α-Phosphanyl amino acids: Diphenylphosphanyl glycines with a chiral N-substituent

Abstract

The first α-phosphanyl amino acetic acids (phosphanyl glycines) with a chiral N-substituent 1a–g have been synthesized by one-pot three-component condensation of diphenylphosphane and enantiomerically pure primary amines of the type (L)- or (R)-R1R2CHNH2 with glyoxylic acid monohydrate in diethyl ether at room temperature. Crystals of the N-(1S)-phenylethyl derivative 1d·MeOH, grown from methanol, contained only the (1S,αS)-diastereoisomer. In [d8]THF or CD3OD solution, however, two diastereoisomers were always observed for 1a–g, usually in a diastereoisomeric ratio in the range 67:33% to 80:20%. This, together with the general sensitivity of the phosphanylglycines to hydrolysis, H-D exchange at the acidic α-CH and facile air oxidation in CD3OD solution and decarboxylation on heating in aprotic solvents are serious shortcomings and militate against the use as ligands, e.g. in Rh- or Ir-catalyzed asymmetric hydrogenations. Nickel catalysts, generated in situ with Ni(COD)2 in THF and/or 1-hexene, proved however more stable and allowed selective oligomerization of ethylene at 100°C, probably by formation of a stabilizing PCC(O)O− hybrid ligand in the catalyst backbone. Complexes and catalysts of this type might thus offer chances for use of this facile available asymmetric ligands.