Direct observation of aziridinium ions in a 2-(N,N-dibenzylamino)- to 1-(N,N-dibenzylamino)phosphonate rearrangement

Aziridinium mesylates stable in the reaction medium for several hours to over a week were observed in a rearrangement of dimethyl (1R,2S)-2-(N,N-dibenzylamino)-1-mesyloxyethylphosphonates substituted at C2 with Bn, i-Pr and t-Bu to the respective 1-(N,N-dibenzylamino)-2-mesyloxyethylphosphonates. Rates of formation of these aziridinium mesylates and rates of their reactions with poorly nucleophilic mesylate anion were governed by steric and electronic factors. The conformation of (2S,3S)-1,1-dibenzyl-2-(tert-butyl)-3-(dimethoxyphosphoryl)aziridinium mesylate in solution was established based on 1H and 13C NMR spectroscopic studies including a NOESY experiment.