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On the Way to a New Class of CatalystsЧHigh-Valent Transition-Metal Complexes That Catalyze Reductions.

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Highlights
Hydrosilylations
On the Way to a New Class of Catalysts—High-Valent
Transition-Metal Complexes That Catalyze Reductions
Werner R. Thiel*
Keywords:
aldehydes · homogeneous catalysis · hydrosilylation ·
oxo ligands · rhenium
Over the last 40 years, homogeneous
transition-metal catalysis has been developed into a versatile tool for fine and
bulk chemical synthesis. If asked to
classify the large number of different
catalytically active species known to
date, most chemists would consider the
catalyzed transformation as a suitable
criterion. If one proceeds in this way and
at the same time initially neglects other
important factors such as the influence
of the ligand sphere, one can readily
obtain a sketch of the periodic table, in
which certain catalytic reactivities can
be assigned to certain groups of elements.
The most active catalysts for reductions, such as hydrogenations, hydrosilylations, hydroformylations and others, contain electron-rich, late transition
metals in low oxidation states. On the
other hand, oxidations are mainly catalyzed by electron-poor species in high
oxidation states. These statements are
not only of relevance for homogeneous
but also for heterogeneous catalysts;
however, they raise a general question:
has this simplified picture, which has
without doubt didactic benefits, not
contributed to us setting limits in the
search for new catalytic reactions or new
catalysts?
Following this argumentation, a review of catalyst research over the last
10–15 years reveals that genuine inno-
[*] Prof. Dr. W. R. Thiel
Institut f"r Chemie
Technische Universit&t Chemnitz
Strasse der Nationen 62
09111 Chemnitz (Germany)
Fax: (+ 49) 371-5311833
E-mail:
werner.thiel@chemie.tu-chemnitz.de
5390
This is to our knowledge not only the
first rhenium-catalyzed hydrosilylation,
but also the first example for a hydrosilylation catalyst whose metal center is
in the oxidation state + v and bears two
terminal oxo ligands. The last two features in particular are remarkable.
Stable hydridorhenium complexes
have been known for a long time.[5]
Probably, the most famous compound
of this type is the nonahydridorhenate(vii) [ReH9]2, which is accessible
from [ReO4] and sodium in EtOH.[6]
In addition, a multitude of phosphanestabilized hydridorhenium species has
been synthesized by treating halogenorhenium–phosphane complexes with
strong
hydride-donating
agents
such as Li[AlH4], KH, Na[BH4], or
Li[BEt3H].[7] However, silanes have
not previously been used as the hydride
source. Some of these hydridorhenium
species have shown activity in the hydrogenation of CC multiple bonds and/or
in CH activation.[8] In contrast, [ReH7(PPhiPr2)2] reacts with ethylene in the
presence of various platinum catalysts to
yield the hydrido–olefin complex [ReH3(C2H4)(PPhiPr2)2], which does not
undergo hydrogenation of the olefin in
the presence of 1 atm H2 but gives the
pentahydridorhenium complex [ReH5(C2H4)(PPhiPr2)2].[9]
In their mechanistic interpretation
of the catalytic cycle of the rhenium(v)catalyzed hydrosilylation of aldehydes
and ketones, Toste et al. proposed a
hydrido-oxorhenium(v) intermediate.
Such compounds have been known since
1989 and have been thoroughly investigated mainly by the groups of J. M.
Mayer and A. Wojcicki. They are accessible by different synthetic routes: 1) decarboxylation of [ReO(HCO2)(RC
CR)2];[10] 2) spontaneous rearrangement (tautomerism) of the hydroxo
complex [Re(OH)(RCCR)3], which
can be obtained by deprotonation of the
corresponding aqua complex, gives
complexes of the type [ReO(H)(RC
CR)2];[11] 3) treatment of [Tp*ReO(OMe)2] (Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate or [TpReO(OEt)Cl]
(Tp = hydridotris(pyrazol1-yl)borate) with BH3·THF gives
[Tp*ReO(H)2] and [Tp*ReO(H)Cl], re-
DOI: 10.1002/anie.200301685
Angew. Chem. Int. Ed. 2003, 42, 5390 –5392
vations were rather rare. Plenty of effort
has been spent on the improvement of
known structural motifs, which has
proved vindicated and in many cases
successful. The ruthenium-catalyzed
metathesis (“Grubbs catalyst”),[1] the
olefin polymerization catalyzed by late
transition-metal complexes,[2] and the
renaissance of N-heterocyclic carbene
ligands for catalysis,[3] are among the few
“really” new aspects that have been
described. Depending on the individual
interests, further examples can be added
to this list.
It is therefore certainly of interest
when an unexpected catalytic reaction is
reported. In a publication and a patent,
F. D. Toste and co-workers described the
hydrosilylation of aldehydes and ketones catalyzed by the rhenium(v) complex [ReI(O)2(PPh3)2] (1) (Scheme 1).[4]
Scheme 1. Rhenium-catalyzed hydrosilylation
of aldehydes and ketones according to Toste
et al.[4] .
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angewandte
Chemie
spectively;[12] and 4) treatment of
[Re(H2)H4Cyttp]SbF6 to give fluorohydrido
complexes
of
the
type
[Re(F)(H)(O)Cyttp]SbF6 (Cyttp = PhP(CH2CH2CH2PCy2)2) by heating to reflux in a mixture of toluene and acetone[13] .
