Nucleophilic substitution reactions of benzyl- and diphenylmethylphosphonamidic chlorides with amines: competition between the usual S-N,2(P) mechanism and elimination-addition with an alkylideneoxophosphorane (phosphene) intermediate
Mjp. Harger, Nucleophilic substitution reactions of benzyl- and diphenylmethylphosphonamidic chlorides with amines: competition between the usual S-N,2(P) mechanism and elimination-addition with an alkylideneoxophosphorane (phosphene) intermediate, J CHEM S P2, (1), 2001, pp. 41-47
Citations number
45
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
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 Et2
ND; these features are in accord with an S(N)2(P) mechanism. The correspond
ing 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 deri
ved from Et2NH in competition experiments, and gives extensively deuterated
product with Et2ND; these features point to an elimination-addition (EA) m
echanism with an alkylideneoxophosphorane intermediate [Ph2C=P(O)NMe2]. The
re is only a modest (13-19 fold) increase in the rate of substitution on go
ing 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 elimina
tion stage of the EA mechanism comprises rapid reversible formation of the
conjugate base followed by rate-limiting expulsion of chloride ion.