Formation, Reactivity and Decomposition of Aryl Phospha-Enolates.

Two lithium phospha-enolates [RP=C(Si i Pr3)OLi]2 were prepared by reaction of triisopropyl silyl phosphaethynolate, i Pr3SiPCO, with aryl lithium reagents LiR (R = Mes: 1,3,5-trimethyl phenyl; or Mes*: 1,3,5,-tri-tert-butyl phenyl). Monomer/dimer aggregation of the enolates can be modulated by addition of 12-crown-4. Substitution of lithium for a heavier alkali metal was achieved through initial formation of a silyl enol ether, followed by reaction with KO t Bu to form the corresponding potassium phospha-enolate [MesP=C(Si i Pr3)OK]2. On addition of water, the enolates are protonated to afford RP=C(Si i Pr3)(OH). For the sterically less demanding system (R = Mes), this phospha-enol rapidly tautomerises to the corresponding acyl phosphine MesP(H)C(Si i Pr 3 )(O), which on heating extrudes CO. In contrast, bulkier phospha-enol (R = Mes*) is stable to rearrangement at room temperature and thermally decomposes to RH and i Pr3SiPCO.