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Carbon Dioxide as an Alternative C1 Synthetic Unit Activation by Transition-Metal Complexes.

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Carbon Dioxide as an Alternative C1 Synthetic Unit:
Activation by Transition-Metal Complexes
By Arno Behr"
The carbon dioxide molecule has been of limited importance as a synthetic unit in organic
chemistry. When it is coordinated to transition metals, however, completely new possibilities arise; CO, can bond to metal complexes in a variety of ways and can enter into insertion and coupling reactions, o r become catalytically attached to other substrates. The formation of C-C bonds between carbon dioxide and unsaturated hydrocarbons under conditions of homogeneous catalysis makes available new synthetic routes to industrially interesting organic compounds.
formed in large concentrations, rather than merely allowing it to escape to the atmosphere.
1. Introduction
Carbon dioxide, whether in free or chemically combined
form, is a considerable natural source of carbon. At present it forms 0.034% of the air by volume.['l The total quantity of carbon present as carbon dioxide, carbonate, and
hydrogen carbonate in the atmosphere, hydrosphere, and
lithosphere has been estimated as 10l6 t.[*l Part of the carbon dioxide is subject to natural circulation processes that
are influenced to some extent by man. Carbon dioxide and
water are converted into carbohydrates and oxygen in
plant chloroplasts; this photosynthetic process forms organic compounds from low-energy carbon dioxide by utilizing sunlight [Eq. (I)]."] A rough estimate has indicated
that ca. 200 billion tons of biomass are produced in this
way per year. In terms of chemical throughput, therefore,
photosynthesis is by far the most important synthetic process.
n C02
+ n H 2 0 e(CH20)" + n 0,
(1)
The respiration of man and animals and the decomposition of organic substances return carbon dioxide to the atmosphere in comparable quantities, and a n equilibrium is
thus set up. This equilibrium has however been considerably shifted in recent years by the activities of man.[4.51The
increasing demand for energy since the industrial revolution has led to the increased combustion of fossil fuels
(coal, oil, gas), forming carbon dioxide; no one can be sure
to what extent this increased C 0 2 emission will affect our
environment. There are many indications that the higher
C 0 2 concentration in the atmosphere can alter the earth's
radiation balance and thus the world climate.'6-'21
A reduction of these negative effects could be achieved
by a change in energy policy (a reduction of CO, emission
by exploiting alternative energy sources). A certain contribution could also be made by chemically converting carbon dioxide into useful products in those places where it is
[*] Priv.-Doz. Dr. A. Behr
['I
Institut fur Technische Chemie und Petrolchemie
der Technischen Hochschule
Worringer Weg 1, D-5100 Aachen (FRG)
['I
New address:
Henkel KGaA
Postfach 1 100, D-4000 Dusseldorf I (FRG)
Angew. Chem. In(. Ed. Engl. 2711988) 661-678
,
I
c
h
f
L L
L4>
A
Biosphere
'cycle
E
Fossil fuels
Carbon dioxide
DDDQ
(coal,petroleum,natural gas)
G
h.
-..
L
L
Organic
Photo-
%*
B
D
synthesis
Biomass
Fig. 1. Carbon dioxide cycle.
Figure 1 depicts the position of carbon dioxide in natural circulation and in processes influenced by man. Until
now fossil raw materials (coal, oil, gas) have been used to
generate energy (route A) and to synthesize chemicals
(route B). The burning of fossil fuels to generate energy
forms carbon dioxide, which is converted by photosynthesis (route C) into biomass; this biomass finds little use in
chemical synthesis, except for a few special applications
(route D). The greater part of the biomass is involved in
natural circulation; most plants die and are reconverted
into carbon dioxide and water by microorganisms in the
soil, but some are eaten by man and animals and converted
into C 0 2 by respiration. Route E shows the formation of
fossil fuels from organic sediments in the course of the
earth's history.
Particularly important for industrial organic chemistry is
route F, the direct transformation of carbon dioxide into
carbon-containing chemicals, which can then be reconverted into COz after use (route C). There are only four
important large-scale industrial processes in which carbon
dioxide is used for organic syntheses:['31the synthesis of
urea, of cyclic carbonates, of salicylic acid (the KolbeSchmitt process), and of methanol (Fig. 2).
0 VCH Verlagsgeselischaji mbH. 0-6940 Weinheim. 1988
0570-0833/88/0505-066/
S 02.50/0
66 1
Fertilizers
Resins
Solvents
@OH
COOH
-
Hydroxy compounds
Acetylsalicylic acid
Methanol
Fig. 2. Industrial organic syntheses with carbon dioxide.
Increased use of carbon dioxide will only be possible if
the relatively inert carbon dioxide molecule can be activated. This goal can be achieved with the aid of transitionmetal catalysts that lower the activation energy of CO,
reactions and thus considerably increase the reaction rate.
Figure 3 shows (purely schematically) how such a metal
catalyst M can influence the bonding of CO, to an arbitrary substrate Su. For instance, step I might be the coordination of the substrate to the metal, forming a reactive
complex; this could react with the carbon dioxide (step 11),
followed by elimination of the required molecule Su-[CO,]
(step 111) and reformation of the starting complex (step
IV). An alternative mechanism is the prior bonding of carbon dioxide to the metal catalyst (step V) and subsequent
reaction with the substrate (step VI). Equally possible is
the simultaneous coordination of CO, and substrate to the
metal (step VII).
s u - [ c02 1
Fig. 3. Possible steps in the reaction of C 0 2 with a substrate Su at a transition
metal M.
This article will discuss the most important of these
reaction steps in more detail:
-
the catalytic combination of CO, with the substrate
(steps 111, IV)
2. Coordination of Carbon Dioxide to
Transition-Metal Complexes
Carbon dioxide is a molecule with several potential
reactive sites: the carbon is a Lewis acid center and the
oxygens are weak Lewis bases. The carbon can thus be described as a n electrophilic center and the oxygen atoms as
nucleophilic centers. In addition, the molecule in its
ground state possesses two equivalent C - 0 71 bonds that
could also play a role in bonding to a transition-metal center. This polyfunctionality of CO, leads to a wide variety
of potential transition-metal C 0 2 complexes (Table 1).
If a metal center reacts with one molecule of carbon
dioxide, adducts 1 to 5 can be formed. Type 1, with a
metal-carbon bond, is sometimes termed a metallacarboxylate; electron-rich metal centers should be particularly
suitable for forming this sort of bond. Structures 2 and 3
(side-on bonding) are very similar to each other, since the
spatial arrangement of the atoms is comparable and only
the bonding type (three-membered metallacycle o r n complex) varies. Adducts 4 (end-on) seem less probable, because the coordination would be solely through the lone
pair of an oxygen atom.
