interactions.
For the comparable phenyl compounds [L2TiPh,], it is possible to demonstrate experimentally a restriction in the rotation
around the Ti-C o-bond upon increasing the degree of substitution on the Cp ligands.r"41 The reduced coordination angle of
permethylated bent metallocene fragments [CpTM] as compared to nonmethylated derivatives [Cp,M] is responsible for
this effect (e.g., [Cp,Zr]: 95"; [CpzZr]: 55").[' "I In the chemistry of vinylzirconium compounds, this reduction in the coordination angle results in the transition from the quantitative formation of diene complexes by reductive elimination to the
primary P-H elimination by a preferred interaction of the 8vinyl C- H bond and the metallocene fragment," 1 6 . '1 leading
to the formation of metallacyclopentene~.[~~~
In contrast to the zirconium compounds, r-H transfers predominate in the case of titanium complexes. The minimum temperatures at which a-H transfer occurs increase with decreasing
steric bulk (see 2 and 3), while H eliminations from phenyl["*]
or alky1[871derivatives require higher temperatures (Scheme 5 ) .
The rather mild conditions required for 1-H eliminations from
1-alkenyltitanium compounds make it possible to demonstrate
the chemistry of the titanaallene [CpTTi=C=CH,] formed;
it resembles that of a titanacarbene "cur?~u/ogous" to the
[L,Ti =CR,] intermediates." 91
As excellent examples for the stabilization of reactive molecules by complexation,[' 201 metallaallenes [L,M=C=CR,] are
very important in the organometallic chemistry of electron-rich
R. Beckhaus
- 25 OC
Scheme 6. The course of 1-H eliminations.
20 OC
70 'C
Scheme 5. Onset temperatures of H eliminations of selected titanocene complexes.
l Z 3 1 I n contrast, isolable vinylidene
complexes of the titanium group metals are thus far unknown,
even though their existence is demonstrated by various trapping
reactions,[90.98,101, 118, 124-1311 as well as catalytic['32] and
preparative application^.^'^' - 361 Comparable a-H transfers
with participation of vinyl groups, as in the case of the
formation of [Cp;Ti=C=CH,], have been shown to occur
only in a few cases, for example in the formation of
[Cp'yTa(=C=CH,)(H)]['041 and [CpIr(=C=CH,)(H),] .[1371 In
addition to the syntheses of vinylidene species from vinyl compounds, the transformation of ally1 derivatives of tantalum" 381
and the deoxygenation of ketene complexes of niobium with
isonitriles['"1 have been reported.
3. Isolable and Intermediary Carbene Complexes
of Electron-Deficient Transition Metals
3.1. Hydrogen Eliminations and the Generation
of [L,M=CR,] Complexes (Scheme 1, Path a)
a-H eliminations proceed especially readily when there is significant steric crowding and where it is possible to transfer an
a-hydrogen atom to a carbanionic acceptor ligand. The reaction
is intramolecular, and first-order reaction kinetics are observed
for organometallic complexes of the titanium group metals that
show C-H bond breaking in the rate-determining step with
either a cyclic reaction path (Scheme 6, path A) or a hydride
intermediate (Scheme 6, path B).
The activation enthalpy and entropy are characteristic of a
o-bond metathesis, which is identified by negative values for
A S *, that is, by a high degree of order in the rate-determining
step,[87.88.1401 A comparison of activation enthalpies (Table 1)
shows that, as expected, a-H abstractions occur more readily for
neopentyl species[1411
than for methyl derivatives.[*'] The values
for the activation entropies indicate a higher degree of order,
corresponding to a lower mobility, for neopentyl, aryl, and vinyl
derivatives than for methyl or benzyl compounds. The strong
isotope effects k,,,, in addition to the negative A S * values are
in agreement with a cyclic transition state (Scheme 6,
path A).[87. 1 8 * 14'- ]'41 In contrast to this, hydride intermediates (Scheme 6, path B) show k,,, values of close to 1
half-life for the thermolysis of [Cp,Ti(CH,CMe,),] at 20 "C is
between 20 and 56 minutes. The [Cp,Ti=CHCMe,] that forms
can be trapped with phosphanes or used in further reactions,
such as intermolecular C-H activations.Li411The steric overloading favorable for a-H eliminations can be achieved by organic moieties (e.g., neopentyl groups) or by bulky ligands. In
addition to permethylated cyclopentadienyl groups or those
with bulky s u b s t i t ~ e n t s , [ substituted
a l k o x i d e ~ [ '14']
~ ~ >or
a m i d e ~ " ~fulfill
* ~ this criterion.
The high selectivity of the methane elimination from the
vinyltitanium compound [Cp;Ti(CH=CH,)CH,] (4) is of particular value in the generation of the vinylidene intermediate
8." 8] The spontaneous intramolecular H transfer between the
two vinyl groups in [CpTTi(CH=CH,),] (2) had been discovered earlier,[981but is proceeds under conditions that prevent the
isolation of 2. The resulting vinylidene-ethylene complex 7
reacts quantitatively to yield methylenetitanacyclobutane 9
(Scheme 7),[90.1 0 1 . 119. 129,1491
The cyclic transition state necessary for the generation of 8
from 4 (Scheme 6, path A) can only form if the vinyl group is
present as rotamer 111 (C-H c e n t r d ) . Nuclear Overhauser ef-
Table 1. Selected kinetic parameters of H eliminatlons
Starting material
[Cp:Ti(CH=CH,)(CH,)] 4
[Cp:Ti(CH,M 6
or product
AH *
[kJ mol-
[C~:TI=C=CH,] 8
[J mol- ' K -
- 11.7(7)
- 54.4(9)
Angen. Chem. Int. Ed. EngI. 1997, 36. 686-713
Carbenoid Complexes
[TiCl,(thf),] with [(Me,Si),NLi] yields [{(Me,Si),N),Ti] as well
[Eq. (a);
as the colorless, diamagnetic compound
tmeda = N,N,N”’-tetramethylethylenediamine] . NMR spectroscopic studies show that the methyl and methylene groups in
10 exchange rapidly. In the crystal, a monomeric [Ti@-CH,),(pCH,)] unit is present in which the titanium center is in a regular
octahedral environment composed of carbon atoms. The Ti-C
distances are unusually long (2.615(4) A; see Table 2) and lie
outside the range of bonding interactions. The comparably
short Ti-H distances (2.229 A) indicate that the carbon atoms
are bound through hydrogen bonds to the titanium atom; however, there is no direct way of proving the existence of a
methylene bridge.
Scheme 7. Synthesis of methylenetitanacyclobutane 9 by a rearrangement of 2
fects (NOE) measurements, however, show the orientation of
the vinyl group in 4 to be exclusively that of rotamer I (C-H
lateral) .I1 The C- H lateral conformation also predominates
in crystal structures of similar complexes.[124’126.
A rotation of the vinyl group (I ---t 111) in 4 results in the immediate
elimination of methane. Hence the rotation of the vinyl group
around the M -C o-bond contributes significantly to the activation energy of the reaction 4 + 8.[lsl1
a-H transfer reactions are usually preceded by agostic interactions.[’52.1 5 3 1 A reliable method for their detection is the IPD
method (IPD = isotopic perturbation of degeneracy).1154’1551
The temperature dependence of the measured values for A , and
A , allows the strength of the agostic interaction to be determined. The reason for this effect can be found in the tendency
of the lighter nucleus (H) to favor the agostic position M t H-C
rather than the alternative interaction M t D-C. Electron-deficient complexes of transition metals often show altered alkyl
group structures when C-H-M interactions occur. This is particularly true for octahedral do complexes, but less so for tetrahedral do complexes.[’561 Studies on substituent effects have
shown that K donors weaken agostic interactions, while K acceptors enhance them, especially when the ligand and the C-H
bond are arranged trans.
Selective a-H eliminations can also be used to generate substituted alkenylating reagents [L,Ti=CHR] (R = C,H, ,[371
~ B u , [ ’ SiMe,[38.411).
For instance, during the thermolysis of
[Cp,Ti(CH,C,H5),],[8s* ‘”I more toluene (a-H elimination
product) is formed than dibenzyl (reductive elimination
product; to1uene:dibenzyl = 9.4:
As the reaction of bicyclopropyltitanocene shows,[391 even p-H-containing compounds of strained ring systems can be used for “alkylidenecyclopropanation” in carbonyl olefination reactions (Scheme 8).
Apart from the above-mentioned a-H eliminations, complex
reactions that form carbenoid compounds of the titanium group
metals are also observed. For example, the reaction of
- Cp,TisL
- 2 LiCl
50 ‘C. 10-15h
- ~H
Scheme 8 Alkylidenecyclopropanarions via TI=C intermediates.
Angew. Chem. Int Ed. Engl. 1997, 36, 686-713
2 ) CH,CI,
I tmeda
3)hexane, THF
3.2. Hydrocarbon-Bridged DimetaUic Complexes
as Carbenoid Reagents (Scheme 1, Path b)
Whereas the Tebbe reagent has already been discussed in
other articles as an effective and useful methylenating
agent,[24, - 6 3 , 6 6 *6 8 , 69. 1601 homologous metal compounds or
substituted alkenylating agents are not accessible directly by
path b. One method of preparation for alkylidene-bridged ZrA1 complexes is based on the reaction of alkenezirconium or
aluminum compounds with the corresponding metal hydride
(Scheme 9).[1611An X-ray structure analysis of 11 revealed a
Scheme 9. Preparation of p-alkylidene zirconium-aluminum compounds 11
remarkably short Zr-C distance (2.158(2) A), clearly indicating
pronounced alkylidene character in this compound. Furthermore, the Zr-C1 bond is significantly longer. The Zr=C species
were generated by cleavage of ClAl(iBu), with hexamethylphosphoric acid triamide.[162]
The dititanacyclobutane (bismethylene complex) 12 reacts
rapidly and quantitatively with PMe, to yield 13.[571The
carbene complex cannot be stabilized by PEt,, PBu,, or PPh,.
