Rh/Ag Bimetallic Catalyzed C-H Bond Olefination of Benzonitriles

Transition-metal-catalyzed C-H functionalization emerges as an atom-economical and highly efficient strategy in the synthetic organic chemistry. In this regard, oxidative Heck reaction, pioneered by Fujiwara and Moritani, represents a great evolvement to the traditional Heck reaction by obviating the substrate preactivation. While palladium was routinely adopted as the catalyst of choice because of its well established catalytic activities, other transition metals, especially rhodium also proved to be suitable candidate in pursuit of such oxidative C-H alkenylation because of its low catalyst loading, high efficiency and function group tolerance. Under such circumstance, the Rh(III)-catalyzed oxidative C-H alkenylation has witness a great advancement in the past decade, with many functionalities such as -COOH, -CONHR, CO2Et, NHAc etc. being explored as suitable directing groups for the initiation of C-H activation. However, most of these directing groups needed prior installation. With our continuing interest in Rh(III)-catalyzed C-H functionalization, herein we would like to present a nitrile based bimetallic Rh/Ag catalyzed C-H alkenylation reaction. In this protocol, nitrile was assumed to transform into imine which then took part into the Rh(III) catalyzed C-H bond olefination reation. With such reaction cascade, a library of ortho-alkenylated benzamides could be directly obtained from benzonitriles in one pot fashion. All the substrates are commercially available and easy to obtain. A general procedure for the bimetallic Rh/Ag catalyzed C-H olefination of benzonitriles: an oven-dried 10 mL Schlenk tube was charged with nitrile 1 (If the nitrile was solid, 0.2 mmol), [RhCp*Cl-2](2) (6.2 mg, 0.01 mmol), AgSbF6 (13.8 mg, 0.04 mmol) in sequence, followed by refilling with N-2. Then olefins 2 (0.5 mmol), nitriles 1 (If the nitrile was liquid, 0.2 mmol) and AcOH were added through syringe. After stirring at 120 degrees C for 24 h, the reaction mixture was cooled to room temperature and filtered through a celite plug. The solvent was removed in vacuo and the residue was purified by column chromatography to afford the desired product 3.