Since carbon dioxide displays up to three potential
bonding modes, it is possible that more than one transition-metal center (perhaps different elements) could coordinate to one CO, molecule. Table 1 shows some examples
(structures 6 to 11) where two metal centers form adducts
with one CO, molecule. Conversely, two molecules of C 0 2
could in principle bind to one metal center (structures 12
to 19). A further possibility is that two CO, ligands in the
coordination sphere of the metal could bond to each other.
Structures 20 to 24 are examples where metals and C 0 2
combine to form rings or chains.
Many transition-metal CO, complexes have been described in the literature. It should however be noted that,
especially in the older work, the identification of the complexes was solely on the basis of their IR spectra, which in
some cases led to errors. An unambiguous classification of
isolated CO, complexes according to the structural categories 1 to 24 is usually only possible if an X-ray structure
analysis is carried out. Some CO, complexes for which this
is indeed the case will now be described.
The first example of type 1 was the rhodiu>n complex
25, isolated by Herskouitz and his c o - ~ o r k e r s . [ ’ ~It. ’ ~is~
[Rh(q ‘-CO,)Cl(diar~)~j
25
likely that the q 1 bonding of CO, is stabilized by additional C-H. . .O interactions with the ligand 1,2-bis(dime0
0
‘C’
- the coordination of C 0 2 to the transition-metal complex (step V)
- the simultaneous coordination of CO, and substrate to
-
the transition-metal complex (step VII)
the insertion of COz into the M-Su bond (step 11)
662
CI
25
Angew. Chem. Int. Ed. Engl. 27 (1988) 661-678
Table I. Potential coordinations of carbon dioxide to transition-metal centers M.
Ratio
metal: CO:
-
Electron transfer
metal
C
Electron transfer
metal + C and 0
-
Electron transfer
0
metal
Formation of a
+
metal
x complex
4
M-O-C=O
0
3
M---ll
/O\
k0
m
c
5
0
‘’
M-
0 = C =0 -M
11
M
0,
C
Il--M--II
o
12
-
0,
,O
C-M’
ON
0
M
0
‘
//” -c\
II
1
0
0
19
M ’0
‘0-c-0
0-M
/O,C/O\
\
/
m
\o/ko/
H -M
M-0
21
22
thy1arsino)benzene (diars). In the iridium complex
[Ir(CO)2Cl(dmpe),]
(dmpe = 1,2-bis(dimethylphosphino)ethane) CO, is probably also q’-coordinated.1’6.’71Since
the oxygen atoms remain accessible in this type of complex, reactions of the coordinated carbon dioxide are possible, e.g., the addition of a methyl cation [Eq. (2)]. This
reaction is an example of step VI shown in Figure 3 with
the methyl cation as “substrate.”
+
0
18
,o-c-0
m
M-C,
0,c
,-
c
II
b0
,O-M
O
c
- d1
--M
/m--C\
20
[Ir(C02)Cl(drnpe)2] FS0,Me
c
II
17
16
0
4
c\
M
,
n:m
15
14
13
d 0‘ II
0
-----t
[Ir(C0,Me)Cl(drnpe)2]FS03
(2)
In 1975 Aresta and his co-workers assigned the structure
type 213 to the nickel complex 26 (Cy = cyclohexyl).[’8.’91
The niobium complex [Nb(CO2)(C,H4Me),(CHZSiMe3)]
and the molybdenum complex [Mo(C02)Cp2](Cp = cyclopentadienyl) also belong to this structure
They
are examples of the ability of the earlier “oxophilic” transition metals to form stable CO, complexes.
L
23
Many structural variants are possible if two transitionmetal centers are available for bonding. The structure 7
has been suggested for the iridium-osmium complex 27
(L = pyridine),
prepared
from
Vaska’s
complex
[IrCI(CO)(PPh,),] and osmium tetroxide in the presence of
pyridine.[”] In this binuclear complex the bridging CO, ligand is formed from C O and oxygen. An X-ray structure
analysis is available for the complex obtained by methylation of the exocyclic oxygen of the bridging CO, in 27
with methyl trifluoromethanesulfonate.
Fewer cases are known in which two CO, molecules are
bound to the same transition-metal center. One example is
the iridium complex 28 isolated by Herskovitz and his coworkers; the carbon of one CO, molecule is bonded to the
oxygen of the second (structure 15).[23.241
This type of COz
dimerization in the coordination sphere of the transitionmetal center is known as “head-to-tail bonding.” The authors assume that the uptake of CO, occurs stepwise and
that the first COz molecule binds to iridium through carbon.
L
Me3PMe3P
26
Anyew. Chem. Inl. Ed. Engl. 27 11988) 661-678
24
27
-
/-I’
ci
28
29
663
A further example is the molybdenum complex 29, as
described by Carmona et al.[2s.261Both C 0 2 molecules are
bonded “side-on’’ and are trans to each other. This structural type can, in simplified form, be represented by formulas 16/17.
With a view to the use of carbon dioxide in syntheses it
is interesting to look for complexes in which two carbon
dioxide molecules are bonded “head-to-head’’ through
their carbon atoms. This linkage leads to oxalato complexes and could represent a first step in a catalytic conversion of carbon dioxide to oxalic acid. Frohlich et al. succeeded in 1983 in preparing the binuclear titanium oxalato
complex [Cp2Ti(C204)TiCp2]
(structure type 24) via such a
dimerization of C02.12’] The synthesis of mononuclear oxalato complexes (corresponding to structure 19) from carbon dioxide has not been reported.
It remains to describe those structural types in which
several metal centers combine with carbon dioxide to form
ring structures. A p3-CO, bridge of the type 21 was demonstrated in the rhenium complex 30.[281
This complex is
not formed directly from C 0 2 , however, but from several
“Re(CO)sOH” units by elimination of water. Structure
type 22 also contains a p3-CO, bridge. To this group belongs the cobalt complex 31, synthesized by Floriani and
his co-workers, in which the carbon atom is bonded to the
transition-metal center and each oxygen enters into interactions with at least one potassium ion.[29-331
This structure
is an interesting example of “bifunctional” CO, activation
by a combination of basic and acid metal centers. It is notable that complex 31 functions as a reversible CO, carrier
and that the Co-C bond is easily broken. Finally, complex
32 is presented as an example of structure type 22, in
which the metal atom assembly is h o m ~ n u c l e a r . ! ~ ~ . ~ ~ ~
\I
suggest that the use of carbon dioxide in homogeneously
catalyzed processes is likely to be successful.
