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 μ–κ:η2 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.