Use of the Nonionic Superbase P(MeNCH2CH2)3N in the Selective Monoalkylation of Active-Methylene Compounds

The symmetric active-methylene compounds CH 2 (CO 2 Et) 2 and CH 2 [C(O)Me] 2 are selectively monoalkylated in the presence of 1.1 equiv of a variety of alkyl halides and 1 equiv of the nonionic superbase P(MeNCH 2 CH 2 ) 3 N in 85−98% yields in 30 min at room temperature. The unsymmetrical active-methylene compound EtO 2 CCH 2 C(O)Me is selectively monoalkylated under the same conditions, except for the temperature, which is 0 °C, in 59−88% yields. The observation of selective C- rather than O-alkylation is rationalized in terms of the formation of an enolate whose negatively charged oxygen is sterically protected by a nearby HP(MeNCH 2 CH 2 ) 3 N + counterion in a tight ion pair.