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 Rh(I) species [RhCl(NHC)(η(2)-HC≡CCH2Ph)(py)] (3) and [RhCl(NHC){η(2)-C(tBu)≡C(E)CH=CHtBu}(py)] (4) and the Rh(III)-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.

The complex [Rh(μ-Cl)(IPr)(η 2 -coe)] 2 {IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene, coe = cis-cyclooctene} efficiently catalyzes the coupling of alkynes and N-vinylpyrazole via C−H activation, leading to Markovnikovselective butadienylpyrazole derivatives under mild conditions. A straightforward approach to cross-conjugated acyclic trienes is also operative through a one-pot alkyne dimerizationhydrovinylation tandem reaction. The proposed mechanism involves C−H activation of vinylpyrazole directed by nitrogen coordination to the metallic center. Subsequent alkyne coordination, insertion, and reductive elimination steps lead to the coupling products.Several key intermediates participating in the catalytic cycle have been detected and characterized, including a κ-N, η 2 -CC coordinated vinylpyrazole complex and a Rh IIIhydride-alkenyl species resulting from the C−H activation of the vinylpyrazole

Forging the lock that autolocks! RhNHC catalysts promote a new access to 4 H‐quinolizine species from 2‐vinylpyridine and terminal and internal alkynes through CH activation and CC coupling reactions (see figure). N‐Bridgehead heterocycle formation is favored for internal‐ over terminal‐substituted butadienylpyridine derivatives in a thermal 6π‐electrocyclization process.