Enantioselective hydrogenation: II. variation of activity and optical yield with experimental variables in methyl pyruvate hydrogenation catalyzed by cinchona-modified platinum/silica (EUROPT-1)

Abstract

The enantioselective hydrogenation of methyl pyruvate, McCOCOOMe, to methyl lactate, MeCH(OH)COOMe, preferentially in the R-(+)-form has been catalyzed in a stirred reactor over the temperature range 253 to 348 K and over the pressure range 10 to 110 bar by 6.3%.Pt/silica (EUROPT-1) modified by cinchonidine and dihydrocinchonidine. Variations of activity and of optical yield are reported as a function of (a) modification procedure, (b) reaction procedure, and (c) temperature. The dependence of optical yield on conversion and hydrogen pressure is also reported. The achievement of high optical yield (ca. 74 to 80%, i.e., 87 to 90% R-(+)-, 13 to 10% S-(-)-ester) is critically dependent on the choice of modifier, the modification procedure, and temperature, whereas conversion, hydrogen pressure, and the presence or absence of solvent are of lesser importance. The observations are interpreted in terms of the mechanism proposed in Part I. Preliminary experiments with a flow reactor show unexpectedly high opti cal yields.