In catalytic reactions of carbon dioxide, the COz molecule bonds to a substrate Su in the coordination sphere of
a transition metal (cf. Fig. 3). This can occur by simultaneous oxidative coupling of CO, and substrate as they add
to the metal (step VII in Fig. 3). It is equally possible that
C 0 2 inserts into a previously formed M-Su bond (step I1
in Fig. 3). More detailed investigations of these important
potential catalytic steps are presented in Sections 3 and
4.
3. Oxidative Coupling with Carbon Dioxide
Oxidative coupling is an important basic reaction in
stoichiometric carbon dioxide chemistry; it leads to products that have been postulated as intermediates in catalytic
processes. In the following discussion oxidative coupling
is understood to be a reaction of carbon dioxide in which a
metal center M, an unsaturated compound X = Y, and
CO, react together to form a metallacycle [Eq. (3)J.
(3)
In all known examples of this type of compound, the
oxygen of the CO, molecule binds to the metal center, not
the carbon. The principle of oxidative coupling has been
known for a long time; Wilkinson and his co-workers described the reaction of platinum phosphane complexes
with oxygen and carbon dioxide in 1967. They obtained a
peroxycarbonato complex [Eq. (4)] as well as a carbonato
Similar reactions have been described for
rhodium.149,
?---K..
;\c/o
LJ
31
30
[(PR3InPt]
o*
‘$0
*+c
-
10-0
iPR31n Pt, o’C90I
(4)
A reaction that has already been presented in Section 2
is the joining of two C 0 2 molecules at an electron-rich
iridium center, forming the complex 28.’233.241
This also
can be regarded as an oxidative coupling [Eq. ( 5 ) ] .
26
32
By far the most important synthetic reactions of CO,
complexes are substitution and addition reactions, which
also play a decisive role in homogeneous transition-metal
catalysis. It is also striking that the synthetic reactions generally occur under mild conditions, nearly always at room
temperature and normal pressure. Homogeneous, transition-metal-catalyzed reactions also generally take place at
low pressures and temperature^.'^^-^^^ These observations
664
A more detailed investigation of oxidative coupling with
C 0 2 has been in progress since the beginning of the 1980s.
It has been shown that, besides the 0-CO, bond formation
discussed above, N-COZ and C-CO, bonds can also be
formed. The predominant transition-metal compounds
were electron-rich Nio complexes.
The reaction of carbon dioxide with aliphatic aldehydes
in the presence of (2,2’-bipyridine)(l,5-~yclooctadiene)nickel(o), [Ni(bpy)(cod)], proceeds at room temperature and normal pressure to form an NiCz02 ring [Eq.
(6)].[51.521
Angew. Chem. Inr. Ed. Engl. 27(1988) 661-678
[Ni(bpy)(cod)]+ R C H G + C G 2
-cod
/L
(bpy) N I
\o/C
I
on treatment with hydrochloric acid. Walther, Dinjus. and
their co-workers were able to show that conjugated dienes
also react with carbon dioxide at nickel(o) c o m p l e ~ e s . [ " ~ ~ ~ ~ ~
I n contrast to the five-membered metallacycles described
above, complexes with a dianionic ligand are formed,
which contain an q3-coordinated allyl group and a carboxylate group [Eq. (lo)]. This C-C bond formation can be
explained in terms of an initial q2 coordination of the 1,3diene. With L^ L = tetramethylethylenediamine and with
2,3-dimethylbutadiene as diene a crystalline complex was
obtained; a crystal structure analysis showed["' that the
monodentate carboxylate group, the allyl C atoms, and a
nitrogen atom of the chelating ligand coordinate the nickel
in a plane, whereas the second nitrogen atom interacts
only weakly with the metal.
(6)
*O
Analogous reactions, but this time with C-N bond formation, occur with azaolefins RCH=NR'.[53-561The use of
c a r b o d i i m i d e ~ [ ~o ~r Ia~adienes[~'lalso leads to N-C bond
formation. Cenini and his co-workers observed an oxidative coupling of CO, with an qz-bonded nitrosobenzene ligand [Eq. (7)].[5y.6"1
Ph
A C-C bond is formed when alkynes (e.g., acetylene, 2butyne, or tolane) are allowed to react with carbon dioxide
and nickel complexes ;[61-631oxanickelacyclopentene derivatives are formed in yields of between 40 and 90%. Olefins can be used instead of alkynes. Open-chain and cyclic
olefins both lead to oxanickelacyclopentanes, which can
either be isolated as complexes o r identified on the basis of
the products obtained by acidic d e c o m p ~ s i t i o n . [ The
~~-~~~
oxidative coupling with olefins is exemplified by the following reactions with cycloalkenes.[681The reaction of the
four-membered ring bicyclo[4.2.0]octatriene, which is in
equilibrium with cyclooctatetraene, leads to an oxidative
coupling of both isomers with CO, [Eq. (S)]. A mixture of
the nickel complexes 33 and 34 is formed: these are de-
+
0
36
composed by hydrochloric acid, giving the carboxylic
acids 35 and 36. Dicyclopentadiene was employed as a
"five-membered ring molecule"; it possesses two rings
with greatly differing reactivities. Equation 9 shows that
the norbornene double bond undergoes an oxidative coupling with CO, and nickel to form complexes 37 and 38,
which yield the diastereomeric carboxylic acids 39 and 40
- ;2
Angew. Chum. In[. Ed Engl 27 (1988) 661-678
The hydrolysis of the complexes obtained according to
Equation (10) with dilute mineral acids leads to 3-pentenecarboxylic acids. The idea of obtaining carboxylic acids
from oxidative coupling of 1,3-dienes and carbon dioxide
has formed the basis for intensive study. 1,3-Pentadiene
can be converted into sorbic
(after workup with
maleic anhydride) and butadiene into 2,4-pentadienecarboxylic
In the presence of pyridine, reaction with a
further C 0 2 molecule yields an a,w-dicarboxylic acid.[761
Since both alkenes and 1,3-dienes can react with carbon
dioxide at nickel(o) complexes, it is interesting to consider
how a molecule with both functional groups would react.
The reaction of 1,3,7-octatriene with (bipyridine)(cyclooctadiene)nickel and carbon dioxide leads to the nickel carboxylato complexes 41 and 42 (Scheme l).[771The decomposition of these complexes with HCI in methanol produces the monomethyl ester 43 and the dimethyl diester
44.The nature of the products is evidence that the reaction
takes place at the diene function of the starting material.
The carboxylato complex 41a containing an q3-allyl group
is formed by oxidative coupling of the diene analogously
to Equation 10; 41a is in equilibrium with 41b, in which a
Ni-C q ' bond is present. A further molecule of CO, can
insert into this bond, forming the dicarboxylato complex
42. This reaction is an interesting example of an oxidative
coupling and a CO, insertion occurring at the same nickel
center.