R. Beckhaus
The cleavage of bis-p-methylene complexes fails, however,
for other metal combinations, such as Ti-Zr, Zr-Zr, or
3.3. Cycloreversions of Metallacyclobutanes
(Scheme 1, Path c)
The use of titanacyclobutanes as the starting material has
the advantage over the Tebbe reagent that additional Lewis
acidic components such as [Me,AlCI.L] are not separated
after the reaction. For the synthesis of titanacyclobutanes,
procedures can be employed that were developed by Grubbs,
using the Tebbe reagent,[58.66’’771
by Bickelh a ~ p t , [1 7~8 ~
- l*8
preferably using 1,3-di-Grignard species, or
by Stryker,“ ”I employing ally1 complexes. Titanacyclobutanes
can also be used to generate substituted[1701or cumulogous
[Cp2Ti=CR,] species. There is a problem, however, namely that
singly substituted titanacyclobutanes such as cc,p-dimethyltitanacyclobutane react preferably under elimination of
butenef7 instead of propene. Hence, “productive” cleavage
reactions are required (Scheme 10).
Scheme 10. “Productive” and “unproductive” cleavage of metallacyclobutanes.
A “productive” cleavage occurs when highly strained cyclic
olefins like 3,3-dimethylcyclopropenesare used in conjunction
with Tebbe’s reagent. The resulting titanacycle is capable of
transferring r-substituted carbenes in carbonyl olefinations.
The reactive titanium carbene can be stabilized with PMe, or
PMe,Ph.[1701The reaction of the cycloaddition product with
cyclopropene can be explained by the strain energy of the cyclopropene. The “nonproductive” cleavage would require the formation of the strained cycloolefin cyclopropene.
Starting from methylenetitanacyclobutane 9 , 8 can be generated by the thermal elimination of ethylene.“0’] In accordance
- [AIMe,Cl]
with this, the electron-impact induced fragmentation of 9 in the
mass spectrometer shows the stepwise release of ethylene and
one C,H, fragment, leading to the formation of 8 and [Cp;Ti].
Theoretical considerations led to the conclusion that the inter-
conversion of 9 and the corresponding vinylidene-ethylene
intermediate 7 must be thermally allowed and that d-A0 participation at the titanium center enables the conversion to take
place. The model geometries that were employed showed 7 to be
40.2 kJmol-’ lower in energy than its cyclic isomer 9. This
is in contrast to the generalized valence bond (GVB)
calculations for the equilibrium [CI,Ti=CH,(H,C=CH,)]/
[CI,TiCH,CH,CH,],[’ 831 which indicated that the methylidene-ethene complex is 46.0 kJmol-’ less stable than the
metallacycle. GVB calculations for [CI,Ti] as metal fragment
show no back bonding at all from titanium (formally d2)to the
olefin ligand, while model calculations for 7 indicate very different strengths of back bonding to the vinylidene and ethene
units.[”’] In agreement with the differing x-acceptor properties
of the vinylidene and ethene ligands, 7 contains a C=CH, moiety that is strongly bound to the Ti, and a significantly weaker
bound C,H, ligand. The alternative cycloreversion of 9 to a
methylidene complex [Cp;Ti=CH,] and allene is not only kinetically (unfavorable overlap populations), but also thermodynamically less favorable. Calculations for the systems
[Cp,Ti=C=CH, (minimum geometry) + free C,H4] and
[Cp,Ti=CH, (minimum geometry) + free allene] show a favorable energy balance of 79.5 kJ mol- for the products of the ring
cleavage 9 + 8.[lol1The experimental findings for 9 show that
thermal decomposition with release of ethylene does not begin
until about 130 “C and reactions that trap the [CpfTi=C=CH,]
formed are only possible in solution at relatively high temperatures (60- 100 “C), in contrast to Grubbs’ findings for [Cp,Ti]titanacyclobutanes. This suggests a high activation barrier,
which, according to model calculations, stems from the cycloreversion step.
3.4. Special Carbenoid Reagents (Scheme 1, Path d)
The Takai reagent has proven to be an effective methylenating
agent in preparative applications. In addition, it can be generated from commercially available components. Mixtures of
CH212,zinc, and a titanium compound (Ti(OiPr)4, Ti(NEt2),,
or TiCI,) yield a reagent that can transform aldehydes,
enolizable ketones,[I8’] esters,“
and other carbony1 corn pound^^^'^^ into the corresponding olefins. Carbene
intermediates [X,Ti=CHR] have also been assumed for this
reaction, even though it has not been possible to isolate them or
identify them spectroscopically.[74]Impurities in the form of
other metals, for example, traces of lead in the zinc used, often
activate carbonyl ole fin at ion^^^ ’1 or deactivate cyclopropanations.[ls81
3.5. Geminal Dimetallic Compounds as Sources
for Carbenoid Reagents (Scheme 1, Path e)
Analogous to the many applications of salt metathesis reactions to the generation of simple organometallic compounds, for
example reactions of transition metal halides with Grignard or
lithium compounds, geminal dimetallic species MLCR, can be
used to prepare carbene complexes. Butyllithium reacts under
mild conditions with diphenylcyclopropene to yield a 1,I,-
Anzew. Chem. hi.Ed. Engi. 1997, 36, 686-713
Carbenoid Complexes
dilithio-3,3-diphenylallene derivative, which can react with
fCp,TiCl,] in the presence of PMe, to yield the first titanabutatriene derivative 17.[771
Ph Ph
+ PMe,
- 2 LlCl
The reaction of cyclopropenes with low-valent metal complex
fragments leads to vinylcarbene complexes, a procedure that is
employed to obtain the corresponding derivatives of the platinum metals.[189-19'1 The fundamental studies were done by
Binger et al., who synthesized 18 in 72% yield.["ll
The 13C chemical shifts of the carbene-C atoms were observed
at 6 = 185- 186. The missing back bonding Ti C leads to the
assumption that the reactivity of these species is different from
classical Ti=C systems.
3.7. Special Paths to Complexes with
Ti-C Double Bonds
During the search for linear chains of carbon atoms between
metal centers, Floriani et al. discovered that the one-pot synthesis of complex 22, which contains a C, bridge, could be achieved
a, R = R = P h
b, R = M e , R ' = P h
The dilithioalkylsulfone [Li,C(SO,Ph)SiMe,] reacts with
[(RO),TiCl,] to yield an intermediate Ti=C derivative of unknown structure, which can be used as carbonyl olefination
TCI, + [EbN,Ll,(thf)J
- 4 LiCl
3.6 Carbene Transfer Reactions (Scheme 1, Path f)
One of the earliest examples of electron-deficient transition
metal chemistry is the reaction of the zirconium(I1) compound
[Cp,ZrL,] (L = PPh,Me) with CH,PPh, to yield the first
methylenezirconium complex 19 observed in solution, which,
however, decomposes within an h o ~ r . [ ' ~ ~ 1
Progress has been made in this area through the use of the
stable "Arduengo-carbene ligand~"."~*
19,- 19a1 For instance,
the reaction with TiCI, allowed the isolation and characterization of the corresponding complexes 20.[1991An X-ray crystal
structure analysis of the product of a subsequent reaction (21)
showed the expected long Ti-C bond (2.194(7); 2.202(7) A).
Owing to the lack of back bonding from the Ti'" center, this
bond length is in the upper range ofknown Ti-C o-bonds.[200J
thf. CH
starting from ethylene.[2011This bridge is in a square-planar
environment composed of two titanium and two lithium cations
and is stabilized by two meso-octaethylporphyrinogen tetraanions (Et,N;-). The metallacumulene forms red crystals and is
paramagnetic (peff= 1.80 pB per titanium center at 290 K). The
main structural characteristic is the Ti-C-C-Ti frame, which is
linear within error limits. The Ti-C distances (1.809(9) and
1.757(7) A) are significantly shorter than those in other
[L,Ti=C] derivatives (see Table 2).[157,2021
The bistitanacumulene [Cp,Ti(PMe,)=C=C=Ti(PMe,)Cp,]
23 is also characterized by a linear [Ti=C=C=Ti] chain, and is
obtained by the reaction of ICp2Ti(PMe3)2]with methylenecy-
AnXru. C'hmi. Int. E d EnXl. 1997, 36. 686-713
R. Beckhaus
Table 2. Selected analytical data of complexes with Ti-C multiple bonds.
Synthetic path
Distances [A]
Angle ['I
Crystal hahit
'H N M R
TI-C 2.194(7). 2.202(7)
Ti=C 1.911(3)
Ti=C-C 158.7(2)
Ti-C-H 85(3)
22 b
Ti-C 2.615(4)
dark violet.
