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

Abstract

The first alpha-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)-(RRCHNH2)-R-1-C-2 with glyoxylic acid monohydrate in diethyl ether at room temperature. Crystals of the N-(1S)-phenylethyl derivative 1d center dot MeOH, grown from methanol, contained only the (1S,alpha S)-diastereoisomer. In [(d(8)]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 alpha-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 THE and/or 1-hexene, proved however more stable and allowed selective oligomerization of ethylene at 100 degrees C, probably by formation of a stabilizing P-C-C(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. (C) 2016 Elsevier Ltd. All rights reserved.