Olefination with phosphorus ylides. I. Mechanism and stereochemistry of the Wittig reaction

Unstabilized P ylides and their betaines added Li salts, losing most of their reactivity, and were reactivated with tert-BuOK, making thus possible the retardation or the acceleration of the Wittig reaction. The C:O olefination often yielded an isomer mixt. contg. .apprx.1:1 cis- and trans-olefins, due to 2 opposite effects. P ylides were condensed with aldehydes, under kinetic control, preferentially into erythro-betaines, and yielded, when the betaine formation was rendered irreversible, mostly cis-olefins. On the other hand, the threo-betaine was thermodynamically more stable, so that the rapid attainment of diastereomeric equil. between erythro- and threo-betaine led mostly to trans-olefins. Preparative methods of the cis or transselective C:O olefination are described. [on SciFinder (R)]