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

摘要

In reactions with R2NH (R = Me or Et) in CHCl3 the Pu0001u0001S substrate Ph2CHP(S)(NMe2)Cl differs markedly fromnits Pu0001u0001O counterpart: the rate of substitution is increased much more by a 4-nitro substituent but is also more sensitivento the bulk of the amine, there is greater discrimination between competing nucleophiles, and with R2ND there isnrelatively little H–D exchange at the α carbon atom prior to substitution. Yet the Pu0001u0001S and Pu0001u0001O compounds bothnseem to react by elimination–addition mechanisms. It is suggested that whereas in the Pu0001u0001O case the conjugate basenis formed rapidly and elimination of chloride to give the methyleneoxophosphorane (phosphene) intermediate isnrate limiting, the conjugate base of the Pu0001u0001S substrate eliminates chloride rapidly giving a relatively stablenmethylenethioxophosphorane (thiophosphene) intermediate.