Stereo- and Regioselective Dimerization of Alkynes to Enynes by Bimetallic Syn-Carbopalladation

Enynes are important motifs in bioactive compounds. They can be synthesized by alkyne-alkyne couplings for which a number of mechanisms have been suggested depending on catalyst type and dominant product isomers. Regarding bimetallic pathways, hydrometalations and anti-carbopalladations have been discussed to account for the formation of geminally substituted and (Z)-configured enynes, respectively. Here we report a bimetallic alkyne-alkyne coupling that yields (E)-configured enynes. An unusual type of acetylide Pd bridging was found in putative catalytic intermediates which is arguably responsible for the regio- and stereochemical reaction outcome. Mechanistic studies suggest that a double mu-kappa:eta(2) acetylide bridging enables a bimetallic syn-carbometalation. Interestingly, depending on the reaction conditions, it is also possible to form the geminal regioisomer as major product with the same catalyst. This regiodivergent outcome is explained by bi-versus monometallic reaction pathways.