Pyridine-Enhanced Head-to-Tail Dimerization of Terminal Alkynes by a Rhodium-N-Heterocyclic-Carbene Catalyst

A general regioselective rhodium‐catalyzed head‐to‐tail dimerization of terminal alkynes is presented. The presence of a pyridine ligand (py) in a Rh–N‐heterocyclic‐carbene (NHC) catalytic system not only dramatically switches the chemoselectivity from alkyne cyclotrimerization to dimerization but also enhances the catalytic activity. Several intermediates have been detected in the catalytic process, including the π‐alkyne‐coordinated RhI species [RhCl(NHC)(η2‐HCCCH2Ph)(py)] (3) and [RhCl(NHC){η2‐C(tBu)C(E)CHCHtBu}(py)] (4) and the RhIII–hydride–alkynyl species [RhClH{CCSi(Me)3}(IPr)(py)2] (5). Computational DFT studies reveal an operational mechanism consisting of sequential alkyne CH oxidative addition, alkyne insertion, and reductive elimination. A 2,1‐hydrometalation of the alkyne is the more favorable pathway in accordance with a head‐to‐tail selectivity. Control plan: Addition of pyridine to rhodium–N‐heterocyclic‐carbene catalysts not only switches the chemoselectivity from alkyne cyclotrimerization to dimerization, but also enhances the catalytic activity for the formation of 1,3‐enynes (see figure). A 2,1‐hydrometalation of the alkyne is the more favorable pathway calculated by DFT.