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Cooperativity in Rh2 Complexes High Catalytic Activity and High Regioselectivity in the Hydroformylation of Olefins.

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HIGHLIGHTS
Cooperativity in Rh, Complexes: High Catalytic Activity and High
Regioselectivity in the Hydroformylation of Olefins
Georg Suss-Fink*
An important step forward in the search for catalytically
active polynuclear metal complexes has been made by
George G. Stanley’s group: The bimetallic complex
cations r~c-[Rh,(nbd),L]~+l r and rne~o-[Rh,(nbd),L]~+l m
(nbd = norbornadiene) which were isolated as tetrafluoroborates (Scheme l),[l1were obtained by reacting bis(norbornadiene)rhodium tetrafluoroborate with a new tetraphosphane
ligand L which exists in R,R, S,S and R,S forms and can be
separated into racemic (R,R/S,S)and meso structures (R,S).[’]
sides the problem of chemical selectivity, the hydroformylation
of higher olefins raises the question of regioselectivity depending
on whether the formyl group is added to the terminal or to the
inner carbon atom of the double bond. This results in a ratio of
linear to branched product (n/i ratio of the aldehyde formed)
[Eq. (a) and (b)]. The industrial processes are all based on
monometallic cobalt or rhodium complexes.
R-CH=CH,
+ CO + H,
4
L
-
--t
R-CH,CH,-CHO
(a)
n-aldehyde
R-CH(CH0)-CH,
(b)
i-aldehyde
rac-(R.R/
Et,P
Et\
/pEt2
- Rh
Rh
- PEt,
<jWP,
‘J
/pvp\ph
Ph
Ph
Ph
rac-lr
meso-(R,S)-L
Scheme 1. The tetrdphosphdne ligand L and the hydroformylation-active dirhodium complex I r .
The racemic complex lr, whose structure was determined by
single-crystal X-ray crystallography, is an excellent hydroformylation catalyst for olefins such as propylene, I-hexene,
and I-octene, combining high activity with high regioselectivity.
Up to 12000 catalytic cycles (10 cycles per minute) were
achieved in a single batch run without observing any decomposition of the catalyst. The conversion of alkene to aldehyde for
the hydroformylation of 1-hexene was 8 5 % , and the n/i ratio
was 96.5 : 3.5.[’]The combination of high activity and high
selectivity can be explained by cooperativity between both metal
centers in the catalytically active bimetallic complex.
The hydroformylation of olefins is the world’s most important homogeneous catalytic industrial process with more than
six million tonnes of 0x0 products produced each year.[3] Be[*] Prof. Dr. G. Suss-Fink
Institut de Chemie, Universitk de Neuchitel
Avenue de Bellevaux 51, CH-2000 Neuchltel (Switzerland)
Telefax: Int. code (38) 214081
+
Angew. Chem. Int. Ed. Engl. 1994, 33, N o . I
0 VCH
Ever since transition metal clusters have been discussed as
catalysts,[4]there have been many attempts to develop polynuclear catalytically active complexes in which the metal centers
interact during formation of the target molecule (“cooperativity”) to control activity and selectivity. In this way, a highly
selective hydroformylation catalyst for propylene was found in
the cluster anion [HRu,(CO), J- (linear to branched product
ratio of butyraldehyde 98.6 : 1.4), however, its activity is very
low with only 57 catalytic cycles in 66 h.I5] The high selectivity
and low activity suggest that the catalytic process occurs via a
sterically demanding, intact trimetallic structure, for which
there is experimental evidence.t6]In contrast to [HRu,(CO), J,
the tetrametallic neutral cluster [Rh,(CO),,] yields high catalytic conversions (84 000 cycles) for hydroformylation of 1-hexene,
however, the selectivity is unfavorable (n/i 54: 46).r71Accordingly, evidence for a catalytic route via monometallic rhodium
complex fragments was found.[’] The catalyst l r described by
Stanley et al. is the only polynuclear metal complex as yet which
compares favorably with the hydroformylation catalysts used
industrially.
The combination of high catalytic activity and high regioselectivity results from the interaction of the two rhodium centers
of l r during the hydroformylation cycle (Scheme 2). Under synthesis gas pressure, the complex cation l r used as a catalyst
precursor is converted into the neutral complex 2r which is
regarded as the active species. Norbornadiene and protons are
eliminated in the process. The coordinately unsaturated species
2r (both Rh atoms have 16 valence electrons) reacts on addition
of an olefin to give 3r, and by insertion of the olefin ligand in the
Rh-H bond forms 4r. The alkyl complex 4r is converted into
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67
HIGHLIGHTS
1
2+
(nbd)
(nbd)
Ph
Ph
2CO
+ ZH,
2nbd
+
2w
2r
4\
R
Ph
Scheme 2. Proposed catalytic cycle of hydroformylation
of oletins with I r as catalyst precursor.
5r
the corresponding acyl complex 5r by incorporation of CO. In
what is generally believed to be the rate-determining step. 5r
rearranges into 6r in which a hydrido and a carbonyl ligand are
arranged semi-bridging between the two rhodium atoms. The
doubly carbonyl-bridged complex 7r forms on aldehyde elimination. This is reconverted with hydrogen into the active
species 2r with elimination of carbon monoxide, thus, completing the cycle.
The key step in the catalysis cycle proposed by Stanley et al.
is the transfer of the hydrido ligand from one rhodium atom to
the other in the bimetallic complex 6r, in which the interaction
of both metal centers is the greatest ("cooperativity")
(Scheme 3). The most important evidence for this hypothesis is
the observation that if the bimetallic complex 2m (meso form),
a diastereomer of 2r, is used as the catalyst precursor, the activity and selectivity drop drastically. The meso form 6m can only
form a singly bridged species on hydride transfer because the
terminal CO group is unfavorably oriented at the acyl-bound
68
(0VCH
~~rlug.~ge~eIlsclmfi
mbH, D-69451 Weinheim, 1994
Rh atom. It is assumed that the interaction of the Rh centers
across the carbonyl bridge in 6r favors intramolecular hydride
transfer. This assumption was confirmed by SYBYL molecular
modeling investigations (molecular dynamics minimizations).
