Evidence for cyclic azaphosphiridine oxide intermediates in the methoxide-induced rearrangements of N-alkyl α-chlorophosphonamidates: formation of phosphoramidates as well as α-aminophosphonates

Abstract

The N-t-butyl α-chlorophosphonamidates RCHClP(O)(NHBu^t)OMe (R = H, Me, or Ph) react with benzyltrimethylammonium methoxide in tetrahydrofuran–methanol to give two types of rearrangement product, the α-aminophosphonates Bu^tNHCHRP(O)(OMe)₂ and the phosphoramidates Bu^t(RCH₂)NP(O)(OMe)₂. For the phosphoramidates it seems necessary to postulate the formation of a cyclic azaphosphiridine oxide intermediate, and its subsequent ring-opening, by nucleophilic attack of methoxide at phosphorus, with cleavage of the P–C bond. The α-aminophosphonates are probably also derived from the azaphosphiridine oxide, by cleavage of the P–N bond. The observation that the proportion of the phosphoramidate (P–C cleavage) increases as R changes from Me to H to Ph accords with this picture, as does the apparent lack of any P–C bond cleavage in the rearrangement of Me₂CClP(O)(NHR′)OMe (R′= Me or Bu^t).