~> 70 ~c
125 (=CH,),
- 12.3 (CH,) [b]
Ti=C 1.809(9), 1.757(7)
C = C 1.301(11)
= 1.80 PPJ
'J(C.H) [Hzl
Z r = C 2.024(4)
Zr=C-C 169.2(3)
Zr-C-H 79.8(23)
A f f
NMR data
dark green
> 232 "C
Ti=C 2.051(2)
C = C 1.253(2)
red powder,
L ~ , > 50 ' C
only in solution
13 a [c]
12 3
only in solution
[a] See Scheme 1; g: complex reaction. [b] CP-MAS N M R spectrum. [c] 13a: [Cp,Ti(=CHCMe,)(PMe,)]
3.8. Spectroscopic and Structural Data of
Isolated [L,M=CR,] Complexes
The M-C bonds of the few structurally characterized
[L,M=CR,] complexes of titanium group metals are shortened
to different extents depending on the ligand system.'' 5 7 ,
Earlier attempts to utilize a-H eliminations in the synthesis of,
for example, isolable zirconium complexes with M =C bonds
By employing a doubly functionalized Cp hgand and
dibenzylmagnesium, it proved possible to isolate and characterize the zirconium carbene complex 24 (85%).[2031The 'H resonance signal at 6 = 8.1 (CHPh) and the I3C resonance signal of
the carbene-C atom (6 = 229.4) can be considered characteristic. The low CH coupling constant of 86.8 Hz indicates an agostic interaction. The crystal structure of 24 shows a Zr-C distance of 2.024(4) A, and the agostic interaction is evident from
the corresponding values for the Zr-H distance (2.07(4) A) and
the Zr-C-C and Zr-C-H angles (169.2(3)" and 79.8(23)", respec-
tively; see Table 2). No rotation of the benzylidene ligand was
observed. Treatment with acetone resulted in carbonyl olefination. Dineopentyltitanium compounds [L,Ti(CH,CMe,),] with
large phosphinoalkoxide ligands L, act as sources of the corresponding alkylidene complexes.r2o51For example, the carbene
complex 25 (R = Ph) is formed at temperatures as low as 5 "C
('HNMR: 6 =12.4; 13C NMR: 6 = 287.3; see Table 2). The
phenyl residue is metalated in solution. A metalation reaction of
this kind can be avoided for R = CH, by creating a chelate
ligand that is as rigid as possible (Z-Z = 2,2-phenylene), and
dark green crystals of the titanium carbene complex 26 are obtained.['571The Ti=C bond length (1.911(3)
is significantly
shorter than that in the bisneopentyl starting material
(2.120(4) A; see Table 2). The Ti=C-C angle of 158.7(2)" and
the Ti=C-H angle of 85(3)", in combination with a Ti-H
distance of 2.05(5) A,indicate an a-agostic interaction in 26,
which is typical for do alkylidenes. Comparison with
[CpV=CHCMe,(dmpe)] (dmpe = MeZPCH,CH,PMe,) shows
a rotation of the alkylidene by 90" relative to the plane of the Cp
ligand, as well as an increase of the V-C-C angle to 173.3(3)"and
a decrease of the V-C-H angle to 65(3)".1206,
3.9. Applications of Hydrogen Eliminations to the
Generation of Complexes with [L,M=X] Moieties
R = P h ; Z=CH,
R = CH,; Z - Z = 1,2-phenylene 26
a-H transfers can be used to generate multiple bonds between
not only transition metals and carbon, but also between
metals and other heteroatoms [Eq. (b)]. In this way, synthetic
Angrn Chem Int Ed Engl 1997, 36, 686 711
Carbenoid Complexes
- R-H
X: NR, PR. 0, S
M: Ti, Zr, (Hf)
building blocks incorporating Ti=Si,[2081 Ti=N,[Z091,
Zr=N,[209,211,212I, Zr,p,[213.2141
507k17 kJmol-'). The ratios of these values to each other
(1.0:1.06:1.72:2.23) correspond to those expected for a titanium single, double, and triple bond, respectively. These
relations therefore also correspond to the increase in
bond energy between a hydrogen molecule (H-H), alkanes
(H-CH,), olefins (H,C=CH,), and acetylenes (HCGCH)
The bond energy in Ti=C complexes
increases with the rotational barrier.[231'2321 STO-3G calculations on titana- or zirconaethylenes indicate a preference of the
H atoms for the coplanar orientation by 54 and 79 kJmol-',
respectively, over the distorted conformer (Scheme 12) .[2331 The
Zr=S[2151bonds can be obtained. The large interest in these
fragments of electron-deficient transition metal complexes,[261
in particular the imido-metal derivatives,[217'2 1 8 1 stems
from the possible applications in 1,Zaddition reactions
Cp 1.876 H
Cp 1.838A
([Cp2Zr=NR];[Z11 . 2 1 9 - 2 2 2 1 [ (tBu ,SiNH),Zr =NSi tBu,] ;I Z 2
M = Ta,[2243
V[zzs,226]).For example, intermolecular C-H ac- H/Ti=C
tivation ofalkanes[z23~224*2261and
arenesis found,[21'.219-2261
217 kJrnol-'
54 kJmol-'
reactions that are of practical and theoretical interest.[227]Studies of [2+2] cycloadditionsf2'
2281 h ave resulted in
catalytic applications, such as the aminations of alkynes.[2211
H 2.123
It was even possible to observe H - H activation with hydrogen
( 3 atm, 60 "C) in which [ (tBu,SiO),(tBu,SiNH)TiH] was
formed as product (NMR: G(Ti-H) = 8.62; IR: v(Ti-H/D) =
79 kJrnol-'
1645/ 1185 cm- 1).[2291 Cycloadditions of the intermediate 28
Scheme 12. Bond lengths and rotational barriers in [H,M=CH,] and
[Cp2M=CH,] complexes (M = Ti, Zr) determined by ab initio calculations.
and alkynes yield azatitanacyclobutenes such as 29 (Scheme 11).
The azatitanacyclobutane 30, formed by the addition of
ethylene, is characterized by its dynamic behavior; at room temchanges in bond lengths are not significant. For [Cp,Ti=CH,],
perature, only one CH, signal of the ethylene molecule is oba considerably higher rotational barrier of 217 kJmol-' is calserved.12091
culated, which is comparable with the barrier of the cis-trans
isomerization in olefins (271.7 kJmol-1).[2341The unexpected
elongation of the Ti=C bond in coplanar [Cp,Ti=CH,] compared to that in the twisted rotamer is ascribed to steric interactions.
In the cumulogous compound 8, the character of the metal
fragment MO b, determines that in the structure 8' only the
C=CH, rotamer shown allows a Ti-C, x-bond ("back bonding"). In accordance with this, model calculations on 8' yield a
rotational barrier of 134 kJmol- l.rlO1] For asymmetrically substituted vinylidenetitanium compounds it is possible to show a
potential rotation around the Ti=C bond in
the titanaallenylidene 17, no structural dynamics are observed,
and NMR data indicate that all C atoms of the cumulene lie in
+ H*C=CH2
a mirror plane of the molecule (Scheme 13).[77]
Detailed ab initio calculations by Cundari and Gordon show
that the contributions of nucleophilic and neutral resonance
Scheme 11. Cycloadditions with [(tBu,SiO),Ti=NSiiBu,1 intermediates 28
4. The Nature of Ti-C Double Bonds
As expected from the M-C bond order, [L,Ti=C] derivatives
exhibit higher bond energies than [L,Ti-C] complexes.[2301
reaction of methane with Ti+ ions in a tandem mass spectrometer allows the derivation of the corresponding dissociation energies (Do(Ti+-H) = 226.9k10.5, Do(Tif-CH,) =
240.7-tl1.7, Do(Tii-CH,) = 391.0t14.6, D"(Ti+-CH) =
Angeu,. Chem. Inr. Ed. Engl. 1997,36.686-?13
Scheme 13 Geometries of titanocenemethylene, -vmylidene, and -allenylidene
R. Beckhaus
structures to the description of Ti=C bonds are dominant
(Table 3) .r231,
2361 The substituent effects established correlate, for example, with solvent effects in ring-opening polymerizations of n o r b o r n e n e ~ . [The
~ ~ ] contributions of nucleophilic
resonance structures increase with the introduction of donor
substituents at the carbene-C atom, while the contributions of
neutral resonance structures decrease.
2 3 5 7
Table 3. Resonance contributions to the description of titanium alkylidene complexes [235,236].
model compound
Resonance contributions ["A]
T i c C
As 8 represents the reactive species of 9 and 4, it i s therefore
of the most interest and its geometry and electronic structure
will be more closely examined. Figure 1 shows the bonding in 8'
(Cp instead of Cp*) for C,, symmetry, for which a C=CH,
moiety is at the center of symmetry, using a correlation diagram
between the known valence MOs of a Cp,Ti fragment['*'1and
a vinylidene unit. The underlying structure with a centralized
vinylidene group, however, is not the minimum geometry of 8'.
The removal of the linear C=CH, unit from the central position
(with a trigonal-planar coordination at the Ti center) by 35"
stabilizes the molecule by 27.2 kJ mol- I . The resulting structure
is shown on the right side of Figure 1. Favorable electronic
conditions for the extremely strong z-acceptor ligand vinylidene
explain the greater extent of the relaxation effect to the C, geometry for 8' (pyramidal for Ti). The electronic structure of 8',
however, clearly indicates a singlet ground state. The two frontier orbitals help to clarify how [Cp,Ti=C=CH,] binds by the
interaction with the K and z* orbitals of ethene, or an electronically similar ligand species. The LUMO of [Cp,Ti=C=CH,] is
ideally oriented in the direction of the "open" coordination site
between the two Cp rings, and its occupation by electron density
from overlap with a ligand donor orbital (such as the K orbital
of C,H4) also populates the p-A0 of the C, atom of the vinylidene unit. The HOMO of [Cp,Ti=C=CH,] is suitable for back
bonding into a ligand-acceptor MO of appropriate symmetry
(such as the x* orbital of C,H,). Neither interaction is particularly strong, as the interaction of the vinylidene ligand with the
Cp,Ti fragment leads to a relatively high LUMO energy and a
relatively low energy for the HOMO of 8'.[lol1
5. The Coordination Chemistry of Reactions with
Carbenoid Metal Compounds of Electron-Deficient
Transition Metals
Kinetic studies and numerous trapping experiments have
shown that the thermally generated [L,Ti=CH,] and
[L,Ti=C=CH,] species represent real
In the following discussion, the reactions leading to their formation will only be mentioned where necessary.
