Mechanism of the pseudo-molecular rearrangement of triethyl N-phenylphosphorimidate to diethyl N-ethyl-N-phenylphosphoramidate

Abstract

Kinetic studies are reported for the pseudo-molecular rearrangement of triethyl N-phenylphosphorimidate to diethyl N-ethyl-N-phenylphosphoramidate in MeCN. This transformation is shown to be readily catalysed by electrophilic reagents such as alkyl halides, zinc halides, I₂, MeCOBr, and halogen acids where Rate =k₂[Substrate][Catalyst]. For alkyl halides, the reaction proceeds via a two-step mechanism involving an ionic intermediate: formation of the intermediate by an S_N2 reaction between the substrate and alkyl halide is rate limiting. Other catalysts effect rearrangement by the intermediate formation of alkyl halides in an initial rapid reaction with the substrate. In the absence of electrophilic reagents, rearrangement proceeds in MeCN at 100 °C by a much slower thermal process which has a second order dependence on [Substrate].

The results are compared with the related rearrangement of N-methylbenzimidates to tertiary amides and discussed in relation to the ambident nucleophilic properties of phosphylamidates. It is suggested that, like carboxylic acid amides, alkylation occurs most readily at the O-atom of neutral phosphylamidates to give a phosphylimidate (kinetic product) which then rearranges in the presence of electrophilic catalysts to an N-substituted phosphylamidate (thermodynamic product).