Laura Rubio-Pérez

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 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.

Selective CH Bond Functionalization of 2‐(2‐Thienyl)pyridine by a Rhodium N‐Heterocyclic Carbene Catalyst

[Rh(μ‐Cl)(H)2(IPr)]2 (IPr=1,3‐bis‐(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) catalyzes the selective functionalization of 2‐(2‐thienyl)pyridine efficiently with a range of alkenes and internal alkynes. A catalytic cycle is proposed on the basis of the identification of key reaction intermediates and the study of their reactivity by NMR spectroscopy. Theoretical calculations at the DFT level support that the reaction proceeds by initial κ1N coordination of 2‐(2‐thienyl)pyridine followed by the loss of H2 to afford the active catalyst. Subsequently, cyclometalation of 2‐(2‐thienyl)pyridine, coordination of the unsaturated substrate (alkyne or alkene) at the vacant position trans to the hydride, and reductive elimination of the thiophene moiety occur. Finally, cyclometalation of the thiophene moiety renders the hydride cis to the unsaturated substrate, which leads to migratory insertion into the RhH bond and subsequent reductive elimination of the functionalized product.

gem-Selective Cross-Dimerization and Cross-Trimerization of Alkynes with Silylacetylenes Promoted by a Rhodium–Pyridine–N-Heterocyclic Carbene Catalyst

The gem‐selective cross‐dimerization and ‐trimerization of silylacetylenes with alkynes through CH activation using a rhodium(I)–pyridine–N‐heterocyclic carbene catalyst have been developed. This reaction is applied to various aliphatic or aromatic terminal alkynes, internal alkynes, and gem‐1,3‐disubsituted enynes to afford the corresponding enynes and dienynes with high regio‐ and stereoselectivities and in good isolated yields (up to 91 %).