5.1. Reactions with Electrophiles
Figure I. Top: Correlation diagram for a [Cp,Ti] fragment (C2J and vinylidene
(C2Jfor "trigonal planar" [Cp,Ti=C=CH,] (C2J(only relevant valence MOs are
shown). Bottom: Decrease in energy for C,-symmetricalS.
In view of the nucleophilic properties of the cr-C atoms in
[L,Ti=C] derivatives, rapid reactions with electrophiles are expected. Reactions of the thermally generated 8 do indeed selectively yield I-alkenyltitanium compounds. The use of CH,OD
in reactions of 9 or 4 yields products that are exclusively deuterated in the cr-vinyl position.['261This is an indication that initially cycloreversion (from 9) or cr-H transfer (from 4) take place
before electrophilic addition reactions can occur. If 9 is allowed
to react with a stoichiometric amount of water, the monomeric
vinyltitanium hydroxide [CpzTi(OH)CH=CH,] 31 is obtained
in 86 % yield as a yellow, microcrystalline product. The selective
protonation of the vinylidene species 8 can also be transferred
to C-H acidic compound^.^'^^] This suggests that the mechanisms of reactions of sterically shielded metal alkyl compounds
with electrophiles should be reconsidered,[z371as they do not
necessarily have to proceed according to an acid- base mechanism as in the case of homoleptic metal alkyl compounds,[238- 2401
Angew. Chem. In!. Ed. Engl. 1997, 36, 686-713
Carbenoid Complexes
5.2. C-H Activation Reactions on [L,Ti=CJ Fragments
In order to coordinatively saturate the [L,Ti=C] derivatives,
intramolecular C-H activation reactions occur in many cases.
For example, 8 spontaneously reacts to yield the dark green
vinylfulvene derivative 32. This behavior is similar to the cyclometalations of other [L,M=C] derivatives with related lig(Ti,[87.157.2051 Hf[881N b [241l Ta[143]
The signal of the methyne hydrogen atom of the vinyl group
in 32 is shifted to higher field (6 = 5.1 1)['281than in other vinyl
compounds.[' 261 The simultaneous broadening observed corresponds to an equilibrium between 32 and 8.
Starting from [Cp,Ti=CH(CMe,)], C-H bonds of benzene
or p-xylene can undergo intramolecular activation and
[Cp,Ti(CH,CMe,)R] complexes are formed.[1411This behavior
is comparable to that of do metal i m i d e ~ ( Z r , [ ~2231
' ~ *Ti,r21o*
V,[226]Ta[2241).Overall, this reaction type corresponds to the
reverse of the original formation reaction by a-H elimination.
For 8 it is possible to show that an intermolecular C-H activation of acetylene takes place, leading to the formation of
[Cp:Ti( CH=CH 2)(C=CR)] complexes 33.[ 241
heteroradialene 34, which spontaneously reacts with further isocyanide to give 35,12431
or with metal carbonyls to yield 36.[2441
The coupling of a carbene ligand with carbon monoxide can
be demonstrated for isolable carbene complexes with bulky,
chelating phosphanealkoxides and a Cp ligand. Ketene complexes are formed in this reaction.['571
5.4. Cycloadditions with Olefins and Acetylenes
Reactions of [L,Ti=C] fragments with olefins lead, in symmetry-allowed reactions, to metallacyclobutanes, which are generally able to undergo further metathesis steps. The behavior of
the metallacyclobutane complex 37 is somewhat different. The
31P resonance signal (6 = - 63.6) indicates that the P atom is
not coordinated. If 37 is heated, no cycloreversion to the carbene complex takes place, but in the presence of excess ethylene
the formation of a crimson colored ethylene complex 38 is observed as well as the release of propene, 3,3-dimethyl-l-butene,
and 4,4-dimethyl-l-pentene as products of P-H and reductive
eliminations.[' 571
5.3. 12 11 Additions
Reactions of metal -carbon multiple-bond systems with isocyanides present a potential synthetic method for the generation
of k e t e n e i m i n e ~ . [This
~ ~ ~reaction,
however, has thus far been
unknown for compounds of the elements of group 4. Reactions
of 8 with cyclohexylisocyanide primarily yield the metalla-
R: C6H11
+ 2 R-N-C
Metallacyclobutenes are found to be the products of [2 + 21
cycloadditions of metal carbenes with a l k y n e ~ , I245~ ~ .2481 as
well as of the ring-opening of c y c l ~ p r o p e n e s . The
[ ~ ~reactivity
of vinylidene compounds of type 8 with alkynes is particularly
important with respect to the mechanisms of polymerizat i o n ~ . Studies
~ ~ ~of ~the. behavior
~ ~ ~ of~ thermally generated 8
towards a number of alkynes have shown that the regiochemistry of the metallacyclobutene ring formation 39 -+ 40 is kinetically controlled and proceeds according to the polarity of the
alkyne employed.[124.2 5 1 1
If acidic (terminal) alkynes (41) are used, a competing reaction occurs, namely the formation of vinylacetylides 33. The use
Angew. Chem. In1 Ed. Engl. 1991,36, 686-713
R. Beckhaus
of unsubstituted acetylene (HC-CH) ,however, leads exclusively to the metallacycle 40, a thermally stable compound. Competing reactions of acetylenes and olefins in cycloadditions with 8
yield exclusively the metallacyclobutene derivatives. Ab initio
calculations for the systems [Cl,Ti=C=CH, + H,C=CH,]
and [Cl,Ti=C=CH, + H C K H ] indicate a preference of
101.5 kJ mol- for the metallacyclobutene ring.['051 Kinetic
studies have shown that the primary alkyne insertion into a
Ti-CH, bond with subsequent y-elimination from 43 to give 44
could be an alternative to the direct [2 + 21 cycloaddition of the
[Ti=C] species to the alkyne (42).1401
5.5. Synthesis and Reactivity of Metallaoxetanes
Metallaoxetanes 46 are discussed as essential intermediates in
the reactions of carbenoid metal compounds with carbonyl
derivatives (45 + 46).[61*62*168, In addition, metallaoxetanes can be considered to be intermediates in the oxidation of
as well in the formation and deoxygenation of
epoxides.[261- 2 6 3 1 Despite some progress towards experimental
proof for metallaoxetanes in the case of Cr, Mo, W,[26432651
Ta compounds,[266.2671 the synthesis of titanaoxetanes remains
a challenge. Theoretical calculations suggest that, as intermediates in carbonyl olefinations (46 + 47), these species should
contain a planar ring structure.
The reactions of the vinylidene intermediate 8 with ketenes or
carbon dioxide indicate the possibility for a synthesis of novel
titanaoxetanes 48 with considerable thermal stability
> 150 oC).1128~1291
The X-ray crystal structure analyses
of 48 b (Figure 2) and 48c confirm the presence of planar, mono-
The reactivity of titanacyclobutenes is characterized by their
principal ability to undergo cycloreversions A or electrocyclic
ring-opening reactions B (Scheme 14). Depending on the substi-
Scheme 14. Reactions of titanacyclobutenes.
tution pattern, reaction path A or B is taken. Alkynes with
particularly bulky ligands (SiMe,) can be replaced by less sterically demanding
This behavior is also observed for
titanacyclobutenes generated from 8.[1241
The electrocyclic ringopening B has so far only been demonstrated indirectly by using
the products (polyacetylene) formed in the r e a c t i ~ n . [ ' ~ ~2 5. 2' 1~ ~ .
The orthogonal orientation of the p orbitals in the metallacyclobutene ring with respect to the acceptor orbitals in the main
plane of the Cp,Ti fragment requires a direct interaction for the
ring-opening B, which can only be realized in the case of metallacyclobutenes with fewer substituents.[" '] This situation
changes fundamentally in the case of heterocyclobutenes, for
which subsequent products of electrocyclic ring-openings are
found.[1251This behavior corresponds to that of tantalacyclobutenes, which can be isolated in the form of vinylcarbene
complexes by the reaction with pyridine.[246.253, 2541 Kinetic
studies on insertion reactions of titanacyclobutenes rule out the
possibility of a primary ring-~pening.['~']
meric metallaoxetane rings. These cycloaddition products represent the first structurally characterized titanaoxetanes with a
planar ring geometry. The T i - 0 (1.983(2) A) and Ti-C bond
lengths (2.121(3) A) in 48b are in the expected range (the values
for 48c are 1.966(3) and 2.119(6), respectively).
Figure 2. Crystal structure of 48b.
