A R T I C L E S
Evans et al.
of the P/S ligands. Under a variety of hydrogenation conditions,
with many different Rh complexes, no hydridorhodium(III)
species were isolated or observed. In many cases where Rh
complexes fail to give stable oxidative addition products, use
of the corresponding iridium complexes allows their observation
or isolation due to the higher propensity for iridium to undergo
oxidative addition and its corresponding reluctance to reduc-
tively eliminate.28 Accordingly, iridium(I) complex 16 was
prepared to take advantage of this property to study the oxi-
dative addition step. The X-ray crystal structure of iridium
complex 16 (Figure 8)29 is nearly identical to that of its Rh
congener 14.
The electronic30 and steric31 effects on the kinetic selectivity
of dihydrogen addition to other iridium complexes have been
previously reported. The four potential cis-dihydride addition
products of 16 are illustrated schematically in Figure 7. If
dihydrogen addition occurs along the axis of the phosphinite,
isomers A and C would be produced; if addition is parallel to
the thioether, B and D would predominate. Furthermore, addition
of dihydrogen may occur either to the top face of the iridium
complex (opposite the S-tert-butyl substituent) to give A or B,
or to the bottom face to give C or D. Upon treatment of iridium
complex 16 with H2 for 1 min at -78 °C, complete conversion
to a dihydride complex 17 was observed. Remarkably, only one
diastereomer of this dihydride is visible in the 1H and 31P NMR
spectra. The relatively low chemical shifts (δ -13.0 and -14.1
ppm) and small phosphorus-hydride coupling constants (J )
20, 22 Hz) of the hydrides indicate that they are both cis to the
phosphorus donor.31,32 Only structures B and D satisfy that
condition, both of which are the product of addition parallel to
the sulfur donor, but these possibilities could not be distin-
guished by NMR spectroscopy.
of the trans influence, the three strongest donors (H, H, and P)
are trans to the three weakest donors (olefin, S, and olefin,
respectively). The trans influence is also manifested in the
increased bond lengths of the olefin and sulfur donors trans to
hydride compared to those in the iridium(I) structure 16.
Interestingly, this isomer is a result of addition of dihydrogen
to the more sterically encumbered face of the complex (i.e.,
syn to the tert-butyl group).34 Addition to the opposite face
would result in strong steric interaction between the migrating
olefin substituent and the pseudoaxial tert-butyl and phenyl
groups in both the trigonal bipyramidal intermediate and the
addition product, and this may be the reason it is not observed.
A possible explanation for the difference in the rate of
addition of dihydrogen to the catalyst-substrate complexes is
illustrated in Figure 9.35 Assuming the selectivity of the oxidative
addition of dihydrogen is the same for the catalyst-substrate
complex 15 as it is for the cyclooctadiene iridium complex 16
(i.e. parallel to the thioether and from the face syn to the S-tert-
butyl group),36 addition of dihydrogen to the major catalyst-
substrate complex 15-maj would give dihydride complex 18.
This isomer is stereoelectronically aligned to undergo migratory
insertion to form the alkylhydride complex 19. On the other
hand, similar oxidative addition to the minor diastereomer would
afford dihydride complex 20, which is not properly aligned to
undergo migratory insertion. Either significant rearrangement
of this intermediate or addition of dihydrogen via one of the
unfavored pathways would be required to produce the alkyl-
hydride intermediate and continue the catalytic cycle. The
putative difference in migration rates may be responsible for
the faster reaction of the major catalyst-substrate diastereomer
and the unique mechanism exhibited by this catalyst.
Hydrosilylation of Ketones
Catalyst Optimization. Given the success of this ligand class
in the hydrogenation reaction, it seemed likely that other
enantioselective Rh-catalyzed processes might be realized with
this ligand family. As an example, we chose the asymmetric
hydrosilylation of ketones. While a variety of ligand classes
have been used for this reaction, application of mixed P/S
ligands has met with little success.37 Preliminary ligand screen-
ing in the reduction of acetophenone with diphenylsilane showed
that Rh complex 14 derived from ligand 3 resulted in moderately
high enantioselectivity (Table 4) with less than 5% enolsilane
formation by 1H NMR spectroscopic analysis. All other catalysts
gave poorer results.
