Table 1 Hydrogenation results (conditions: 20 mL hexane, 8 psi H2 and room temp. except where noted. Reaction time: A 30 min; B,C, 16 h)
Substrate
Support
Solvent
Temp
Anion
Conv. (%)
Ee (%)
A
A
A
B
B
B
C
C
C
MCM-41
None
None
MCM-41
None
None
MCM-41
None
None
Hexane
Hexane
MeOH
Hexane
Hexane
MeOH
Hexane
Hexane
MeOH
R.T.
R.T.
R.T.
R.T.
R.T. (50 psi)
R.T. (90 psi)
R.T.
R.T. (40 psi)
R.T. (90 psi)
OTf
OTf
OTf
OTf
BArF
OTf
OTf
> 99
> 99
> 99
> 99
92
> 99
> 99
26
99
87
> 99
98
93
96.2
98
a
a
BArF
OTf
85
96
99
a The lipophilic BArF2 anion is more soluble than OTf2 and provided more consistent results than the OTf salt. See also ref. 13.
complex onto the surface of the MCM-41 and the surface-
sorbed complex is recyclable and stable to leaching from the
surface in non-polar solvents. The results show that binding
[(R,R)-Me-(DuPHOS)Rh]OTf to an MCM-41 surface has a
beneficial effect on enantioselectivity and activity in the
hydrogenation of prochiral enamides when compared to the
homogeneous catalyst.
This work was carried out with Laboratory Directed Research
and Development funds at Los Alamos National Laboratory,
operated by the University of California for the U.S. Depart-
ment of Energy under contract W-7405-ENG-36. R. D. B
thanks the National Science Foundation (CAREER Award). We
thank J. Rau, N. Clark, G. Brown and C. Hijar (all LANL) for
materials synthesis and characterization.
Fig. 3 Substrates utilized for catalytic hydrogenation studies.
complex to silica by hydrogen bonding.6 Unfortunately,
because of the low level of loading of the highly active catalyst
(see below), we could not confirm the presence of hydrogen
bonding using IR spectroscopy. Further work on other catalyst
systems is underway. Other evidence that surface hydroxy
groups may be involved in immobilization was found on studies
of MCM-41 supports that were pretreated with trimethylsilyl
chloride to protect the hydroxy groups. These supports were
much less effective in immobilizing complex 1 (1.9 vs. 6.7 wt%
based on Rh). We have found that other silica supports such as
commercial silica gel, which are known to contain fewer surface
silanols,3 also led to significantly lower loadings of 1.
Notes and references
1 H.-U. Blaser and B. Pugin, in Supported Reagents and Catalysts in
Chemistry, ed. B. K. Hodnett, A. P. Kybett, J. H. Clark and K. Smith,
RSC, Cambridge, 1998, vol. 216, p. 101.
The immobilized complex 2 was found to exhibit high
catalytic activity, selectivity and recoverability for the hydro-
genation of three prochiral a-enamide esters, used as test
substrates. Hydrogenation of enamide A (Fig. 3) in hexane
using 2 led to complete conversion with high enantioselectivity.
As shown in Table 1, the immobilized catalyst 2 led to higher
activity and selectivity than the homogeneous catalyst in hexane
for the b,b-disubstituted substrates B and C,15 and rivals the
enantioselectivity reported in MeOH.15 For example, B was
hydrogenated with 98% ee with 2 as the catalyst in hexane while
the optimized reaction with unsupported 3 gave 96% ee in
MeOH and 93% ee in hexane.15 Even more striking, the
conversions for enamide C were significantly higher using the
immobilized catalysts rather than the homogeneous analog in
hexane, where conversion was only 26% (85% ee) with
unsupported 3 in hexane after 22 h at 40 psi, while the reaction
with 2 was complete (98% ee) after 16 h at 8 psi. Few reports11
of such a positive influence on activity and selectivity for
heterogenized catalysts exist. Other silica supports, including
commercial silica gels, can be used to immobilize 1; however,
decreased loading (and therefore activity) was observed.