Angew. Chem. Int. Ed. Engl. 1997,36,686-113
Carbenoid Complexes
Considering that the driving force of carbonyl olefinations
stems from the formation of a titanium oxide fragment,1256.268*269]
the fact that the planar oxetanes 48 can be
isolated[128.129.2701 must be attributed to the reduced electrophilicity of the transition metal center due to the strongly
basic Cp* l i g a n d ~ . [ ~2721
~ *This
assumption is supported by the
observation that titanaoxetanes similar to 48, but with unsubstituted Cp ligands, have never been detected, even by spectroscopic means.[1351If the energy of the CO bond to be cleaved is
increased by the use of metal carbonyls instead of heterocumulenes, it becomes possible to generate nonoxophilic oxetanes
from 8. Depending on the resulting reaction pattern, these
metallaoxetanes can be classified as “classical” (46 + 47) or
“nonclassical” (46 + 45). Mass spectrometry shows the formation of [Cp:Ti=O] fragments for the metallaoxetanes 48, so that
their classification as “classical” oxetanes is justified. As well as
the effect of the Cp* ligands, the position of the exocyclic double
bond in the metallacyclic ring is also essential for the high stability of the metallaoxetanes 48. If the double bond is in the p
position, such as in cycloaddition products of [Cp:Ti=O] with
allene, rapid ring-opening occurs in solution, with the subsequent formation of fulvene e n o l a t e ~ . [ ~ ~ ~ ]
5.6. Synthesis of Heterodinuclear Carbene Complexes
with a Titanaoxetane Substructure
The “nonclassical” metallaoxetanes can be obtained by the
use of metal carbonyls as heterocumulene units in reactions with
In contrast to metal-mediated cycloadditions of metal
carbonyls with a r ~ n e s , [ ~ ~ dienes,[278-2851
and heteroolefin~[~’~]
at metallocene fragments of group 4
metals,[288.2891 the corresponding reactions of Schrock carbene
fragments with metal carbonyls are little known thus far
(Scheme 15). In a few cases, corresponding intermediates have
+ [L,M=C=O]
The spectroscopic data for 49 clearly indicate the titanaoxetane substructure. In the solid state, these “nonclassical” oxetanes 49 are stable up to temperatures of about 130°C. Solutions, however, show a much higher reactivity, which is quite in
contrast to the “classical” oxetanes 48. For example, the rapid
release of the heterocumulene occurs even at low temperatures,
especially when potential rc-acceptor ligands (ethylene, isocyanates) or even traces of moisture or oxygen are present
(Scheme 16).[2741This means that the Fischer carbene com+ H,C=CH,
Scheme 16. Reactions of Fischer carbene complexes 49 with fitanaoxetane substructures.
Scheme 15. Top: Metal-mediated cycloadditions to metal carbonyls; bottom:
[2 21 cycloadditions of [L.M=CR,] to metal carbonyls. M = Ti, Zr; M’ = Cr,
Mo, W. Mn,Re, Fe. R h ....
either been observed spectroscopically~2901
or their existence has
been postulated based on subsequent products.[2911For example, the reaction of 4 permits the isolation of the reaction
products 49 (acac = acetylacetone) in the form of intensely colored
Angew. Chem. In!. Ed. Engl 1997.36, 686-713
plexes 49 with a titanaoxetane substructure represent another
effective source for the Schrock carbene fragment 8. Inasmuch
as complexes comparable to 49 are known at all, other decomposition reactions dominate, which exclusively proceed
with the formation of metal-oxygen bonds (M = Ti;r2901
M = Ta[29’52921).
This is in accordance with the expectations
for metallaoxetanes.[2601
The nonclassical reaction behavior observed for heterodinuclear Fischer carbenes 49 with a titanaoxetane substructure
,L,, iTC
,/.. .c=c..,
70-110 OC
R. Beckhaus
leads to an unusual destabilization of the titanium vinylidene
moiety, which can undergo a rapid vinylidene-acetylene rearrangement.['28*244, 2741 Complex 49 isomerizes quantitatively to
50 without the appearance of any detectable intermediate prodUCtS.[2741
In contrast to classical Fischer carbene syntheses with dinuclear metal carbonyls (e.g., [Mn,(CO),,], [Re,(CO),,]),[293 -2981
an exclusively axial arrangement of the carbene ligands is observed in 49e and 49f, which remains in place even during the
isomerization to give 50. This preferred axial substitution can be
taken as proof for the formation of 49 as a result of a [2+2]
c y c l o a d d i t i ~ n .The
[ ~ ~location
of the carbonyl bands in the IR
spectra of 49 and 50 indicates a stronger acyl character for the
four-membered rings 49 than for the five-membered species 50
(Scheme 17).[244*2741
The electron delocalization connected with this metalladiene
structure is reflected in the alternating bond lengths.
The rearrangement 49 + 50 is only found for the dimetallic
compounds. The classical metallaoxetanes 48 do not exhibit
such a ring expansion reaction. Apparently, the second transition metal destabilizes the Ti=C=CH, unit by interaction with
the C=C bond (52). This implies a removal of the Ti=C double
bond and must lead to a spontaneous vinylidene-acetylene rearrangement (53),as the orbital arrangement in 8 only allows
Cp;Ti +
t ./
one rotamer. Hence the isomerization 49 + 50 can be considered as one of the few examples, in which a metal-coordinated
vinylidene fragment undergoes a reaction that is also typical for
a [:C=CH,] particle in the gas
acyl resonance form
Scheme 17. Mesomeric structures of Fischer carbene complexes with titanacyclic
The formation of different acyl resonance structures is also
suggested by the molecular structures of 50a (Figure 3) and 50i
(L,M = {Cp'Mn(CO),] (Cp' = C,H4Me));['Z8*274150aexhibits
a relatively long T i - 0 bond (2.013(2) A) as well as a short
C(3)-0(1) distance of 1.286(3) A. These characteristics are less
5.7. Synthesis and Reactivity of Heterometallacyclobutenes
Cycloadditions of nitriles with Ti=C bond systems ought to
lead to the formation of azatitanacyclobutenes (55). The electrophilicity of the titanium center, however, also influences the
reaction behavior of the primary reaction product in this
case,[44s4 8 * leading mainly to subsequent products stemming
from intermediary vinylimido derivatives (56)
In contrast, the corresponding heterotitanacyclobutenes can be prepared starting from 8.[1251
The reaction with acetonitrile, pivalic acid nitrile, or cinnamic
acid nitrile at room temperature leads to the near quantitative
formation of the products 57a-c. In the case of compound 57 b,
Figure 3. Crystal structure of 50a
apparent in the Ti/Mn compound 50i. T i - 0 and C - 0 distances of 1.83 and 1.43 A, respectively, can be considered characteristic for homonuclear titanaoxacyclopentenes, as shown in
the cycloaddition product of [Cp,Ti(RC=CR)] with ace3011 In contrast to these complexes, a planar titanaoxacyclopentene ring, which results from the conjugated arrangement of the C=C and M=C bonds, is found to be structural
characteristic. Hence the binuclear Fischer carbene complexes
50 can also be considered as 1,3-substituted 9 '-vinylcarbenes.
57a, R = CH,
57b,R= C(CH&
57c. R = trans-CHCHPh
Angew. Chem. Int. Ed. Engl. 1997, 36, 686-713
Carbenoid Complexes
a single-crystal X-ray structure analysis was able to confirm
the structures determined in solution by NMR spectroscopy
(Figure 4).
c 42
found in free butadiene. The Ti-X and C-X distances (X = N,
P) and the small angles (Ti-X-C) indicate an sp2-hybridized X
atom with a noncoordinated lone pair of electrons.[48.3 1 i -3141
Doxsee et al. observed that the reactivity 55 + 56[47,48.
501 for
products stemming from [Cp,Ti=CH,] can be viewed as a consequence of the participation of the lone pair of electrons on the
nitrogen atom in the bond to the titanocene center. This in turn
results in an increase of the Ti-N-C angle and an opening of the
azatitanacyclobutene ring (+ 56) 5 , 3 1 61 Only in the presence
of excess nitrile and preferably at higher temperature, does the
insertion of a further nitrile molecule into 57 occur, leading to
Figure 4. Molecular structure of 57b in the solid state
In an analogous reaction, the use of 2-tert-butyl-I-phosphaacetylene enables the synthesis of the heterometallacyclobutenes 58 and 59. Compound 58 is the sterically favored
isomer, while the formation of 59 is kinetically favored.
The different regioselectivities in reactions with nitriles and
phosphaalkynes arises from the frontier orbital constellations of
the reactants. While in nitrile complexes, end-on coordination
dominates,[302-3061 phosphaalkyne ligands prefer a side-on geometry, [307-3101 as they posses a HOMO with n-character due
to the larger n-n splitting. After the primary coordination of the
nitrile (A), the cycloaddition occurs from the suprafacial orientation B (Scheme 18).
The vinylimido species that is formed in the course of the
reaction of [Cp2Ti=CH,] with nitriles by an electrocyclic ringopening of 57 can be trapped by reaction with further substrates
such as ketones, nitriles, or p h o s p h a n e ~ . [The
~ ~ ] azazirconacyclobutenes 61a, which are comparable to 55, can be formed
3171 They are characterfrom imido complexes and alkynes.[2113
ized by the ability to undergo cycloreversion reactions instead of
the above-mentioned electrocyclic ring-openings.121 This behavior is similar to that of the azatitanacyclobutane 61 br3I8]
and must be attributed to the altered hybridization of the nitrogen atom in the ring.
5.8. Structural Data for Small Titanacycies
B, X:
N, P
Scheme 18. Side-on (A) and end-on coordination modes (B, C).
The structural data found for 57 b and 58 show in each case
a planar heterotitanacyclobutene. The alternating bond lengths
in the sequence Cl-C2-C3-X (see Figure 4 for assignments) are
comparable to a conjugated n-bonding system, similar to that
Angew. Chem. Int. Ed. Engl. 1997,36, 686-113
The numerous four-membered metallacyclic compounds that
can be prepared from 8 allow a comparative discussion of their
molecular structures (Table 4). All a-methylenetitanacycles are
characterized by a planar four-membered ring. The bond angles
indicate a stretching of the four-membered ring, which is reflected in the relatively short distances c (2.47 (9)-2.80A (58)).
Particularly in 9, this leads to an extreme high-field shift of the
corresponding signals in the NMR spectrum.
