Formation, Reactivity and Decomposition of Aryl Phospha‐Enolates

Two lithium phospha‐enolates [RP=C(Si( i )Pr(3))OLi](2) were prepared by reaction of triisopropyl silyl phosphaethynolate, ( i )Pr(3)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( t )Bu to form the corresponding potassium phospha‐enolate [MesP=C(Si( i )Pr(3))OK](2). On addition of water, the enolates are protonated to afford RP=C(Si( i )Pr(3))(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 )Pr(3)SiPCO.