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

The unsaturated hydride complex RhClH{kappa(2)-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.