3460 Organometallics, Vol. 19, No. 18, 2000
Communications
Ta ble 1. Hyd r ofor m yla tion of Ar om a tic Olefin s
w ith Ca r ben e Ca ta lyst 1a
for either the branched or the linear isomer (depending
on the application) is critical. Optimizing the branched/
linear ratio is generally accomplished by altering the
nature and the number of ligands on rhodium, which
are usually phosphorus-based.7 We report herein the
first example of carbene rhodium complexes 1 and 2 as
catalysts for the hydroformylation reaction.8 These
complexes are highly selective for the hydroformylation
of styrene derivatives, giving up to 50:1 selectivity for
the branched isomer (eq 1).
entry
Ar
loading (%)
yieldb (%)
B:Lc
1
2
3
4
5
Ph
p-MePh
p-ClPh
p-MeOPh
p-i-BuPh
1.0
1.0
0.8
1.0
1.1
85
88
94
91
95
96:4
94:6
98:2
95:5
97:3
a
Reaction conditions: substrate (0.8 mmol), catalyst (ca. 1%),
and solvent (5 mL, dry, deoxidized benzene) were treated with 500
psi of CO and 500 psi of H2 at 60 °C for 20-22 h, except for entry
b
5 (16 h). Isolated yield of chromatographically and spectroscopi-
cally homogeneous material. c B:L ) branched:linear ratio.
Ta ble 2. Hyd r ofor m yla tion of Styr en e w ith
Va r iou s Ca ta lystsa
entry
cat.
additive
conversnb (%)
TOFc
B:Ld
Rhodium complex 1 was prepared by treatment of
IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene)9
with freshly recrystallized Wilkinson’s complex ([RhCl-
(PPh3)3]). The resulting yellow solid was characterized
1
2
3
4
5
6
1
2
2
2
none
none
1 PPh3
2 PPh3
10 PPh3
2 PPh3
78
38
74
79
80
27
7
4
8
8
8
3
95:5
94:6
96:4
96:4
96:4
88:12
by H and 31P NMR. In the H NMR spectrum, the two
ortho methyl residues on the IMes ligand are inequiva-
lent. Saturation transfer experiments revealed that
these two methyl groups are undergoing slow exchange
on the NMR time scale.10 In the 31P NMR spectrum, two
resonances with distinct 31P-103Rh coupling constants
were observed at 48.7 and 35.4 ppm, attributable to
triphenylphosphine ligands that are trans and cis to
IMes. It is likely that the large bulk of the IMes ligand
overrides the usual preference of [LRhCl(PPh3)2]-type
derivatives of Wilkinson’s complex to have the two
phosphine ligands in a trans arrangement.11 Complex
1 proved to be relatively sensitive to oxygen in solution,
decomposing to give a green solution that contained
uncomplexed PPh3 along with triphenylphosphine oxide.
As phosphine lability is often beneficial to catalytic
activity, we decided to assess the catalytic behavior of
the IMes catalyst 1 in the hydroformylation reaction.
1
1
2
3e
a
Reaction conditions: see Table 1 footnote, 0.5% catalyst
loading employed for 19 h, except entry 1, which was run for 20
b
h. conversn ) conversion. c TOF ) turnover frequency per hour.
B:L ) branched:linear ratio. e Catalyst 3 ) Rh(CO)Cl(PPh3)2.
d
As can be seen in Table 1, aromatic olefins are hydro-
formylated with extremely high branched to linear
selectivities. The selectivities range from 94:6 for p-
methylstyrene to 98:2 for p-chlorostyrene.12 Although
1 displays good selectivity, its reactivity is marginal. As
shown in Table 2, under the conditions studied, ap-
proximately 7 turnovers/h are obtained.
During hydroformylation, the originally bright yellow
suspension of 1 in benzene was converted into a
homogeneous, pale yellow solution that underwent no
appreciable color change with time, provided that an
inert argon atmosphere was maintained. This prompted
us to prepare and isolate the corresponding carbonyl
complex. Indeed, rhodium complex 2, a pale lime yellow
powder, could be readily prepared by passing a stream
of CO through a THF solution of 1. The resulting
complex (2) is highly stable, and no special precautions
were required in its manipulation. In the 1H NMR
spectrum of complex 2, the two ortho methyl substitu-
ents on IMes are equivalent. The 31P NMR spectrum of
2 contained, as expected, only one phosphine doublet
at 31.7 ppm, with a 31P-103Rh coupling constant of 115
Hz, consistent with a phosphine ligand trans to the
IMes.13 X-ray-quality crystals were grown by layering
(8) For the reaction of Rh-IMes catalysts with H2 and CO, see:
Huang, J .; Stevens, E. D.; Nolan, S. Organometallics 2000, 19, 1194.
A related rhodium complex with Ph2CH substituents on the imidazo-
lium nitrogens has been reported: Kocher, C.; Herrmann, W. A. J .
Organomet. Chem. 1997, 532, 261. For hydroformylation and other
catalytic chemistry using saturated carbene ligands see: Lappert, M.
F. In Transition Metal Chemistry; Muller, A., Diemann, E., Eds.; Verlag
Chemie: Weinheim, Germany, 1981. For an excellent review of carbene
chemistry with a description of catalytic applications see: Herrmann,
W. A.; Kocher, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2162.
(9) Arduengo, A. J ., III; Harlow, R. L.; Kline, M. J . Am. Chem. Soc.
1991, 113, 361.
(10) Doyle, M. J .; Lappert, M. F. J . Chem. Soc., Chem. Commun.
1974, 679.
(11) McAuliffe, C. A.; Levason, W. Phosphine, Arsine, and Stibine
Complexes of the Transition Elements; Elsevier: Amsterdam, 1979; p
137. It should be noted that Lappert has prepared complexes of this
same type employing less sterically hindered carbene ligands: Doyle,
M. J .; Lappert, M. F.; McLaughlin, G. M.; McMeeking, J . J . Chem.
Soc., Dalton Trans. 1974, 1494. Cardin, D. J .; Cetinkaya, B.; Doyle,
M. J .; Lappert, M. F. Chem. Soc. Rev. 1973, 2, 99. Complexes of the
type RhL3Cl (L ) saturated carbene) have also been prepared:
Cetinkaya, E.; Hitchcock, P. B.; Kucukbay, H.; Lappert, M. F.; Al-J uaid,
S. J . Organomet. Chem. 1994, 481, 89. Cetinkaya, B.; Hitchcock, P.
B.; Lappert, M. F.; Shaw, D. B.; Spyropoulos, K.; Warhurst, N. J . W.
J . Organomet. Chem. 1993, 459, 311.
(12) Hydroformylation of aliphatic olefins was also performed with
limited success. For example, with 1-octene as substrate, a linear to
branched ratio of 2.7:1 was obtained. Conversion was quantitative after
16 h; the yield of the aldehyde was determined to be 97% (NMR yield,
with hexamethylbenzene as internal standard) and was not ac-
companied by isomerization.
(13) Doyle, M. J .; Lappert, M. F., Pye, P. L.; Terreros, P. J . Chem.
Soc., Dalton Trans. 1984, 2355.