| Ligands | Substrates | Solvents | Anchored
Precursors |
Large
Scale Continuous Use |
Most
of the early work using anchored homogeneous catalysts was concerned with
the enantioselective hydrogenation of methyl 2-acetamidoacrylate (1).
This substrate was selected because of its commercial availability, the
rapid rate of its hydrogenation, the ease of chiral gas chromatographic
analyses of the product and the fact that virtually all chiral homogeneous
hydrogenation catalysts have been successful in this hydrogenation giving
generally very good enantiomeric excess (ee) results. 
Another
substrate used in the early stages of this investigation was methyl 2-acetamidocinnamate
(2). Both of these substrates are representative of the class of
compounds known as dehydroamino esters, materials which have been extensively
studied as prochiral substrates for a variety of enantioselective hydrogenations.
This class of compound has an advantage for screening purposes in that hydrogenation
of them over virtually every chiral homogeneous hydrogenation catalyst gives
reasonably good to excellent ee data for the products. 2-Acetamidocinnamic Acid
There
was some concern raised concerning the ability of these anchored catalysts
to promote the hydrogenation of unsaturated carboxylic acids. We have successfully
used rhodium complexes containing several different ligands anchored to
either alumina or carbon for this hydrogenation. The best ligand appears
to be DuPhos regardless of the support. The amount of rhodium found in the
reaction mixtures ranged from 1.5 to 6.0 ppm. No tungsten (from the PTA)
was detected. Dimethyl Itaconate
In
contrast to the hydrogenation of the dehydroamino acids and esters, the
enantioselective hydrogenation of dimethyl itaconate (4) (DMIT) was
more demanding with respect to the ligand and reaction conditions used.
This hydrogenation was run primarily using Rh(DuPhos) complexes anchored
to either alumina or carbon. Ru(TMBTP)/PTA/Al2O3 catalyzed
hydrogenations of DMIT were run eight times over the same catalyst with
no loss of activity or product enantioselectivities. Carvone
The
hydrogenation of carvone (5) to either the dihydro- (6) or
tetrahydro (7 and 8) species was examined over several anchored
rhodium complexes. Complete hydrogenation to the tetrahydro stage was readily
accomplished using an anchored Rh(PFdppe) complex. Apparently, the electronic
character of the perfluorophenyl groups on the ligand facilitate the hydrogenation
of the conjugated double bond. The anchored Rh(dppb) was more selective
than the anchored Wilkinson catalyst toward dihydrocarvone formation, particularly
in IPA and EtOAc. |
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2002 CAC