Nucleophilic substitution reactions of benzyl- and diphenylmethyl-phosphonamidic chlorides with amines: competition between the usual SN2(P) mechanism and elimination–addition with an alkylideneoxophosphorane (phosphene) intermediate
Abstract
The 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 SN2(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 (
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 α 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.