38
665
II
41 a
II
0
41 b
0
2
/o-c3-
(bpylNi
\ 0-
HCI / MeOH
c
V/
c
C
O
O
M
e
COOMe
b
42
44
Scheme 1. Coupling of a molecule contalning mono- and diene functions with COz
4. Insertion Reactions of Carbon Dioxide
4.1. COz Insertion into the M-H Bond
In the last two decades, many insertion reactions of carbon dioxide into M-C, M-H, M-0, and M-N bonds have
been described in the literature. Scheme 2 provides a survey of some of the products.
The insertion of CO, into the M-C bond generally leads
to a carboxylato complex via the formation of a new C-C
bond. Insertions in the opposite sense, forming a new C - 0
bond and thus a 1-metalated formate ester, have only been
observed in a few special cases. Similarly, the insertion
into M-H bonds can lead to either formato complexes or
1-metalated formic acids. Insertion into M - 0 bonds preferentially forms alkyl or aryl carbonato complexes; insertion into M-N bonds forms carbaminato complexes.
In insertion reactions the first step is the coordination of
CO, to the M-X fragment (X = H, 0, N, C). In the case of
metal hydrido complexes, hydridometal-CO, intermediates can be formed (Scheme 3), which then rearrange by
hydrogen migration to formato complexes (path A) or 1metalated formic acids (path B).
H
,
[‘--;*“I I
M.... 0
1-
M
+
co2
-
,.*?A
M,. >C-OR
-
’0
L[y----i
,0.
M----C
’.d/
M/ >C-CR3
M0
0
Pc - 0 - CR3
M..
.p- NR2
’0
.
‘0
-3
R=alkyl. aryl. H
-
P
-
Scheme 2. Adducts formed by insertion of COz into M-X bonds (X
N, C).
=
H, 0,
Several review articles covering these insertion reactions
have been p ~ b l i s h e d . [ ’ ~Here
- ~ ~ ] we shall discuss C 0 2 insertion in terms of some typical examples, showing the
wide range of applications of this reaction type.
666
CI
M-0
M-C /
__
,o‘3
’
M,.C; ; ,
-H
’0
0-H
\O
*O
Scheme 3. lnsertion of C 0 2 into the M-H bond.
,O
M’‘, .;/
>C-H
.*’
40
Most transition-metal hydrido complexes react with
COz to give formato complexes. Important examples are
the hydridometal carbonyls [MH(CO),]’ (M = Cr, Mo,
W)[86-s*1as well as hydrido complexes of r h e r ~ i u m , [ ~ ~ . ~ ~ ]
iron,[”l r u t h e n i ~ m , ’ ~ and
~ - ~ ~ 1o p p e r . [ ~
There
~ , ~ is
~ ’only one
piece of indirect evidence for the existence of a l-metalated formic acid: Kolomnikov, Volpin, and their co-workers
found such a species in a yield of 17% from the reaction of
a nitrogencobalt complex with C02.178,971
As seen from
Equation 1 I. the identification was based on the product
[ c o ( N 2 l H ( PPh3 131
+
C02
-ICo
-N2
MeI
I COOH I I PPh3131
MeOH
(1 1)
Me-COOMe
B F3
Anyen. C l w n . I n , .
W . Enyl. 27 (1988) 661-678
ato species are obtained. Examples of alkyl carbonato
of subsequent reaction with methyl iodide and methanol/
complexes thus obtained are known for zirconium,['06'
BF,. However, the reactions of hydridocobalt complexes
tantalum,"081
molybdenum,['o91 and
with COz generally lead to formato c o m p l e ~ e s . ~ ~ ~ ~ ~ vanadiurn,[lo7]
~~
tungsten,['Io1 and also for the lanthanoids."
Alkyl
Transition-metal hydrido complexes are not limited to
carbonato complexes can be synthesized "in situ" from
the formation of M(0,CH) or M-COOH complexes on
a suitable precursor complex, an alcohol, and carbon dioxreaction with CO,, as demonstrated by the following exide~[104,112-114]H ydrogen carbonato complexes have been
amples.
established for (in particular) rhodium and iridium." I s . ' 16]
The hydridozirconiuin complex [ZrCp2C1H] reacts
In some cases the formation of metal carbonato complexes
with excess CO, to form [(ZrCp,Cl),O] and formaldewas also observed.[117,1181
An interesting
.
dimeric carbonato
hyde.[l"0-'021A reduction of the hydrido complex to the zircomplex, in which the C 0 3 ligand functiom as an q',qlconium methanolato complex can be achieved by using the
bridging group, is obtained from the reaction of the hyhydrido complex in excess. The different reaction products
droxoplatinurn complex [PtPhL,(OH)] with C 0 2 [Eq. ( 1 9 ,
may be formed via a binuclear intermediate with a n
L = P(CH2Ph)3].[1191
0 - C H , - 0 bridge [Eq. (12)].
/
Cp2Zr,
H
-
+
co2
Another unusual reaction was studied by Eisenberg et
al.; a hydrido-bridged dirhodium complex reacts with CO,
to form a dirhodium complex with a formato bridge [Eq.
(13)].[10~~
0
The insertion of CO, into copper alkoxo complexes is
worthy of particular attention in this respect. It leads not
only to the expected alkyl carbonato complexes,"20~'211
but
also, by various routes, to a copper hydrogen carbonato
complex["'^ '231 that decarboxylates in water or organic solvents and transfers the released CO, to an organic substrate. Scheme 4 shows the reaction with cyclohexanone as
an example of such a transcarboxylation. The copper hydrogen carbonato complex functions here as a reversible
CO, carrier, which could win importance in catalytic reactions.
1
0
x
0
co2
co
IL, Cu- 02 C O t B u 1
co
[ L,CU
-orBu]
The reaction of CO, with hydridoplatinum complexes
allowed an interesting observation; starting from the complexes [PtHZL2] in benzene o r toluene, a monoformato
complex is
whereas in acetone o r acctonitrile
a binuclear cationic complex with a free formate counterion is formed [Eq. (14), L = PEt3].11051
Scheme 4 Insertion of C 0 2 into copper alkoxo complexes I
4.3. CO, Insertion into the M-N Bond
, , are
Many examples of insertion of CO, into M-P
known for the early transition metals (as was a '
11 to
be the case for insertion into M 3 bonbs). Meta'
Jminato complexes are formed in nearly all such rea
s (cf.
Scheme 2). Chzsholm and Extrne investigated COertion
into dimethylamine complexes of titanium and zirconium
and observed a rapid exchange of l3C-labelld carbon
dioxide [Eq. ( 16)].[12412'] The carbon dioxide obviously
only loosely bound; the carbaminao complexes, like the
copper hydrogen carbonato species shown in Scheme 4,
may be considered to be "C02 carriers."
