Transformation of Arylmethylamines into α-Aminophosphonic Acids via Metalated Phosphoramidates:  Rearrangement of Partly Configurationally Stable N-Phosphorylated α-Aminocarbanions

Abstract

N-Benzyl phosphoramidate was protected at nitrogen with a TMS, p-toluenesulfonyl, Boc, lithium carboxylate, or diethoxyphosphinyl group and metalated with s-BuLi or LDA at -78 degrees C at the benzylic carbon. For the latter three protecting groups, the intermediate alpha-amino(phenylmethyl)lithiums isomerized to N-protected alpha-aminophosphonates (phosphoramidate-aminophosphonate rearrangement). (R)-N-[1-(2)H(1)]Phenylmethyl phosphoramidate in combination with Boc or (EtO)(2)P(O) was used to demonstrate that metalation occurs with a high primary kinetic isotope effect (k(H)/k(D) 13-50) and migration of the diethoxyphosphinyl group with retention of configuration at carbon. Furthermore, the short-lived carbanion lithium pairs are partly configurationally stable as the aminophosphonates formed with the two protecting groups have enantiomeric excesses of 79 and 24%, respectively. When homochiral lithium amides derived from (R)-N-isopropyl-1-phenylethylamine and (R,R)-N,N-di(1-phenylethyl)amine were used to induce a phosphoramidate-aminophosphonate rearrangement, chiral nonracemic alpha-aminophosphonates were formed (ee 26-35%). Three racemic aminophosphonates were deprotected with hot 6 M HCl and purified by ion-exchange chromatography on Dowex 50W,H(+) to exemplify the transformation of phenyl-, p-tolyl-, and (1'-naphthyl)methylamine into aminophosphonic acids via lithiated phosphoramidates.