Stanley et al. have attempted to verify the hypothesis of intramolecular hydride transfer in the bimetallic hydroformyla-
OC
-
Rh
4
Scheme 3. Intramolecular hydride transfer in complexes 6r (left) and 6m (right).
The complexes are shown without the aldehyde ligands.
0570-0833~94/0l0l-006X
S /0.00+ ,2510
Angpw. Chem. h
i.
Ed. Engl. 1994, 33, N o . 1
HIGHLIGHTS
tion catalyst through a well-designed systematic investigation.
For this, the monometallic complex 8 (“half’ complex) and the
bimetallic complex 9 (“spacer” complex), both of which are
analogous to lr, as well as other similar complexes were synthesized. All these model complexes either do not catalyze the hydroformylation or catalyze the hydroformylation with only very
low activity and selectivity. Complexes 8 and 9 are comparable
12+
7
plains the difference in catalytic characteristics compared to lr.
Finally, the isolation and characterization of the bimetallic complex 7r (postulated in the catalytic cycle) from the reaction of l r
with CO and H,, suggestive of a sensitive equilibrium between
2r and 7r, adds additional support to the mechanism proposed
by Stanley et al.
The results of Stanley et al. indicate that a breakthrough in
the development of tailor-made homogeneous catalysts may be
expected. It now seems possible to fix two or more cooperating
metal centers in the geometry required for a particular catalytic
process with the aid of a suitable ligand matrix, with the aim of
controlling selectivity and activity of the catalyst.
German version: Angew. Chem. 1994, 106, 71
[I] M. E. Broussard, B. Jumd, S . C. Train, W.-J. Peng, S . A. Laneman. G. G. Stanley, Science 1993, 260, 1784.
(21 S. A. Laneman, F. R. Fronczek, G. G. Stanley, J. Am. Chem. SOC.1988, 110,
5585.
9
0
to l r both electronically and sterically ; however, an intramolecular hydride transfer is not possible either in the monometallic
complex 8 or in the bimetallic complex 9, in which both the Rh
atoms are kept further apart by phenylene spacers, which ex-
[3] K. Weisserrnel, H.-J. Arpe, Chimie orgunique indusfrielle, 1st edition, Masson,
Paris, 1981, p. 113; lndustrielle Orgunische Chemie, 2nd edition, Verlag Chemie,
Weinheim, 1978, p. 120.
[4] B. F. G. Johnson, J. Lewis, Pure Appl. Chem. 1975, 44, 43; E . L. Muetterties,
Bull. Soc. Chim. Belg. 1975, 84, 959; Science 1977, 196, 839.
(51 G. Suss-Fink, G. F. Schmidt, J: Mot. Chem. 1987, 42, 361.
[6] G. Suss-Fink, G. Herrmann, J: Chem. SOC.Chem. Commun. 1985, 7 3 5 .
[7] R. Lazzaroni, P. Pertici, S. Bertozzi, G. Fabrizi, J. Mu[. Catal. 1990, 58, 7 5 .
[XI C. Fyhr. M. Garland, Organometulhcs 1993, 12, 1753.
Oxidation of Weakly Activated C -H Bonds**
Oliver Reiser*
The selective oxidation of alkanes remains a challenge in organic synthesis. The development of economical processes for
the functionalization of hydrocarbons is of tremendous technical importance. In nature such reactions are mediated efficiently
by various enzymes. The systems related to cytochrome P-450,
which for example detoxify lipid-soluble compounds in the human liver,“’ have received the most attention.
The fundamental problem in the functionalization of saturated hydrocarbons is that their components, carbon and hydrogen, do not have lone electron pairs, and the molecules do not
have orbitals of sufficient energy that are easily accessible. Thus,
very reactive reagents and/or extreme reaction conditions are
typically required, for example for the oxidation of alkanes.
However, the initial products are almost always more reactive
[*] Dr. 0. Reiser
Institut fur Organische Chernie der Universitlt
Tammannstrdsse 2. D-37077 Gottingen (FRG)
Tekfax: Int. code (551)39-9475
[**I This work was supported by the Deutsche Forschungsgemeinschaft (Habilitation fellowship).
Angen.. Chem. I n [ . Ed. Engl. 1994, 33. No. 1
than the starting compounds, and undesired side reactions may
occur.
Other complications arise if the molecule to be oxidized contains different types of C-H bonds. Since formation of tertiary
radicals and carbenium ions is favored and they are also more
stable than their secondary and primary analogues, functionalization processes that proceed via these intermediates generally
have the selectivity for C atoms in the order tertiary > secondary > primary. For steric reasons, however, the attack of
bulky reagents at primary C atoms may be preferred; the best
example here is the oxidative addition of transition metal complexes.
The metal-mediated oxyfunctionalization of organic compounds, of olefins in particular, is becoming increasingly important.I2] Research on metal catalysts that activate elemental oxygen and make it usable as a selective oxidizing agent is therefore
especially worthwhile. Inspired by the enzyme cytochrome P450, in which the active center is an oxoiron(1v) unit, researchers
have examined numerous iron-containing reagents for the oxidation of alkanes. For example, the so-called Gif systems[3]
0 VCH Verlagsgesellschafr mbH,’D-69451 Weinheim, 1994
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hydroformylation, high, catalytic, olefin, regioselectivity, cooperativity, activity, rh2, complexes
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