)
4.2. C 0 2 Insertion into the M - 0 Bond
A variety of transition-metal complexes with M-OR
groups undergo insertion reactions with CO,. If R is an
organic moiety, alkyl or aryl carbonato complexes are
formed (cf. Scheme 2); if R is hydrogen, hydrogen carbonAngew. Chem. I n [ . Ed Engl. 27/1988) 661-678
i d
667
’2c02
-
noted for the M-H bond); the “normal” insertion to form
a carboxylato complex and the “inverse” insertion to form
a I-metalated formate ester. However, this is a somewhat
oversimplified description; C 0 2 insertion into the M-C
bond will be considered here in more detail.
ITI (0212C-NMe2 1 ~ 1
C 0 2 insertion into the M-N bond has been investigated
not only for metals of the titanium group,[’z6-’281but
also for metals of the vanadium[’291 and chromium
g r o ~ p s . [ ’ ~ ” -M
’ ~echanistic
~~
studies show that small
amounts of free amine accelerate the reactions markedly.
It may thus be assumed that amine and C 0 2 form a carbamic acid, which then, in a second step, reacts with the
metal amine complex [Eq. (17)]. This would imply that the
reaction should strictly be classified as a ligand exchange
rather than a C 0 2 insertion.
Me2NH
+
CO2
f---L
+ l M - N Me21
-
4.4.1. Insertion into Metal-Aryl and Metal-Alkyl Bonds
The insertion of CO, into aryltitanium complexes was
investigated in the 1970s.[143~’451
Kolomnikou et al. observed
the formation of the metallacycle 45 (Scheme 5 ) by the
reaction of diphenyltitanocene with CO,; the product was
identified by X-ray structure determination. An arynetitanium complex was assumed to be an intermediate. Treatment of the crude product with methanol/BF, leads not
only to the expected methyl benzoate but also to small
quantities of dimethyl phthalate, the product of a double
insertion
Me2NCOOH
(17)
IM-02CNMe21
t
Me2NH
-
I-
Reactions of C 0 2 with amine complexes are also known
for the metals uranium,”351
~ i n c , [ ” ~ . and
’~’~
c ~ p p e r , [ ’ ~ ‘ , but
’ ~ ~not
- ’ ~for
~ ~the metals of the iron, cobalt,
and nickel groups. The carbaminato complexes of the noble metals are however accessible by an indirect synthetic
route, namely the reaction of metal complexes with amines
and CO,; the ruthenium hydrido complex [RUH(PR~)~]PF,
reacts with dimethylamine and carbon dioxide to form the
carbaminato complex [Ru(o,CNMe,)(PR,),]PF,
(R = Me,
Ph), and the dimethylpalladium complexes [PdMe,L,]
react
similarly
to
give
[PdMe(OZCNMeZ)L2]
(L = p~~).[141.1421
J
co2
OIcooue
45
COOMe
Cp2Ti
4.4. C o t Insertion into the M-C Bond
COOMe
0
As shown in Scheme 2, there are two modes of CO, insertion into complexes with an M-C bond (as already
Scheme 5. Insertion of COz into diphenyltitanocene.
[Ln N I - P h ]
::
,
L N 1 CH2 CH2 P h 1
[Ln N i - 0 - C - P h ]
)I
lL,NI-HI
[LnNi-O-C-CH2
t
P h - CH=CH2
+[ L n N1- H )
I
I I
Ph-CH2CH3
CH2 P h ]
+[LnNI -HI
Ph-BU
MeOH/
fBF31
P h - CH 2 CH 2-COO Me
Ph - COOMe
Scheme 6. Reactions with phenylnickel complexes. For L see “461.
668
Angew. Chem. Int. Ed. Engl. 27 11988) 661-678
Phenylnickel complexes can also undergo CO, insertion
to form nickel benzoato complexes, which are then esterified to release methyl benzoate (Scheme 6). It is also
known that ethene can insert into the nickel-phenyl bond.
A (2-pheny1ethyl)nickeI complex is formed, which undergoes !?-elimination of styrene. Double insertion leads to a
(4-pheny1butyl)nickel complex ; the subsequent elimination
product, n-butylbenzene, was also identified.
Particular attention should be drawn to the first example
of a combined insertion of ethene and carbon
The initial product may be the (2-phenylethy1)nickeI complex, into whose Ni-C bond the CO, then inserts. The subsequent treatment with methanol/BF, releases methyl pphenylpropionate (Scheme 6).
4.4.2. Insertion into Metallacycloalkanes
Insertion reactions of CO, into metallacycloalkanes are
important because metallacycles are often postulated as intermediates in numerous transition-metal-catalyzed reactions forming C-C bonds, e.g., the dimerization of cyclopropene derivatives and methylenecy~lopropane,~'~~~
the
oligomerization of a11ene,1'481and the dimerization of
ethene.1'4y1
The nickelacyclobutane 46 is presented as an example
of a metallacycle that undergoes insertion reactions; it is
generated "in situ" from (bipyridine)(cyclooctadiene)nickel(o) and quadricyclane (Scheme 7). Its reaction with CO,
at room temperature forms the nickel carboxylato complex
47 in 92% yield.'681The decomposition of 47 with acid releases the endo-nortricyclane-3-carboxylicacid 48.
bered ring) does not occur. However, complexes of other
transition metals-e.g., anionic complexes of tungsten [Eq.
(19)J-form seven-membered rings of sufficient stability." 5 11
I
i l
1
L
4.4.3. Insertion into M-C Bonds Generated in situ
In Sections 4.4.1 and 4.4.2 carbon dioxide insertion reactions into q' transition-metal-carbon bonds of well-defined complexes were described. The possibility of generating the M-C bonds in situ, by oxidative addition of cycloalkanes to transition-metal complexes, was also mentioned (see Scheme 7). Here a further "in situ" method is
presented; oxidative addition of compounds with acidic
C-H functions can lead to intermediate alkylmetal complexes that then undergo C 0 2 insertion.
Zttel, Tolman et al.[1521
studied a hydrido-naphthyl complex of iron, to which eH2CCN (from MeCN) could be
added oxidatively, naphthalene being eliminated. The insertion of C 0 2 into the resulting Fe-C bond led to a carboxylato complex, which was identified spectroscopically
and which reacted with methanol/BF, to give methyl cyanoacetate [Eq. (2O)J. An analogous reaction in iridium
chemistry was described by English and H e r s k o ~ i t z . " ~ ~ ~
These reactions demonstrated the feasibility of forming
C-C bonds between acidic hydrocarbons and carbon dioxide in the presence of transition-metal complexes.
