The kinetic data indicate that the addition of tertiary
phosphines
to α-methylene lactones in acetic acid is strongly accelerated
in comparison to the reactions of related open-chain esters. Six-membered
α-methylene-δ-valerolactone exhibited a more pronounced
rate increase than five-membered α-methylene-γ-butyrolactone.
The use of α-methylene-γ-butyrolactam as a nitrogen analogue
of α-methylene-γ-butyrolactone resulted in a total loss
of the reaction acceleration. The observed reactivities were rationalized
by DFT calculations at the RwB97XD/6-31+G(d,p) level of theory, showing
that the intramolecular interaction between phosphonium and enolate
oxygen centers provided by the locked s-cis-geometry
of the heterocycles plays an important role in the stabilization of
intermediate zwitterions. The reactivity is also controlled by the
conformational flexibility of the heterocycle. The geometries of five-membered
and, especially, six-membered lactone cycles are slightly changed
upon the nucleophilic attack of phosphine, leading to the stabilizing
stereoelectronic effect by the Ρ···Ο interaction.
The addition of phosphine to α-methylene-γ-butyrolactam
significantly distorts the initial geometry of the heterocycle, making
the nucleophilic attack unfavorable. The application of the stereoelectronic
effect to enhance the efficiency of the phosphine-catalyzed Michael
and Pudovik reactions of α-methylene lactones was demonstrated.