Pyridine-enhanced head-to-tail dimerization of terminal alkynes by a rhodium-N-heterocyclic-carbene catalyst
Laura Rubio-Pérez1, Ramón Azpíroz, Andrea Di Giuseppe
1Departamento Química Inorgánica, Instituto Síntesis Química y Catálisis Homogénea, Universidad de Zaragoza-CSIC, C/Pedro Cerbuna 12, 50009 Zaragoza (Spain).
Abstract:
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.
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