[ F e ( n a p h t h y l i (dmpe),Hi
+
*
MeCN
-
IFe ldmpei,(CH,CN) H I
CO2
I F e ( d r n p e 12 1 02C - CH2 CN 1 H I
tOOH
48
Scheme 7. Insertion of C 0 2 into a nickelacycle
The manganaphosphacyclobutane 49 reacts with C 0 2 to
form the insertion product 50 in yields of u p to 40% [Eq.
(1s)].['
C10H8
[BFjl
Me OH
(20)
NC
-
CH2 - C 0 2 Me
In order to understand this reaction type better, further
experiments were carried out using iridium and rhodium
complexes and the strongly C-H-acidic malonodinitrile.['541The complex 51 can be isolated from the reaction
between hydridotetrakis(trimethy1phosphane)iridium and
equimolar amounts of malonodinitrile and carbon dioxide
[Eq. (21)J.
51
49
SO
Whereas the manganacyclobutane forms a six-membered ring on CO, insertion, an analogous insertion into
the manganaphosphacyclopentane (to form a seven-memAngew Chem. Int. Ed. Engl. 27 11988) 661-678
The distorted tetrahedral arrangement of the phosphane
ligands inferred from spectroscopic data was confirmed by
a n X-ray structure analysis (Fig. 4); this also showed that
the dicyanoacetate is dimeric. The central carbon atom,
669
noacetate migrates to a carboxylate oxygen and forms a
C-deprotonated acid that dimerizes by hydrogen bond formation.
The question arises as to how the reaction that forms the
C-C bond actually proceeds. One possibility (path A in
Fig. 5 ) involves the oxidative addition of the C-H-acidic
Fig. 4. Unit cell and molecular structure of complex 51
which bears the cyano and carboxylate groups, is sp2-hybridized [see also Eq. (23)], and the distance of 258 pm between oxygen atoms in neighboring monomeric units is
typical of carboxylic acid hydrogen bonding.
The formation of C-C bonds between malonodinitrile
and CO, takes place not only with metal hydrido complexes but also with metal chloro complexes, e.g.,
[Ir(depe),CI] (depe = bi~-1,2(diethylphosphino)ethane). In
this case, however, octahedral complexes are formed in
which the chloro ligand and the newly added hydrogen
atom are mutually trans [Eq. (22)].
R
co2
Fig. 5. Possible mechanisms in the reaction of C-H-acidic substrates with
C 0 2 in the presence of electron-rich transition-metal complexes.
32
M=Ir.R=Et
33
M = Rh. R = Me
The generalized Equation (23) summarizes the reactions
of the iridium and rhodium complexes with malonodinitrile and carbon dioxide. The first step is the formation of
a positively charged metal hydrido complex, stabilized in
solution by a dicyanoacetate counterion. Malonodinitrile
and CO, then undergo the expected C-C bond-forming
reaction; however, they do not, as previously assumed, remain bonded to the transition metal as a ligand, but form a
free anion in solution. On crystallization, a further remarkable feature is observed; the methine proton of the dicyaf L n M I * CH2ICNI2
+ C02
-
1
M
=
Ir.Rh
L = PMe3, dmpe. depe
n = 2.4
670
I
[LnM-HI'IICNIZCHCOOl0
IL,M-H
in s o l u t i o n
(23)
J @ [ l C N ) 2 CCOOH
in t h e crystal
lo
1,
compound R-H to the transition-metal complex L,M,
forming an alkyl-hydrido complex. The insertion of CO,
into the metal-carbon bond then leads to a carboxylatohydrido intermediate, which rearranges to the ionic species
[L,MH]OIRCOO]" because of the electron-withdrawing effect of the R group. The second possible reaction path (B
in Fig. 5) involves a proton transfer from R-H to the transition-metal complex L,M, leaving a carbanion Re, which
then reacts with the electrophilic CO, to form the anion
R-C0OQ. Path B, which may be described as a "transition-metal-initiated" C-C bond formation, cannot be considered as a CO, insertion; it is however a feasible alternative explanation for the formation of the complexes
[L, M] '[ RCOO]' .
4.4.4. Insertion into Metal-Olejin Bonds
CO, reacts with the bis(ethene)molybdenum complex
[Mo(C,H,),(PMe,),] in a remarkable way.['551This complex
possesses an electron-rich metal center surrounded by labile trimethylphosphane ligands. Carbon dioxide reacts
with the coordinated ethene, forming a new C-C bond,
and the product is a binuclear molybdenum complex with
two bridging acrylic acid ligands (Scheme 8). Hydrogenation leads to a hydrido-propionato complex, which on
treatment with n-butyllithium and ethene releases lithium
propionate and re-forms the starting complex. This reaction sequence is of particular interest because it outlines a
possible future catalytic synthesis of industrially important
acrylic acid from ethene and carbon dioxide.
Angew. Chem. I n t . Ed. Engl. 27j1988) 661-678
Teuben et al. suggest an alternative electrocyclic mechanism, coupled with an allyl migration, for the reaction of
the allyltitanium complexes with CO, [Eq. (24), R = H,
CH,I.
t
nBuLi
L
H
Bis(q3-allyl)nickel reacts with CO, at room temperature,
forming q3-allyl(vinylacetato)nickel, which releases y-butyrolactone on heating to 140°C [Eq. (25)].L1"01
0
i
Scheme 8 Insertion of C 0 2 into a Ma-ethene bond. L = PMe,.
4.4.5. Insertion into Metal-Ally1 Bonds
The insertion of CO, into q3-allyl complexes of titanium
has been studied in detai1.['56-'591
Sato et al. prepared the
crotyltitanocene complex from [TiCp,Cl,], butadiene, and
a Grignard reagent (Scheme 9) and allowed it to react with
C 0 2 yielding a crotonato complex. Subsequent hydrolysis
leads to 2-methyl-3-butenoic acid and [TiCp2C1], which
can be reconverted into [TiCp2C12]. This stoichiometric
reaction sequence corresponds to a catalytic synthesis of
@
,CI
-
HCI
+
-
02
H20
&I,
@
K'
Jolly, Wilke et al. were able to show that bis(q3-2-methyla1lyl)nickel reacts quantitatively with carbon dioxide at
-30°C in the presence of basic phosphanes to form a carboxylato complex [Eq. (26)].[16i1
Allylpalladium complexes have also been treated with
C 0 2 . It0 et al. carried out the reaction of bis(q3-allyl)palladium and bis(q3-crotyl)palladium with CO, and obtained
the corresponding carboxylic acids after treatment with
mineral acid [Eq. (27a), (27b)].['621
Joiiy, Wifke et al. were able to trap the intermediates in
this reaction by adding basic p h ~ s p h a n e s . " ~With
~ ' trimethyl- or tricyclohexylphosphane, complexes were formed
that contained both an q 1 and an q3 allyl ligand. The insertion of CO, into the Pd-C G bond led to an allyl-carboxyIato complex, the ligands of which could be displaced by
carbon monoxide [Eq. (28), R = Me, Cy].
