Homogeneous hydrogenation of tetrasubstituted alkene moieties in prochiral didehydro amino acid derivatives catalysed by iridium complexes

Abstract

The hydrogenation of the tetrasubstituted alkene moieties of the prochiral didehydro amino acid derivatives (1a and b), (2a and b), (3b), (+)- and (–)-(6), (+)-(7), and (–)-(9) has been successfully achieved under very mild homogeneous conditions (1 atm of H₂; 293 K) by using iridium complexes of the type [Ir(cod)(bzn)(L)][ClO₄][cod = cycle-octa-1,5-diene, bzn = benzonitrile, L = tricyclohexylphosphine, (–)-neomethyldiphenylphosphine, or (+)-phenyl-(o-methoxyphenyl)methylphosphine] as catalyst precursors. The presence of chiral groups in the catalysts or in the unsaturated substrates produces little optical induction; the enantiomeric excesses obtained were less than 27% in all cases.