Approaches to the synthesis and detection of a transient palladium(0) alkylidene

Article abstract of DOI:10.1021/om00132a022

Treatment of palladium enolate (PPh₃)₂BrPdCH₂C(O)-t-Bu with t-BuOK in tetrahydrofuran at -63°C affords Ph₃P=CHC(O)-t-Bu (2) in good yield. Kinetics demonstrated the reaction to involve rate-determining dissociation of phosphine. In the presence of added phosphine, the reaction exhibited fractional direct dependence on the concentration of t-BuOK and fractional inverse dependence on the concentration of added phosphine. Isotopic labeling studies showed that the t-BuOK did not function as a Br?nsted base up to the rate-determining step. Crossover experiments demonstrated that the post-rate-determining step involving phosphorus-carbon bond formation occurred by an intramolecular mechanism. Reactions of (Z)-BrCH=C(OSiMe₃)-t-Bu, Br₂CHC(O)-t-Bu, (Z)-BrCH=C(OLi)-t-Bu, and N₂CHC(O)-t-Bu with Pd(PPh₃)₄ each provided phosphorane 2. The mechanism for formation of 2 and the possible intermediacy of low-valent palladium alkylidenes are discussed.