Scheme 9. Insertion of C 0 2 into a titanium-ally1 bond
methylbutenoic acid from butadiene and COz, although
HCI and Grignard reagent are consumed. Particularly notable is the fact that chiral cyclopentadienyl ligands cause
an asymmetric insertion of C 0 2 ; by use of a crotyltitanocene complex with a menthyl-substituted cyclopentadienyl
ligand, the (S)-2-methyl-3-butenoic acid was formed with
an optical yield of 18.9%.
Angew. Chem. Int. Ed. Engl. 27(1988) 661-678
The binuclear octadienediylpalladium complex 54 displays some remarkable reactions (Scheme 10).['64.1651
The
addition of one equivalent of triisopropylphosphane leads
selectively to complex 5 5 , in which only one palladium
center has acquired a phosphane ligand. Complex 55
reacts with CO, to give 56, which reacts with hydrochloric
67 1
acid, releasing 3,8-nonadiene- 1-carboxylic acid. The catalytic hydrogenation of this acid yields pelargonic acid.
5. Homogeneously Catalyzed Reactions of
Carbon Dioxide with hydrocarbon^"^'^
F 3 C m C F 3
5.1. Reactions with Alkynes
;Pd
iPr3P 9
oc:
Pd.
i
JJ_ F3C
The reaction of alkynes with carbon dioxide was first reported in 1977 by Inoue et a1.['671The reaction between
1-hexyne and CO, led to 4,6-dibutyl-2-pyrone in 50%
yield [Eq. (30)J. Analogous reactions were observed with
alkynes bearing nonterminal triple bonds (e.g., 3-hexyne,
4-0ctyne).['~*]Nickel complexes with bidentate phosphane
COOH
CF3
transition-metal allyl complexes with asymmetric ally1 ligands always took place at the secondary carbon atom C 3
[Eq. (29), path A] (see Scheme 9 for titanium complexes,
Eq. (27) for palladium complexes). This necessarily led to
branched-chain products. The insertion into complex 5 5 ,
however, takes place at the terminal carbon atom C1 [Eq.
(29), path B] and the workup leads to a straight-chain
product.
The mechanism of this reaction type was intensively
studied, and two possible reaction paths (A and B in Fig.
6) were discussed. In path A, the first step is the oxidative
coupling of two alkyne molecules, forming a nickelacyclopentadiene species. The insertion of CO, into a Ni-C bond
would then form a cyclic carboxylato complex, which
would release the pyrone by reductive elimination. This
catalytic cycle could however not be confirmed by comparative stoichiometric studies with nickelacyclopentadiene
complexes. Path B involves the reaction of an alkyne and a
CO, molecule to form an oxanickelacyclopentene complex, which then reacts with a further alkyne, expanding to
a seven-membered ring. The use of a stabilizing ligand
(e.g., the bidentate, basic N,N,N',N'-tetramethylethylenediamine ligand) made possible the isolation of both catalytic intermediates. This is reliable evidence that path B is
the actual mechanism of nickel catalysis.
1
L .&
R
C-0-MLn
(02
R-CEC-R
L x N ~ R2 R
R
2 R-CZC-R
h
Fig. 6. Suggested mechanisms for the nickel-catalyzed reaction of alkynes with C02.
67 2
Angew. Chem. Inr. Ed. Engl. 27 11988) 661-678
Aresta and A / b ~ n o [studied
' ~ ~ ~ the reaction of C 0 2 and
propyne with the catalyst [Rh(dppe)(q-BPh,)] (dppe = 1,2bis(dipheny1phosphino)ethane). They observed the formation of 4,6-dimethyl-2-pyrone in yields of u p to 3%. A
mechanism was suggested in which the active intermediates were.not metallacycles but rhodium complexes with
noncyclic substituents.
5.2. Reactions with Monoenes
Transition-metal-catalyzed reactions of carbon dioxide
with monoenes are appreciably more difficult than those
with alkynes. Lupidus et a1."701reported in 1978 a rhodiumcatalyzed reaction of CO, and ethene, forming propionic
acid and ethyl propionate. Pressures of 700 bar and a catalytic system consisting of Wilkinson's complex and a hydrogen halide were necessary to achieve yields of up to
38%. Besecke and S ~ h r i i d e r " ~patented
'~
in 1981 a method
of synthesizing esters from propene and carbon dioxide in
the presence of ruthenium catalysts and alcohols [Eq. (31)].
Mechanistic studies with I3C-labeled methanol, however,
showed that the carbon of the carboxyl group came not
from the carbon dioxide but from the
Methylenecyclopropanes are more reactive than linear
olefins. Inoue et al. succeeded in 1979 in converting substituted methylenecyclopropanes and carbon dioxide into ylactones by using palladium catalysts [Eq. (32)].["31
R).(R+
c02
I
Pd
I
0
R =Me
R . R = -ICH2)5-
In the presence of palladium complexes containing
monodentate phosphane ligands, the p-methylene lactone
is preferentially formed, whereas the use of [Pd(dppe),]
leads to the a$-unsaturated lactone as the main product.
4:- Pd
L
+c02
Fig. 7. Suggested mechanism for the reaction of rnethylenecyclopropane with
CO?.
Angew Chem. Int.
Ed Engl. 27 (1988) 661-678
The suggested mechanism (Fig. 7) involves a trimethylenemethane-palladium intermediate. The CO, insertion forms
a cyclic carboxylato species from which the lactone is released.
Binger et a1.11741
have carried out extensive investigations
on the reaction of carbon dioxide with the unsubstituted
methylenecyclopropane. Optimum reaction conditions allow the formation of 3-methyl-2-butene-4-olide in 80%
yield. In addition to this "co-dimer," however, the further
reaction of the lactone with the starting material leads to
co-trimers, co-tetramers, and co-pentamers.
