Methylenethioxophosphorane (thiophosphene) intermediates in the reactions of diphenylmethylphosphonamidothioic chlorides with amines. Differences between the elimination–addition mechanisms of nucleophilic substitution for PS and PO substrates

Abstract

In reactions with R₂NH (R = Me or Et) in CHCl₃ the PS substrate Ph₂CHP(S)(NMe₂)Cl differs markedly from its PO counterpart: the rate of substitution is increased much more by a 4-nitro substituent but is also more sensitive to the bulk of the amine, there is greater discrimination between competing nucleophiles, and with R₂ND there is relatively little H–D exchange at the α carbon atom prior to substitution. Yet the PS and PO compounds both seem to react by elimination–addition mechanisms. It is suggested that whereas in the PO case the conjugate base is formed rapidly and elimination of chloride to give the methyleneoxophosphorane (phosphene) intermediate is rate limiting, the conjugate base of the PS substrate eliminates chloride rapidly giving a relatively stable methylenethioxophosphorane (thiophosphene) intermediate.