Here, only the synthesis of [ReO(H)(RCCR)2] (point 2) and the reactivity of the tris(3,5-dimethylpyrazol1-yl)borate
complex
[Tp*ReO(H)Cl] (point 3) will be briefly mentioned: [ReO(H)(RCCR)2] is alternatively accessible by treatment
of
[ReO(OEt)(RCCR)2]
with
Bu3SnH, a mild hydride-transfer agent.
In contrast, the use of silanes such as
Et3SiH or Et2SiH2 gives low yields.[10]
[Tp*ReO(H)Cl] can be converted readily to the corresponding triflate complex
[Tp*ReO(H)(OTf)] by treatment with
AgOTf, which undergoes an insertion of
acetaldehyde or a terminal olefin into
the ReH bond to yield [Tp*ReO(OEt)(OTf)] or [Tp*ReO(CH2CH2R)(OTf)], respectively (Scheme 2).[12]
These reactions correspond to the
insertion of an aldehyde or ketone into
the ReH bond of the hydridorhenium(v) intermediate, obtained by Toste
et al. in the addition of a silane to the
rhenium complex 1. In the reaction of 1
in the absence of a suitable substrate, a
resonance signal occurs at d = 6.60 ppm
in the 1H NMR spectrum, which was
assigned to the hydridorhenium species
2 (Scheme 3). The position of this signal
is consistent with the 1H NMR spectroscopic data of [Tp*ReO(H)Cl] (d =
Scheme 2. Insertion of alkenes and aldehydes
into the ReH bond of [Tp*ReO(H)(OTf)].
The hydridotris(3,5-dimethylpyrazol-1-yl)borate(Tp*) ligands are depicted in a simplified
form for clarity.
8.64 ppm) and [ReO(H)(EtCC
Et)2] (d = 6.05 ppm). In constrast, the
OH ligand of the tautomeric rhenium(iii) hydroxo complex [ReI(OH)(OSiR3)(PPh3)2] (3) should give a proton resonance signal at higher
field ([Re(OH)(EtCCEt)3]: d =
2.53 ppm). It still remains to be established whether the silane adds to one of
the Re=O bonds by a [2+2] addition
leading to the formation of a H-ReV-OSi
structure (2), or whether a HO-ReIIIOSi primary adduct (3) forms by a [2+3]
addition to the O=Re=O moiety, which
subsequently undergoes a rapid tautomerism. This question will probably be
best answered with the aid of quantumchemical calculations. A HO-ReIII-OSi
intermediate may also play a role in the
Scheme 3. Mechanism of the rhenium-catalyzed hydrosilylation of aldehydes. The reaction of 1
with R3SiH leads to 2, presumably via 3 as the primary adduct. Comlpex 5 is formed by coordination of the aldehyde and insertion into the ReH bond. The catalytic cycle is completed by
elimination of the product and re-formation of 1. * = PPh3.
Angew. Chem. Int. Ed. 2003, 42, 5390 –5392
www.angewandte.org
addition of the aldehyde or ketone, as it
would open up a free coordination site
for the docking of the substrate (4). The
final elimination of the product is slow
(5!1): the intermediate 5 could be
detected by 1H NMR spectroscopy.
The results presented by Toste et al.
will open up a discussion on whether
compound 1 will remain a unique example for a high-valent transition-metal
complex that catalyzes a reducing reaction. In my opinion, additional related
systems will soon follow. If one considers the structural and electronic features
of 1, one can readily identify a series of
alternative candidates. First, these compounds should have a coordination
number of five or less, since it is
increased to six by the addition of the
SiH unit (1!2). The metal center
should be present in a high and stable
oxidation state. A reduction by two units
should be unachievable by mild reducing reagents such as silanes. The presence of at least one oxo ligand would be
advantageous, since the addition of
SiH is favored by the formation of a
stable SiO bond. However, M=S and
M=N groups may do the same job.[4b]
Looking at the neighbors of rhenium in
the periodic table, it appears that compounds of the type [CpMoO2X] (M =
Mo, W; X = halogen) could be suitable
candidates and may show analogous
catalytic activity. The coordination number of six in these compounds could be
lowered by the exchange of the cyclopentadienyl (Cp) ligand for an indenyl
ligand (Scheme 4, top), which is known
to undergo ring slippage (h5Qh3) much
easier than Cp. Alternatively, other
bidentate ligands carrying one negative
charge may be used. Scheme 4 (bottom)
Scheme 4. Examples for oxo complexes with
metal centers in high oxidation states, which
can be considered as candidates for hydrosilylation catalysts. M = Mo, W; Do = optional donor function.
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5391
Highlights
shows two complexes with bispyrazolyl
ligands, which when necessary perhaps
could be equipped with a third, labile
donor site.
This is just a small and subjective
collection, which may be completed by
other complexes of Group 5, 8, or 9
elements in mid to high oxidation states,
as well as by di- and polynuclear complexes. Variations in the ligand sphere of
1 should lead to further improvements
in the catalytic performance of this type
of compound. The exchange of the
phosphane ligands for N-heterocyclic
carbene ligands or the introduction of
chiral (chelating) phosphane ligands for
enantioselective hydrosilylation will certainly be investigated in the near future.
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