Rhodium(I)-N-Heterocyclic Carbene Catalyst for Selective Coupling of<i>N</i>-Vinylpyrazoles with Alkynes via C–H Activation

Abstract

The complex [Rh(μ-Cl)( I Pr)(η 2 - coe)] 2 { I Pr = 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 Markovnikov-selective butadienylpyrazole derivatives under mild conditions. A straightforward approach to cross-conjugated acyclic trienes is also operative through a one-pot alkyne dimerization-hydrovinylation 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 III -hydride-alkenyl species resulting from the C–H activation of the vinylpyrazole