5.3. Reactions with Dienes
Both cumulated and conjugated dienes can undergo
reactions with C 0 2 . The palladium-catalyzed reaction of
CO, with allene leads to the synthesis of two esters and a
pyrone [Eq. (33)]. Since pyrones are also formed from aln
I
0
kynes and CO,, it was assumed that the allene first isomerized to propyne. Experiments with propyne instead of allene showed, however, that the alkyne does not react with
carbon dioxide under comparable reaction conditions."751
The optimum catalyst for the allene-CO, reaction is a
combination of 1,2-bi~(~~-alIyl)palladium
and bis(cyclohexy1phosphino)ethane. The synthesis presumably occurs
via metallacyclic intermediates. Cyclic nickel carboxylato
complexes were indeed isolated from stoichiometric reactions between allenes and carbon
The first studies of the catalytic reactions of 1,3-dienes
with CO, were carried out by the research groups of
Inoue~"7~17x1
and M U S C O [ ' ~at~the
~ ' end
~ ~ 'of the 1970s. They
succeeded in synthesizing lactones and esters in small
quantities from dienes and C 0 2 , and isolated the products
using preparative gas chromatography or column chromatography. Since these first results, preparative methods
have been developed that allow the synthesis of a variety
of products from dienes and carbon dioxide on an industrial scale.['x1-18x1
The reaction of butadiene with CO, leads to various
products, depending on the catalyst. Figure 8 shows that
several connected catalytic cycles can be postulated, starting from the transition-metal complex ML,. Cycle A applies to palladium, ruthenium, or nickel catalysts. The dimerization and addition of two butadiene molecules leads
to the bis(q3-allyl) complex 57. Carbon dioxide inserts into
one ally1 bond, forming the allylcarboxylato complex 58,
from which the products 59 to 63 are released, regenerating the catalyst ML,. Ring closure leads to the 6-lactone
2-ethylidene-6-hepten-5-olide 59 and the y-lactones 60
and 61; the esters 62 and 63 are obtained by addition of
two further butadiene molecules.
673
I
58
62
59
t
I
61
63
67
Fig. 8. Catalytic reaction between butadiene and carbon dioxide; products and postulated mechanism with intermediates. L = PR,
Cycle B is observed with rhodium catalysts. Three molecules of butadiene coordinate to the metal and form complex 64;this reacts with carbon dioxide to give the intermediate 65,from which the y-lactone 66 is formed.
Cycle C describes a variant of the reaction catalyzed by
rhodium. Rhodium compounds can bind the 6-lactone 59,
opening the ring, and simultaneously coordinate a further
molecule of butadiene, leading to the intermediate complex 67; C-C bond formation converts this into another
intermediate 65,the precursor of the y-lactone 66.
The crucial intermediates in the reaction of butadiene
with CO, are carboxylato complexes. To support the proposed mechanisms (Fig. s), the isolation of such a carboxylato species in a comparable stoichiometric experiment
was attempted. The reaction of bis(acety1acetonato)palla-
Fig 9 Molecular structure of complex 68.
614
dium with triisopropylphosphane and the &lactone 59 allowed the isolation of the complex bis(2-ethylidene-4,6heptadienoate)bis(triisopropylphosphane) 68. The X-ray
structure analysis of this complex (Fig. 9) shows that both
phosphane ligands and both carboxylato ligands are trans
at the square-planar-coordinated palladium atom and that
the carboxylato ligands are monodentate.
Neither isoprene nor 1,3-pentadiene (piperylene) reacts
with CO, in the presence of those catalysts that promote
good yields ( > 50%) with the butadiene-CO, reaction.
However, it proved possible to react either isoprene or piperylene with butadiene and carbon dioxide; these composite syntheses lead to the lactones 69, 71,and 73 (Fig.
10) although the yields from such “three-component systems” are poor (maximum 5%).
The palladium-catalyzed reaction of isoprene with butadiene forms products that result from the linkage of butadiene with either end of the isoprene molecule. The corresponding intermediates should therefore be formulated as
bis(q3-allyl)palladium complexes with the additional methyl group in either the 3- or 2-position. The subsequent
insertion of carbon dioxide occurs exclusively in the allyl
group generated from butadiene.
The reaction between piperylene, butadiene, and carbon
dioxide forms only the “tail-coupling” products 73. The
alternative “head-coupling,” to produce the lactones 75
and 76, is not observed. The C 0 2 insertion, in exact analogy to the isoprene reaction, also occurs exclusively at the
nonsubstituted allyl group.
Further three-component syntheses have been carried
out with dienes, carbon dioxide, and epoxides. These reacAngew. Chem.
Int.
Ed. Engl. 27 (1988) 661-678
+
Head-coupling
Tail-coupling
Tail-coupling
Head-coupling
t
t
1
tions represent the attempt to extend the catalytic principles formulated in Figure 3 to reactions of carbon dioxide
with two entirely different substrates Su.
Butadiene, carbon dioxide, and ethylene oxide react in
the presence of palladium catalysts, forming 77, the glycol
ester of 2-ethylidene-4,S-heptadienoicacid [Eq. (34)].1's9J
The corresponding reaction with propylene oxide is analogous, but produces two isomeric esters 78 and 79 with different positions of the additional methyl group.
It is remarkable that the octadienyl esters 62 and 63 and
the glycol esters 77 to 79 are derivatives of the same carboxylic acid. It is probable that the mechanisms of their
formation are similar (Fig. 11). The first steps in the latter
O-0
'FiLx
0
58
0
59
ij
Fig. I I . Suggested mechanism for ester synthesis from butadiene, ethylene oxide, and carbon dioxide. M
L = PR,.
Angew. Chem hi.Ed. Engl. 27 (1988) 661-678
=
Pd, Rh:
675
reaction are the oxidative coupling of two butadiene molecules and the insertion of the C02, forming the palladium
carboxylato complex 58. The epoxide coordinates to this
intermediate product, undergoes ring-opening, and inserts
into the P d - 0 bond. The catalyst is regenerated by elimination of the glycol ester.
6. Concluding Remarks
How will the chemistry of carbon dioxide develop? A
wide variety of different possibilities were recognizable at
the “NATO Summer School on Carbon Dioxide,” which
took place in June 1986 in Pugnochiuso (Italy).~’901
The electrochemical exploitation of CO, has become the
object of great efforts. The electrochemical reduction of
carbon dioxide to carbon monoxide, formic acid, o r oxalic
acid, and the electrocarboxylation of olefins, aromatic species, and carbonyl and halogen compounds are both being
intensively studied. The photochemistry of COz has also
made great strides in the last few years, although an industrial application cannot yet be foreseen. A further possibility is that bacteria could be used to convert carbon dioxide
into organic products.
Carbon dioxide undergoes a series of industrially important catalytic reactions. The activity and selectivity of heterogeneously catalyzed CO, reactions are often higher
than those of corresponding reactions with carbon monoxide. The coordination chemistry of CO, and the related homogeneous catalysis by transition metals still seem particularly promising areas. “Tailor-made” metal-complex catalysts are able to control the reactions of C 0 2 and allow
high selectivity in the synthesis of organic chemicals. This
interesting research area still has many novelties to be discovered.
Received: August 4, 1986;
revised: January 14, 1987 [A 671 IEI
German version: Angew. Chem. I00 (1988) 681
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