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

摘要

The substitution reaction of PhCH2P(O)(NMe2)Cl with Me2NH or Et2NH in CHCl3 is very sensitive to the bulknof the nucleophile (u0001200 times slower with Et2NH), affords only the product derived from Me2NH in competitionnexperiments, and gives largely undeuterated product with Et2ND; these features are in accord with an SN2(P)nmechanism. The corresponding reaction of Ph2CHP(O)(NMe2)Cl is relatively insensitive to the bulk of thennucleophile (5 times slower with Et2NH), gives some of the product derived from Et2NH in competitionnexperiments, and gives extensively deuterated product with Et2ND; these features point to an elimination–naddition (EA) mechanism with an alkylideneoxophosphorane intermediate [Ph2Cu0002u0002P(O)NMe2]. There is only anmodest (13–19 fold) increase in the rate of substitution on going to ArPhCHP(O)(NMe2)Cl (Ar = 4-NO2C6H4)nbut with R2ND there is now very rapid H/D exchange at the α carbon atom. This suggests that the eliminationnstage of the EA mechanism comprises rapid reversible formation of the conjugate base followed by rate-limitingnexpulsion of chloride ion.