The planarity of the four-membered rings is lost if the exocyclic methylene group is localized in the /Iinstead of the c(
position (63, 64). The automerization equilibria corresponding
R. Beckhaus
Table 4. Selected structural parameters of four-membered metallacyclic compounds
9, X = CH,, Y = CH,
@a, X = C-Me, Y = C-Me
40b, X = C-SiMe,, Y = C-Ph
57b,X = N,Y = C-tBu
58, X = P, Y = C-tBu
48b, X = 0, Y = C=NC6H,,
48c, X = 0, Y = C=CPh,
62, X = S , Y = C=NC,H,,
2.1 02(6)
2.1 34(2)
Distances [A]
Angles ("1
1.701( 5 )
83 l(4)
57 b
1.318(9) fi-ex-CH,
1.376(9) 8-exo-CH,
- -
94.5( 1)
to the ring foldings (22.5" 63; 33.0(9)O 64) cannot be frozen out
in NMR spectra for either 63[2731
or 6 4 . I 3 I 9 ] Neither compound
undergoes cycloreversions.
with [Cp,TiCl,] and its derivatives[64* show an isotope effect
kH,Dof 2.9 and a large negative entropy value, which is in agreement with a cyclic transition state 65. The polarization of the
Ti -C1 bond by the aluminum center increases the basicity of the
Al-CH, bond, which in turn increases the acidity of an cr-H
atom of the Ti-CH, group. This rules out the primary formation
of a [Cp,Ti=CH,] species with subsequent complexation by
[CIAIMe,] during the course of the formation of the Tebbe
The Ti-C distances a and b in 9 are different (2.068(6) and
and 2.137(7) A, respectively), in agreement with the different
hybridizations of the carbon atoms. The equal C-C bond
lengths cf, e) (1.520(10) and 1.521(10) A, respectively) rule out a
partially ring-opened structure (-7). In the metallacyclobutene
40a, inner C-C bond, which can be conjugated, shortens significantly. Bulky substituents facilitate cycloreversions, which is
reflected in the longer distances e and b in 40 b. The exocyclic
a-methylene groups exhibit characteristic bond lengths of
1.318(8) A for 48c and 1.377(4) A for 40a. The increase of the
angle y in 9 to 152.4' is noteworthy. The distances b to the
heteroatoms (0,S, P) are in the expected range, while for 57b,
a slightly shortened Ti-N bond is found, which is in accordance
with the observed reactivity in electrocyclic ring-openings, as
well as a lengthening of the M-C bond ( a = 2.134 A).
5.9. The Structure of Dimetallic Compounds
The Tebbe, Grubbs, and Petasis reagents are well-established
reagents in organic synthesis. Let us first look at the Tebbe
reagent, the most important source of a titanium methylene
compound. Ab initio calculations (STO-3G) on [H,Ti=CH,]/
[CIAIH,] show that for the Tebbe reagent a strong bonding of
[Cp,TiMeCI AIMe,l
- CH,
- AIMe,CIl -
Other compounds that are particularly interesting from a coordination chemistry viewpoint are also accessible from
[L,Ti=CR,] and suitable metal fragments. Depending on the
nature of the interacting metal fragments, a variety of structures
can be obtained, which exhibit symmetric or asymmetric bridges
66-68. A distinct carbenoid character is revealed by the reduced
the Lewis acid [ClAlMe,] must be assumed, in contrast to the
much-discussed weak a s s o ~ i a t i o n . [The
~ ~ l presence of a strong
bond is also shown in the course of the reactions of the Tebbe
reagent as compared to the Petasis or Grubbs variants.[241Kinetic studies on the formation of 14 by the reaction of [M%A11
Carbenoid Complexes
Table 5. Selected structural and NMR data of heterodinuclear p-methylene and related complexes.
Crystal hahit
Distances [A]; Angles ["I
dark green crystals
Ti-Rh 2.827(1), Ti-CH, 2.076(4),
Ti-C, 2.403(4), Rh-CH, 2.131(4);
Ti-CH,-Rh 84.4(1), CH,-Ti-C, 89.9(1)
6.92 (s)
189.4 (dt)
red crystals
Ti-Pt 2.962(2), Ti-CH, 2.066(18),
TI-CI 2.427(5), Pt-CH, 2.112(17);
Ti-CH,-Pt 93.3(7)
8.09 (d)
179.2 (d)
red-orange crystals
Ti-Pt 2.776(1), Ti-CH, 2.115(7),
Ti-CH, 2.395(8), Pt-CH, 2.078(7);
Ti-CH,-Pt 82.9(3)
7.42 (d)
180.0 (d)
7.48 (s)
186.5 (td)
red crystals
Ti-CH, 2.018, Ti-Cl 2.493,
Rh-CH, 2.133; Ti-CH,-Rh 92.0
orange-yellow crystals
Ti-Rh 2.835(1), Ti-CH, 2.147(5),
Ti-CH, 2.294(6)
crimson platelets
7.24 (s)
185.4 (dt)
8.72 (s)
235.8 (t)
red-orange crystals
Ti-CH, 2.146(3) [2.169(4)] [a];
CH,-Ti-CH, 84.1(2) [83.7(2)]
2.50 (s)
red crystals
CH,- and C1-group disordered
8.28 (s)
[a] Two symmetry-independent molecules
Ti-C distances and a notably low-field shift of the p-CH, signa1.[56,58,64,321-325]
Table 5 summarizes selected examples.
A participation of the mesomeric structure 68 is discussed in
particular for the Ti-Pt compounds 70 and 71. For example, in
contrast to dititanacyclobutane 12, the silatitanacyclobutane 74
shows no carbenoid activity.['65. 166]
The p-methylene complexes 70 and 72 do not only act as more
or less activated [Ti=CH,] equivalents, but can undergo a variety of different subsequent reactions. For example, by varying
the bridging ligand novel structural types can be obtained such
as those with p-phenyl or p-methyl groups and thus providing
information about the nature of these ligands. Compound 69,
the first example of a dinuclear phenyl species that is asymmetrically bridged by the @so-Catom of the phenyl
be synthesized by the reaction of 72 with phenyllithium derivatives (cod = 1,5-cyclooctadiene).
majority of known p-vinylidene complexes belong to the structural type 75.[136,326-3421
Some homo-[343- 3471 and heterodinclear[^^^] p-vinylidene complexes indicate the possibility of
side-on bridges (77). A semi-bridge (76), which is characteristic
for CO-[349-3531
or CS-bridged[3541
heterodinuclear complexes
with early-lute metal combinations[3541
has been reported for
comparable vinylidene complexes (Scheme 19) in just one
Scheme 19. Dimetallaethylene (75) as well as semi- and .ride-on-bridged structures
(76 and 77, respectively) of dinuclear p-vinylidene complexes.
When 8 reacts with complexes of group 11 metals (78a-d),
the p-vinylidene complexes 79 can be isolated.[3551In the com-
o-200c- LiCl
R: NMe,
[X-ML] 78
The syntheses of complexes of the types 75-77 are of interest
in the understanding of the ring expansion reactions 49 + 50
and the related possibilities of the formation of p-vinylidene
compounds as well as in the study of this bridging ligand. The
Angew. Chem. Int. Ed. Engl. 1997, 36, 686-713
78a, [CI-CuPMe,]
78b, [Ph-C=C-CuPMe,]
78C, [ C I - C U P P ~ ~ I
78d, [CI-AUPPh,]
R. Beckhaus
plexes 79a-79d, the signal of the a-vinylidene carbon atom is
shifted to very low field 6 =300-330; Table 6). This is in
accordance with a semi-bridging structure 76 (M = Ti,
M' = Cu, Au). The possibility of a side-on bridge (77) can be
ruled out due to the orientation of the x-bonding planes in 8.
The bridge type 76 is formed prefcrentially when 8 reacts with
strongly basic transition metal complexes such as 78 a-78d that
can form Cu Ti or Au Ti
Table 6. Selected NMR data of heterodinuclear pvinylidene complexes 79.
fi(pC(=CH,)) B(pC(=CH,))
'J(H,H) [Hz] J(P,H) [Hz] G(pC(=CH,))
9.4 (d)
8.6 (d)
9.3 (d)
6.16, 5.70
6.16, 6.00
6.16, 5.72
6.71, 5.62
Of experimental and t h e ~ r e t i c a l [ ~ ' ~interest
- ~ ' ~ ~ is the
olefination of carbene ligands by intramolecular coupling
80 -+ 81[360-3691
as well as the possible cleavage of the C-C
double bond 81 + 80[2'7s370-37'1in
the coordination sphere of
transition metals. Proof for an intermolecular, heterodinuclear
C-C coupling reaction 82 -+ 83 has only recently been obtair~ed.[~~~]
\ /
\ I
Figure 5. Crystal structure of 85
The synthesis of the reaction products 85 and 86 is in accordance with a carbene-carbene coupling with the intermediate
formation of the allene complex 88. Its formation can be explained by the primary interaction-already indicated for 87between the nucleophilic and electrophilic carbene-carbon
When 84 is allowed to react with 8, an unusual coupling
reaction of the Fischer carbene complex 84 and 8 takes place,
forming the novel heterodinuclear complex compounds 85 and
86 by a metal-mediated linking of a vinylidene, a carbene, and
a carbonyl carbon atom.[37y1
Cycloaddition products, which are
typical for metal c a r b o n y l ~ , [ ~are
~ ~not
I observed. The NMR
signals of the carbene carbon atoms in 85 and 86 are in the
expected range (6 = 336.7 and 328.0, respectively). The singlecrystal X-ray structure analysis of 85 shows that the formation
of the chelate structures by a coordination of the OCH, group
leads to a distortion of the octahedral environment of the
chromium center (Figure 5). The bicyclic system is nearly
atoms, supported by the CO coordination to the oxophilic titanium center. A titanium-mediated cycloaddition of the resulting
allene molecule with the remaining Cr(CO), fragment forms the
product 85 directly, and 86 is formed after a rapid addition of
CO. This reaction is the first instance of an intermolecular coupling reaction of inversely polarized carbene ligands and is a
further example of a reaction that leads to the metal-mediated
linking of several C atoms stemming from different subs t r a t e ~ . [ ~ 38'
' ~1-
6. Stoichiometric and Catalytic Reactions of
Carbenoid Metal Compounds of Electron-Deficient
Transition Metals
The most important applications of carbenoid complexes of
electron-deficient transition metals lie in the areas of organic
synthesis and catalytic reactions, such as carbonyl olefination,[", 5 9 ,63,741 the generation of cyclopropane derivatives by'
the oxidation of metallacyclobutanes[386~ or cyclopropanation,[l8'I the preparation of heterocyclic four-membered
ring c~mpounds,[~''~
3891 and the ring-opening polymerization[20. 2 4 . 2 5 , 7 2 , 3 9 0 - 3941 to obtain special polymers, such as
Angew,. Chem. Int. Ed. E q I . 1997. 36. 686-713
Carbenoid Complexes
39s1 or “conducting”[3961
polymers. In the follow“living”L171,
ing sections, selected examples are presented to document the
principles of the applications of Ti=C derivatives.
