Asymmetric hydrogenation catalyzed by rhodium complex with a new chiral bisphosphine derived from L-threonine.

Abstract

A new chiral bisphosphine, (2R, 3S)-1, 2-bis (diphenylphosphino)-3-tBoc-aminobutane (RS-5), was prepared from L-threonine. Mesylation of tBoc-L-threonine methyl ester (2) and subsequent reduction with sodium borohydride gave the alcohol (10), which was treated with potassium carbonate to afford a key intermediate, (2S, 3S)-1-tBoc-3-methyl-2-aziridinemethanol (SS-7b). Mesylation of SS-7b, followed by treatment with sodium diphenylphosphide afforded the new chiral bisphosphine (RS-5). The structure of RS-5 was confirmed by the X-ray analysis of its crystalline CuCl complex (RS-12). The cationic rhodium (I) complexes prepared from RS-5 and RS-12 are efficient asymmetric hydrogenation catalysts for N-acyldehydroamino acids, giving (S)-N-acylamino acids in high optical yields (83-94% ee).