Nucleophilic substitution reactions of benzyl- and diphenylmethylphosphonamidic chlorides with amines:: competition between the usual SN,2(P) mechanism and elimination-addition with an alkylideneoxophosphorane (phosphene) intermediate

The substitution reaction of PhCH2P(O)(NMe2)Cl with Me2NH or Et2NH in CHCl3 is very sensitive to the bulk of the nucleophile (greater than or equal to 200 times slower with Et2NH), affords only the product derived from Me2NH in competition experiments, and gives largely undeuterated product with Et2ND; these features are in accord with an S(N)2(P) mechanism. The corresponding reaction of Ph2CHP(O)(NMe2)Cl is relatively insensitive to the bulk of the nucleophile (5 times slower with Et2NH), gives some of the product derived from Et2NH in competition experiments, and gives extensively deuterated product with Et2ND; these features point to an elimination-addition (EA) mechanism with an alkylideneoxophosphorane intermediate [Ph2C=P(O)NMe2]. There is only a modest (13-19 fold) increase in the rate of substitution on going to ArPhCHP(O)(NMe2)Cl (Ar = 4-NO2C6H4) but with R2ND there is now very rapid H/D exchange at the alpha carbon atom. This suggests that the elimination stage of the EA mechanism comprises rapid reversible formation of the conjugate base followed by rate-limiting expulsion of chloride ion.