Structural requirements in chiral diphosphine-rhodium complexes. III. Small scale method for fresh preparation of cationic diop-rhodium complexes and comparison with neutral diop-rhodium complexes

(-)-2,3-Isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane (DIOP) was used in cationic rhodium(I)-diphosphine complexes, freshly prepared on a milligram scale, to catalyze the asymmetric hydrogenation of (Z)-α-acetamidocinnamic acid. These cationic complexes were found not to be air stable over a period of time, but when utilized after storage under argon reproducible kinetic results were noted, and the optical purity of the reduction product was found to be independent of the age of the cationic complex. The cationic rhodium(I)DIOP complex was found to be a more reactive complex than the corresponding neutral chlororhodium(I)-DIOP complex when used in asymmetric hydrogenations, but both gave the same optical yield.