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Hydride–Rhodium(III)‑N‑Heterocyclic Carbene Catalyst for Tandem Alkylation/Alkenylation via C–H Activation

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posted on 2019-09-18, 14:08 authored by Ramón Azpíroz, Andrea Di Giuseppe, Asier Urriolabeitia, Vincenzo Passarelli, Victor Polo, Jesús J. Pérez-Torrente, Luis A. Oro, Ricardo Castarlenas
The unsaturated hydride complex RhClH­{κ2-N,C-(C11H8N)}­(IPr) {IPr = 1,3-bis-(2,6-diisopropylphenyl)­imidazolin-2-carbene} (2) has been prepared via C–H activation of 2-phenylpyridine and fully characterized by spectroscopic methods and X-ray diffraction analysis. Complex 2 efficiently catalyzes the isomerization of terminal and internal olefins under mild conditions to give preferentially the E regioisomers. Complex 2 also catalyzes the hydroarylation of terminal olefins with 2-phenylpyridine to yield selectively mono-ortho-alkylated derivatives. Tandem isomerization–alkylation processes were observed for internal olefins. In contrast to olefins, double alkenylation is operative for internal alkynes. The marked complementary reactivity of olefins and alkynes allows for a tandem alkylation/alkenylation of 2-phenylpyridine to yield substituted styrenes. These heterobiaryl compounds exhibit axial chirality. The rotational barrier has been experimentally calculated and corroborated by density functional theory (DFT) calculations. A catalytic cycle for hydroarylation reactions has been proposed based on the identification of key reaction intermediates and H/D exchange experiments. The reaction seems to proceed by initial C–H activation of 2-phenylpyridine, subsequent insertion of alkene or alkyne, and reductive elimination steps. According to experimental results, DFT calculations have shown a higher energy barrier for bis-alkylation processes than for bis-alkenylation ones that display a feasible activation energy. Moreover, it has been found that reductive elimination is the rate-limiting step for alkene hydroarylation, whereas migratory insertion is the rate-limiting step for alkyne hydroarylation processes.