Formation, Reactivity and Decomposition of Aryl Phospha-Enolates.

Two lithium phospha-enolates [RP=C(Si Pr )OLi] were prepared by reaction of triisopropyl silyl phosphaethynolate, Pr SiPCO, with aryl lithium reagents LiR (R=Mes: 1,3,5-trimethyl phenyl; or Mes*: 1,3,5,-tri-tertbutyl 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 Bu to form the corresponding potassium phospha-enolate [MesP=C(Si Pr )OK] . On addition of water, the enolates are protonated to afford RP=C(Si Pr )(OH). For the sterically less demanding system (R=Mes), this phospha-enol rapidly tautomerises to the corresponding acyl phosphine MesP(H)C(Si Pr )(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 Pr SiPCO.