The recyclability of the immobilized catalyst was demon-
strated using standard procedures. After completion of the
initial hydrogenation of enamide A, the reaction mixture was
filtered and the filtrate was tested for activity by adding more
enamide; no further conversion was observed indicating the
absence of highly active soluble catalyst leaching from the
support. In a second set of recycling experiments, the materials
were reacted under standard conditions for 30 min and the
contents were then decanted leaving solid 1 in a small amount
of solvent.16 The bottle was recharged, and the reaction repeated
four times, with the final run differing in that the catalyst was
stored in hydrogen-free hexane for 16 h prior to the final
reaction. Under these conditions, the catalyst remains active
with no loss of conversion or enantioselectivity.
2 B. Pugin, J. Mol. Catal. A, 1996, 107, 273; D. Brunel, N. Bellocq, P.
Sutra, A. Cauvel, M. Lasperas, P. Moreau, F. Di Renzo, A. Galarneau
and F. Fajula, Coord. Chem. Rev., 1998, 178, 1085; A. Sayari and P. Liu,
Microporous Mater., 1997, 12, 149; K. Moller and T. Bein, Chem.
Mater., 1998, 10, 2950.
3 J. Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem., Int. Ed.,
1999, 38, 56.
4 R. H. Grubbs and L. C. Kroll, J. Am. Chem. Soc., 1971, 93, 3062; S. C.
Bourque, F. Maltais, W.-J. Xiao, O. Tardif, H. Alper, P. Arya and L. E.
Manzer, J. Am. Chem. Soc., 1999, 121, 3035.
5 S.-G. Shyu, S.-W. Cheng and D.-L. Tzou, Chem. Commun., 1999, 2337;
H. Yang, H. Gao and R. J. Angelici, Organometallics, 2000, 19, 622.
6 C. Bianchini, D. G. Burnaby, J. Evans, P. Frediani, A. Meli, W.
Oberhauser, R. Psaro, L. Sordelli and F. Vizza, J. Am. Chem. Soc., 1999,
121, 5961.
7 C.-Y. Chen, H.-X. Li and M. E. Davis, Microporous Mater., 1993, 2, 17;
J. H. Clark and D. J. Macquarrie, Chem. Commun., 1998, 853.
8 M. J. Burk, J. E. Feaster, W. A. Nugent and R. L. Harlow, J. Am. Chem.
Soc., 1993, 115, 10 125.
9 The MCM-41 was synthesized by a previously published procedure.3 A
summary of experimental details is provided as ESI†.
10 M. Kruk, M. Jaroniec and A. Sayari, J. Phys. Chem. B., 1997, 101, 583;
S.-S. Kim, W. Zhang and T. J. Pinnavaia, Catal. Lett., 1997, 43, 149; C.
Liu, Y. Shan, X. Yang, X. Ye and Y. Wu, J. Catal., 1997, 168, 35;
C. P. Mehnert and J. Y. Ying, Chem. Commun., 1997, 22; R. Anwander
and R. Roesky, J. Chem. Soc., Dalton Trans., 1997, 137; R. Burch, N.
Cruise, D. Gleeson and S. C. Tsang, Chem. Commun., 1996, 951.
11 R. Augustine and S. Tanielyan, Chem. Commun., 1999, 1257; A.
Corma, M. Iglesias, C. del Pino and F. Sanchez, J. Chem. Soc., Chem.
Commun., 1991, 1253.
12 K. D. Behringer and J. Blümel, Z. Naturforsch., Teil B, 1995, 50,
1723.
13 M. Brookhart, B. Grant and A. F. Volpe, Organometallics, 1992, 11,
3920.
14 Triflate is known to be a hydrogen bond acceptor, while BArF2 cannot
bond in this fashion: S. Spange, A. Reuter and W. Linert, Langmuir,
1998, 14, 3479; O. Kristiansson and M. Schuisky, Acta. Chem. Scand.,
1997, 51, 270.
15 M. J. Burk, M. F. Gross and J. P. Martinez, J. Am. Chem. Soc., 1995,
117, 9375.
16 We found that if all the solvent is removed, after two recycles the
activity starts to decrease. Examination of the catalyst reveals that the
catalyst becomes fouled with the product of the hydrogenation.
This work clearly shows that a chiral cationic rhodium
catalyst can be simply and efficiently sorbed onto silicas
without any ligand modification, a method that in principle
could be applied to a wide variety of cationic catalysts. The
surface-bound triflate counter ion immobilizes the cationic Rh
1798
Chem. Commun., 2000, 1797–1798