Metal Modified by a Soluble Chiral Compound

Modification of a metal hydrogenation catalyst by addition of a naturally occurring chiral compound is another classical method the first successful attempts were published about 60 years ago (for reviews see [l-3,7,8]). Continuous progress led to efficient chiral solid catalysts affording up 97-98% enantiomeric excess (ee). 

Pd modified by cinchona, vinca, or ephedra alkaloids is a moderately efficient catalyst but Pd is still the catalyst of choice for the enantioselective hydrogenation of olefins with a functional group in the a position [8,20]. Modification of Pd with cinchonidine is as simple as for Pt, but Pd requires a considerably lower substrate/ modifier ratio than Pt, probably because of weaker adsorption and/or partial degradation (hydrogenation) of the modifier during reaction. Another drawback is that the reactions are not accelerated but decelerated by the chiral modifier (by a factor of up to 140 [21]). This phenomenon can rationalize the moderate performance of chirally modified Pd. 

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