6.1. Carbonyl Olefinations with Tebbe, Grubbs,
and Petasis Reagents
Many cdrbonyl compounds, such as esters, ketones, and
amides, can be methylenated with various titanium-based
reagents. The most important of these are, in particular, the
Tebbe reagent, the Grubbs reagent, and the Petasis reagent. The
application of the Tebbe reagent in particular has found broad
interest in organic chemistry, owing to its early discovery.lz2.24. 6 1 - 6 3 . 3971 The use of [Cp,TiMe,] is particularly
advantageous, as this compound is not pyrophoric, is stable
towards air and water, and no Lewis acidic aluminum by-components have to be separated later, which enables its use in
olefinations of silyl esters, anhydrides, carbonates, and acylsilanes.[42.3981 A number of different, sometimes contradicting,
have been proposed for the reactions that occur,
which will be discussed by using the carbonyl olefination of
esters as an example (Scheme 20). In the absence of an additional base, the reaction between the Tebbe reagent and the ester is
Cp Ti
+ Base; + RCOOR
- [Cp,Ti=O]
R: 0
- [MefilCI-Base]
3( =<
- [Cp,Ti=O]
i :R
first-order with respect to the titanium reagent employed. The
strongly negative activation entropy observed is in agreement
with a six-membered intermediate. In the presence of bases
(pyridine), the reaction proceeds more rapidly and shows zeroorder kinetics with respect to the ester and first-order kinetics
with respect to the titanium reagent. The use of titanacyclobutanes (Grubbs reagents) in carbonyl olefinations is characterized by the primary formation of a metallaoxetane, which spontaneously decomposes to give the desired olefin and a Ti=O
compound.[241Based on the observed H/D exchange, Petasis
and Bzowej favored a methyl addition mechanism;[361
they reported deuterium scrambling in reactions of esters with
Detailed kinetic studies show that the olefination of esters
with [Cp,TiMe,] proceeds via the titanium carbene
[Cp,Ti=CH2].[1421This is proved by: 1) the absence of H/D
exchange or 13C scrambling when labeled esters or titanium
compounds are employed; 2) the finding of zero-order kinetics
with respect to the ester, and first-order kinetics with respect to
the titanium compound; 3) the reaction of ethyl acetate or dodecyl acetate with [Cp2Ti(CD3),]yielding a k,,, effect of 9-10; 4)
esters with different electronic and steric environments exhibiting comparable reactivities. The stoichiometry required for the
Petasis reagent (1:2, substrate to Ti) can be explained by the
necessity of reactions which trap the Ti=O fragment formed.
The products were obtained in better yields with the Tebbe
reagent than with the Wittig reagent. This is especially significant when sterically hindered ketones are to be olefinated. The
Tebbe reagent can also be used in less basic media. which precludes racemizations of substrates with enolizable, chiral centers.[611The above-mentioned formation of enol ethers by the
methylenation of esters and lac tone^[^^. 661cannot be performed
with phosphorus ylides.[’601 For the generation of special
vinylsilanes, there are effective reagents in the form of
[Cp,Ti(CH,SiMe,),] and [CpTi(CH,SiMe,),] . 1 3 8 1 The introduction of a benzylidene residue can be successfully performed
by using [Cp,Ti(CH,C,H,),] .r371Substitutents at the m-position of the phenyl ring (C1 or F) increases the yield. For example, in this way aryl-substituted enol ethers 89. which also
find use as bioactive prostaglandin analogs, can be formed
- [Cp,TiO]
‘ k c H 3
+ [Cp,TiMe,]
- [(Cp,TiMe),Ol
6.2. Carbonyl Olefinations with Other Reagents
Scheme 20. Mechanisms ofcarbonyl olefinations 1. with theTebbe reagent, 2. with
Grubbs reagents. 3 with the Petasis reagent.
Angew. Chem. Int. Ed. En@. 1997. 36. 686-713
There are derivatives related to the Tebbe reagent, in which
one of the metal components has been replaced, for example
aluminum by zinc ([Cp,TiCH,~ZnX,~(thf),]).i591
This reagent
can also be used in carbonyl ole fin at ion^.[^^^] When [Cp,ZrCl,]
in THF is treated with dibromomethane and zinc, a reagent is
obtained that rapidly transforms aldehydes and ketones into the
R. Beckhaus
corresponding ole fin^.[^^^] Olefinations with zirconium carbenes can proceed under steric control as long as Schiff bases are
used as carbonyl analogs.r4011 However, reactions of 1alkenylzirconium or 1-alkenylaluminum compounds with aluminum or zirconium hydrides, respectively, generate only low
yields (20 %)[4021 of the substituted p-alkylidene complexes 11
(see Scheme 9), which are potentially useful in carbonyl olefinations. The Takai reagent 90, which is prepared from 1,l-dibromoalkanes and a mixture of TiC1, and Zn, can also be utilized
in carbonyl olefinations [Eq. (c)] .174,4031 Esters are preferentially transformed into 2-enol esters, while silyl esters and amides
are transformed into silyl enolates and enamines. Organochromium compounds (91), formed from 1,l-dihaloalkanesand
[CrCI,] exhibit high chemoselectivity towards aldehydes, leading to (E)-alkenes [Eq. (d)].['"] Reagents with the composition
CH,I,/Zn/Ti(OiPr), or CH,I,/Zn/Me,Al are effective methylenating agents for aldehydes. The selective methylenation of ketones can be achieved by the addition of Ti(NEt,),
r R~.m,crc121
Reactions of S-alkylthioesters with the Takai reagent in T H F
at 25 "C selectively yield the corresponding (2)-alkenyl sulfides
(which are important as disguised carbonyl compounds) in excellent yields, and ketenedithioacetals as well as enamines are
also accessible.r1871Other metal-containing carbonyl olefination reagents can be obtained by the reaction of the metal chlorides MoCl,, (EtO),MoCl, , [MoOCl,(thf),], (EtO)MoOCl, ,
MoOCl,, MoO,Cl,, [WOCl,(thf),], and WOC14 (but not
WCl,, or Mo" or Mo"' compounds) with MeLi (1:2) in
The NMR data indicate the formation of p-methylene
complexes.[4051Molybdenum carbene complexes, which are
readily prepared in situ, have proven to be suitable reagents for
c a r b o n y l o l e f i n a t i ~ n s,07]
~ ~ ~as~they
exhibit excellent chemoselectivity as well as low sensitivity towards hydroxyl groups.r4o81
Depending on the olefin empioyed, yields of up to 93 % in carbonyl olefination reactions are possible,r4074091 although the
course of the reaction is strongly influenced by external conditions (rate of addition, solvent). Tungsten carbene complexes
also undergo analogous reactions with the formation of carbonyl olefination products.[4101This method is suitable for the
preparation of di-, tri-, and tetrasubstituted olefins, as well as
for enol ethers and enamines. The reactivity decreases with different substrates in the order aldehyde > ketone > formate >
ester > amide.
lizable ketones, on the other hand, can react in a 1,5-sigmatropic
intramolecular H shift to yield titanium enolates,[2s7*411*4121
reaction that is particularly noteworthy with respect to stereoselective C-C linkage^.[^^^-^^'] The majority of studies carried
out to date have used NMR spectroscopy to characterize such
substrates, and only recently have studies of the solid-state
structures been performed.r127.1 2 9 * 4 2 0 * 4 2 1 1When [CpfTiMe,]
(6), [CpfTi(Me)(CH=CH,)] (4), or [Cp~TiC(=CH,)CH,CH,]
(9) reacts with enolizable ketones, the titanocene enolates
9214"] and 93["'] are exclusively obtained. The formation of
products exhibiting A '-and E-configurations is always preferred for both 92 and 93. The observed high regio- and stereoselectivity results from the geometric conditions at the [CpfTi=C]
intermediate 94, which apparently only permit selected deprotonations of the acidic ketones. Theoretical studies show that for
[Cp,Ti=CH,] fragments, the end-on coordination 95 is 0.96 eV
This in turn apmore favorable than the side-on mode 26.[2561
parently leads to the observed preference for the enolate formation over the formation of oxetanes.
The single-crystal X-ray structure analysis of complex 93 a
(R = C,H, ; R = H, Figure 6) shows that the enolate group is
solely coordinated by the oxygen atom. The short titanoceneoxygen distance of 1.859(2) 8, is noteworthy, particularly in
combination with the distinctly increased Ti-0-C(4) angle of
c 35
C 36
C 8
C 26
6.3. Ene Reactions
Reactions of Ti=C intermediates with carbonyl species can
lead to carbonyl olefinations, as discussed in Section 6.1. Eno706
C 27
Figure 6. Molecular structure of 938
Angew. Chem. Inl. Ed. Engl. 1997, 36, 686-713
Carbenoid Complexes
165.9(2)”. This indicates that the oxygen atom is nearly sphybridized, which can be explained by a strong d,-p, interaction with partial double bond character along the T i - 0
structural data for 93a clearly show that the
enolates 93, which are so readily accessible by the ene-reaction,
exhibit a distinct alkoxide character. This reduces the nucleophilic properties of the methylene group and the oxygen atom
in the enolate group, so that typical reactions with elect r o p h i l e ~ / ~ ~such
~ , ~as
~ ’methyl
iodide or benzaldehye, do not
occur.[4111This is also the case for 92.
