Palladium Vinylidenes in Catalysis

The sole report of homogeneous palladium catalysis invoking vinylidene intermediates comes from the laboratory of Buono and coworkers [38]. They discovered a reaction unique to air-stable palladium catalysts 122, which form from the self-assembly of secondary phosphine oxides with Pd(II) (Equation 9.12). 

Using optically pure 121, available from resolution by HPLC, the authors demonstrated a rare example of asymmetric catalysis involving metal vinylidenes (Equation 9.13). The synthesis of 126, enriched in a single olefin geometrical isomer, was achieved in 59% ee. 

Whereas, in the presence of PtBr2, internal alkynes of type 127 were cleanly transformed to vinyl naphthalenes (129), similar terminal alkynes were converted primarily to indenes (131). The authors quickly discovered that the observed C—H insertion reaction is very substrate specific. Only tertiary benzylic C—H bonds could be 

The intermediacy of a platinum vinylidene in Yamamoto s reaction was supported by the results of isotopic labeling studies. D FT calculations were used to further probe the proposed reaction mechanism. In contrast to the prevailing model of alkyne/ vinylidene interconversion for Rh(I)-catalysts, direct Ca Cp 1,2-H-migration is implicated in the formation of vinylidene 130. Direct C—H insertion via a single 

The reactions of Yamamoto and Liu are sufficiently dissimilar that different modes of catalysis may indeed be operative. 

---