hence the modus operandi of the monophosphine catalysts is
reliably established.
This work was financially supported by Chemistry GCOE
program of Tokyo Institute of Technology. EA acknowledges
financial support from the Regione Autonoma della Sardegna,
L.R. 7Agosto 200, n. 7.
Notes and references
z Single crystals of complex 1 were grown from a saturated CH2Cl2
solution.
Crystal data for 1: C66H53Cl9F6P2RhSb, M = 1565.82, orthorhombic,
a = 11.1730(13) A, b = 22.609(3) A, c = 25.378(3) A, V = 6410.7(13) A3,
T = 93 K, space group P212121, Z = 4, m(Mo-Ka) = 11.608 mmꢀ1
,
47 975 reflections measured, 14 633 unique reflections (Rint = 0.0444). The
final R1 [I > 2s(I)] was 0.0510 and the final wR(F2) was 0.1278 (all data).
The goodness of fit on F2 was 1.000. The Flack parameter was ꢀ0.03 (3).
CCDC 846908.
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Scheme 5 Catalytic cycle of asymmetric hydrogenation catalyzed by 2 in
the low temperature hydrogenation of 4 and reactions of 6a,b with MAC.
unstable and involved in some exchange even at ꢀ90 1C that
prevented accurate structure elucidation. Other characterized
intermediates are the 2 : 1 catalyst–substrate complex 913 and
the final catalyst–product complex 10 (Scheme 5).
The most remarkable changes in the 31P and 1H NMR
spectra are observed when, after the substrate is completely
1
hydrogenated (Fig. 3d, confirmed by the H NMR spectrum),
the temperature is raised to ꢀ50 1C and the substrate reappeared
again—in the form of complex 9. This observation suggests that
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catalytic cycle (Scheme 5).4b It is also in accord with the perfect
ee of the product obtained in this experiment. Since re-4 and si-4
do not interconvert at ꢀ90 1C, complete absence of the second
enantiomer of the product requires the intermediacy of species
with non-coordinated double bonds.
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Previously with PHANEPHOS-Rh it had been established that
the hydrogenation process is reversible until the formation of an
agostic intermediate that is the immediate precursor of the
monohydride species.14 In the present case, the reversibility seems
to be pushed further ahead until the stage of the monohydride and
only reductive elimination is then left as the irreversible step of the
catalytic cycle. It is known since long that the reductive elimina-
tion step can occur slowly enough for observable accumulation of
a monohydride intermediate15 but to our knowledge this is the
first example of an apparently reversible migratory insertion step.
It should be noted that the highest ee achieved so far
in the hydrogenation of MAC catalyzed by 1 was 90%
(toluene, 25 1C, S/C 100)8b that is significantly lower than the
>99% observed in our stoichiometric experiments. Since the
low-temperature hydrogenation of MAC using catalytic amounts
of 2 gave the product with 89% ee, the lower ee in catalytic
reactions may be consequent to the presence of a large excess of
the substrate that affects the equilibria of the catalytic cycle.
In conclusion, we have characterized for the first time several
intermediates of the catalytic cycle of monophosphine-Rh
catalyzed asymmetric hydrogenation. All of them are L2Rh species,
15 (a) J. M. Brown, P. A. Chaloner and G. A. Morris, J. Chem. Soc.,
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c
2188 Chem. Commun., 2012, 48, 2186–2188
This journal is The Royal Society of Chemistry 2012