6.4. Complex Reactions
By the appropriate arrangement of various synthetic steps
and by using carbenoid complexes of electron-deficient transition metals, it is possible to achieve useful syntheses. For
example, vinyl-substituted lactones can undergo a titanium-mediated carbonyl olefination which, followed by a Claisen rearrangement, leads to ring expansion. In this way it is possible to
obtain the cyclooctanoid natural product (_+)-precapnelladiene
(97) (Scheme 21), an unusual sesquiterpene that has been shown
to exist in soft coral^.[^^^^
Scheme 22. Organic syntheses employing titanacyclobutenes.
1-chloro-1-fluorocyclopropane in good yields. Studies of the
ratio of synlanti products show that free carbenes rather than
carbenoid species are involved.[4301Reactions of [Cp,Ti=CH,]
precursors with allenes yield the methylenetitanacyclobutanes
which can undergo spontaneous carbonyl olefination
reactions to form the higher substituted allenes 105.r1351
Scheme 21 Synthesis of the natural product (+)-precapnelladiene 97 by titaniummediated carbonyl olefination and subsequent Claisen rearrangement.
Disubstituted titanacyclobutenes 98 (R, = R, = CH,,
CH,CH,) react with two equivalents of nitrile under mild conditions to yield diazacyclooctatetraenes, a result of nitrile insertion into the titanium-alkyl bond as well as into the titaniumalkenyl bond. Subsequent hydrochlorination of these products
yields the tetrasubstituted pyridines 99 (Scheme 22) .I461 Steric
bulk leads to the exclusive formation of monoinsertion products. Insertions into the metal-vinyl bond are favored over
those into the metal-alkyl bond, when metallacyclobutenes
with small substituents (methyl) are used. Diphenyltitanacyclobutenes, however, react with nitriles or ketones to yield solely
products of monoinsertion into the titanium-alkyl bond, which
can be worked up to yield the unsaturated products 100 and 101,
respectively.[46.4 2 9 1 Reactions of 98 with ketones can be used to
generate dienes 102[491or, after the reaction with RPCl,, the
phosphacyclobutenes 103.[45.3 8 8 1
The reaction of CFCl, with a low-valent titanium compound
generated from TiCl, and [LiAlH,] allows the preparation of a
chlorofluorocarbene at 0”C, which reacts with olefins to give
Angeus. Chem. Inr. Ed. Engl. 1997, 36, 686-713
R 1 ~ 2 ~ - ~ - c ~ 2
- Cp2Ti&
+ R3R4C=0
behavior stems from the intermediate appearance of a vinylidene fragment [Cp,Ti=C=CR,]. A titanaoxetane, which could
also be present in these reactions, could not be observed spectroscopically.[’341
6.5. Catalytic Reactions
Catalytic applications of precursors for [Cp,Ti=CR,] intermediates in the context of ring-opening metathesis polymerizations (ROMP) have already been summarized and discussed
in various articles.[390-394.431,4321
New studies show the
effective use of [Cp,TiMe,], [CpTiMe,], [CpTiClMe,], and
[Cp,Ti(CH,SiMe,),] as carbenoid
The important
R. Beckhaus
advantages in these cases are similar to those already discussed
for stoichiometric reactions, namely the easier preparation and
handling of the corresponding titanaalkyl compounds as compared to the Tebbe or Grubbs reagents. For example, the ringopening polymerization of norbornene can be carried out at
high temperatures in the presence of these reagents (Scheme 22).
When T H F is used as solvent, the reaction does not take place,
even though carbonyl olefinations proceed more rapidly in T H F
than in toluene or h e ~ a n e . [ This
~ ~ ] effect is attributed to the
nucleophilicity of the carbene-C atom. Other
a slower rate of chain growth for THF or pyridine as solvents.
In general, however, THF has only a small effect when [CpTiMe,] and [CpTiMe,Cl] are employed. In the case of
[Cp,Ti(CH,SiMe,),], THF nearly completely inhibits the ringopening polymerization. The different ROMP activities correspond to the electronic structure of the [Ti=CR,] derivatives
according to studies by Cundari and Gordon.[231.2 3 5 , 2361 These
indicate, for example, that [Cp,Ti=C(H)SiMe,] is a particularly
effective metathesis initiator. Furthermore, when electron-withdrawing substituents are present on the carbene-C atom, an
increased activity can be expected, which is demonstrated by
results obtained with [Cp,Ti(CH,Ph),] complexes.[371While unsubstituted [Cp,Ti(CH,Ph),] is suitable for carbonyl olefinations. it is unable to initiate ROMP reactions.
h P
f Y+ ~Ph,C=O
c z R
Scheme 23. The course of the ring-opening polymerization of norbornenes.
Vinylidene derivatives of electron-deficient transition metals
will be discussed as intermediates in catalytic reactions. Alt et
al., for example, have suggested a vinylidene intermediate (107)
in the formation of trans-polyacetylene 110 from the acetylenecomplex 106 and excess acetylene.“ 321 This proposed mechanism is supported by the observation that it is possible, starting
from methylenetitanacyclobutene 40 (Cp* instead of Cp), to
achieve the polymerization of acetylene to pure trans-polyacetylene.“ 241 The transformation of a metal-coordinated
acetylene to a vinylidene ligand (106 -107) has so far not been
experimentally verified in an electron-deficient transition metal
7. Summary and Outlook
Transformations of alkyl- and alkenyltitanium complexes under mild thermal conditions have significantly improved the
access to short-lived carbene complexes of electron-deficient
transition metals. Such transformations have made the isolation
and characterization of compounds with Ti-C and Zr-C
double bonds possible. This has widened the spectrum of
preparative and catalytic applications of precursors for
[L,Ti=CH,] intermediates considerably. Convenient syntheses
of the titanaallene complex [Cp:Ti=C=CH,] 8, mainly by H
transfers starting from titanium vinyl species, form the basis for
the study of a multitude of four-membered titanacyclic compounds. For example, it was possible to isolate and structurally
characterize unusually stable complexes by the [2 + 21 cycloaddition of the Ti-C double bond in 8 to various multiple bond
systems. The resulting titanaoxetanes show variable reaction
behavior, which depends on the carbonyl species employed and
which allows a classification into “classical” and “nonclassical”
compounds. The high stability of the [2 + 21 cycloaddition products obtained from 8 can also be attributed to the reduced electrophilicity of the transition metal centers due to the Cp* ligands. The planarity of these metallacyclobutanes, -butenes, and
four-membered heterocyclic species is favored by the sequence
of sp2-hybridized ring atoms. The observed ring expansions for
dinuclear Fischer carbene complexes with titanaoxetane substructures-which are formed by the cycloaddition of 8 and
metal carbonyls--can be attributed to a vinylidene-acetylene
rearrangement that is unusual in metallaallenes. The future syntheses of classes of compounds, which have thus far been accessible only with difficulty, are becoming conceivable. A particularly stimulating area for future work should be the application
of 8 to the synthesis of radialenes or molecules with radialene
4351 The possibilities of obtaining novel complex geometries by using carbenoid complexes of electron-deficient transition metals have certainly not yet been fully explored. The discovery of unusual reaction patterns, such as the
carbene-carbene coupling reaction discussed herein, is a reminder of the continuing challenge for organometallic chemists
and synthetic chemists, alike, to bring novel ideas into a field of
preparative chemistry that is developing rapidly.14361
I wish to express m y particular thanks to m y very active cowiorkers, Dr. Jiirgen Oster, Dr. Javier Sang, Dipl. Chem. Isabelle
StrauJ, Dipl.-Chem. Jiirgen Heinrichs, and Dipl.-Chem. Martin
Wagner.for the large number of results obtained, as well as to the
temporary members of the research group which are mentioned in
the references. I am indebted to all m y colleagues for constructive
discussions and suggestions, in particular to Dr. Uwe Bohme
Angew. Chem. Int. Ed. Engl. 1997,36,686-113
Carbenoid Complexes
( Universitat Bergakademie Freiberg) for ab initio calculations.
I thank my .family for their understanding and support of my
scientific endeavors. I especially wish to thank the Institut fur
Anorganische Chemie der R W T H Aachen, where I was a guest
and have curried out research for the last five years, the Arbeitskreis Kristallstrukturanalyse ( RW T H Aachen) , and in particular
to Dr. Tri.xie Wagner .for the numerous structure determinations.
I want to gratefully acknowledge that m y research was supported
by the Deutsi,he Forschungsgemeinschaft and the Fonds der
Chemischen Industrie. The Buyer AG Leverkusen and the
Degussa AG ur'e ulso acknowledged f o r their generous financial
Received: May 7, 1996 [A166IE]
German version: Angew Chem. 1997, 109, 694-722
Translated by Dr. W. C. Wilisch. Ansbach, Germany
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[436] Note added in proof (February 24. 1997): After submission of this contribu-
tion several important publications have appeared on the formation of intermediates with T I - C multiple bonds: The thermolyses of [Cp*TiMe,] yields a
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suitable reactions (J 1.Poke, R. A. Andersen, R. G. Bergman, J Am. Chem.
Soc. 1996,118. 8737-8738).
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