Figure 7. Possible kinetic products of dihydrogen addition to 16.
No change in the composition of the dihydride was observed
upon warming to room temperature, indicating that the ther-
modynamic oxidative addition product is the same as the kinetic
adduct. Crystals of [(3)Ir(cod)H2]SbF6 (17) were obtained and
analyzed by X-ray crystallography (Figure 8).33 This structure
corresponds to isomer D in Figure 5. In keeping with the dictates
(27) This has recently been claimed to be true for a bisphosphetane ligand on
the basis of circumstantial evidence, but no structural data has been
presented to bolster this claim. See Marinetti, A.; Jus, S.; Genet, J.-P.
Tetrahedron Lett. 1999, 40, 8365-8368. Some skepticism has been
expressed as to their conclusions, see footnote 21 in Landis, C. R.; Feldgus,
S. Angew. Chem., Int. Ed. 2000, 39, 2863-2866
(33) Crystals of 17 (C30H43F6IrOPSSb) were grown by slow diffusion of Et2O
into a solution of 17 in CH2Cl2 to yield colorless needles. The compound
crystallizes in the monoclinic crystal system, space group P21; a ) 10.4130-
(16) Å, b ) 11.5788(18) Å, c ) 13.786(2) Å, R ) γ ) 90°, â ) 96.320(3)
°; V ) 1652.1(5) Å3; Z ) 2; R ) 0.0314, GooF ) 1.034.
(28) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
2nd ed.; Wiley-Interscience: New York, 1994.
(34) Although diastereomeric equilibration is not impossible, previous studies
of iridium dihydrides have shown that isomerization only occurs above
-50 °C, see refs 30, 31.
(29) Crystals of 16 (C30H41F6IrOPSSb+C4H10O) were grown by slow diffusion
of Et2O into a solution of 16 in CH2Cl2 to yield red prisms. The compound
crystallizes in the tetragonal crystal system, space group P41; a ) b )
10.2022(8) Å, c ) 36.270(4) Å, R ) â ) γ ) 90°; V ) 3775.2(6) Å3; Z
) 5; R ) 0.0310, GooF ) 1.032.
(35) We cannot eliminate the possibility that it is simply the predominance of
the observed isomer, rather than its faster rate of reaction, that is responsible
for the enantioselectivity.
(36) Note that the native selectivities of the P/S ligand and the substrate are
reinforcing in these complexes, that is, the former prefers addition parallel
to the thioether, and the latter prefers addition parallel to the olefin. See:
Brown, J. M.; Maddox, J. J. Chem. Soc., Chem. Commun. 1987, 1278-
1280.
(37) For other mixed P/S ligands in asymmetric hydrosilylation, see: (a) Hiraoka,
M.; Nishikawa, A.; Morimoto, T.; Achiwa, K. Chem. Pharm. Bull. 1998,
46, 704-706. (b) Nishibayashi, Y.; Segawa, K.; Singh, J. D.; Fukuzawa,
S.; Ohe, K.; Uemura, S. Organometallics 1996, 15, 370-379.
(30) (a) Burk, M. J.; McGrath, M. P.; Wheeler, R.; Crabtree, R. H. J. Am. Chem.
Soc. 1988, 110, 5034-5039. (b) Deutsch, P. P.; Eisenberg, R. Chem. ReV.
1988, 88, 1147-1161.
(31) (a) Ha¨lg, W. J.; O¨ hrstro¨m, L. R.; Ru¨egger, H.; Venanzi, L. M. Magn. Reson.
1993, 31, 677-684. (b) Kimmich, B. F. M.; Somsook, E.; Landis, C. R. J.
Am. Chem. Soc. 1998, 120, 10115-10125.
(32) Hydrides trans to the phosphorus donor generally have coupling constants
of 80-100 Hz. See ref 31.
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3540 J. AM. CHEM. SOC. VOL. 125, NO. 12, 2003