Phosphoramides. Part 9. An attempt to describe the mechanism in the reaction of benzyl alcohols with hexamethylphosphoric triamide

Abstract

The reactions of p-methoxybenzyl alcohol, m-nitrobenzyl alcohol, and diphenylmethanol with hexamethyl-phosphoric triamide (HMPT) at 190–210 °C to form NN-dimethylbenzylamines were followed by quantitative ¹³C n.m.r. spectroscopy and were found to be autocatalysed. A competition experiment between m-nitrobenzyl alcohol and diphenylmethanol with HMPT, and the observed quenching of the reaction by addition of NaOAc to the reaction mixture, indicated the metaphosphate anion as a reactive intermediate. Mono- and bis-p-methoxy-benzyl phosphates, which were believed to be formed from the corresponding alcohol and metaphosphate, were detected in the ¹³C and ³¹P spectra of the reaction mixture. The intermediately formed benzyl phosphates were believed to undergo an S_N1 or an S_N2 substitution reaction with dimethylamine present in the reaction mixture yielding the NN-dimethylbenzylamines.