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S. Bhaduri et al. / Journal of Organometallic Chemistry 606 (2000) 151–155
3. Conclusion
4.2. Synthesis of 1a
The work described here shows that ion-pairing a
chiral quaternary amine with an anionic rhodium car-
bonyl may be a viable method for designing asymmetric
heterogeneous catalysts for the hydrogenation of dehy-
droamino acids. Among the amines and complexes
studied so far, cinchonine plus [Rh(CO)2Cl2]− couple
gives the best result. We also find that this particular
combination results in maximum rate (and enantio-
selectivity) as is normally observed in LAC.
To a pale yellow solution of [Rh(CO)2Cl2]− (0.2 g) in
methanol (10 ml) chloromethylated 20% DVB cross-
linked polystyrene beads quaternized with cinchonine
was added and the mixture was stirred for 24 h at 25°C
under nitrogen. Complete anion exchange took place to
give pale brown beads of 1a, which were filtered,
washed thoroughly with methanol and vacuum dried.
Catalysts 1b–f were prepared by this general procedure
using resins quaternized with different amines. Catalyst
2a was synthesized by this general procedure starting
from 2 and cinchona-functionalized resin.
4. Experimental
The salt CH+ [1] (C=cinchonine) was made and
crystallized by adding excess (1.0 g) CH+Cl− to a
solution of [Rh(CO)2Cl2]− (0.2 g) in methanol (2 ml)
and cooling the resultant solution overnight at 0°C.
RhCl3·3H2O was purchased from Johnson Mathey,
London. All the amines and chloromethylated divinyl
benzene cross-linked (20%) polystyrene were purchased
from Fluka, USA. Infrared spectra were recorded on a
Nicolet FT spectrophotometer. Atomic absorption
measurements were carried out on an IL 751 spec-
trophotometer. Microanalyses were carried out with a
Carlo–Erba 1106 model C, H, N analyzer. Complexes
1 and 2 were synthesized according to literature re-
ported procedures as the H+ and Na+ salts, respec-
tively [18,19]. NMR spectra were recorded on a Bruker
300 MHz instrument. Hydrogenation experiments were
carried out in a Parr autoclave (Parr, USA). Conver-
sion of a-acetamidocinnamic acid to N-acetylphenylala-
nine was monitored by gas chromatography (GC) using
an SC-30 (mesh, 1000–2000) GC column with FID
detection.
Optical rotations were measured using a JASCO
model CT-10 polarimeter (JASCO, Japan). Optical ro-
tation measurements were carried out on purified prod-
ucts. The product was passed through a short silica gel
column. The NMR spectrum of the purified material
showed it to be N-acetylphenylalanine not contami-
nated with any other organic compounds. The observed
optical rotation values could thus be ascribed solely to
optical enrichment of N-acetylphenylalanine. All the
solvents and organic compounds were thoroughly dried
and distilled under nitrogen before use.
4.3. Use of decarbonylated 1a as a catalyst and
recarbonylation of the used catalyst
For a typical catalytic experiment 1a (0.1 g) was
heated for 6 h at 60°C under vacuum. The IR spectrum
of a portion of the solid showed the total disappearance
of the inorganic carbonyl bands. A methanol (5 ml)
solution of a-acetamidocinnamic acid (410 mg, 2 mmol)
in a glass vessel was placed in the Parr pressure reactor
of 100 ml capacity. Thermally activated 1a (0.15 g) was
added to this solution and stirred magnetically under a
pressure of 500 psi of hydrogen for 24 h at 60°C.
Samples were taken out at regular time intervals and
analyzed by GC. At the end of the catalytic run the
used catalyst in methanol was subjected to a CO pres-
sure of 500 psi for 24 h. The solid was then filtered and
the IR spectrum recorded again.
Acknowledgements
Financial support from the Department of Science
and Technology, New Delhi, India is gratefully
acknowledged.
4.1. Functionalization of the polymer
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