Synthesis of Medium-Sized Carbocycles by Gallium-CatalyzedTandem Carbonyl–Olefin Metathesis/Transfer Hydrogenation

The first examples of a catalytic tandem process involving a ring-closing carbonyl-olefin metathesis and a transfer hydrogenation are described. 1,4-Cyclohexadiene has been used as an H2 surrogate to reduce the cyclic alkenes formed after the metathesis step. The same cationic gallium(III) complex, [IPr·GaCl2][SbF6], performs the two steps with functional group tolerance. This stereoselective reaction leads to 1,2-cis-disubstituted cyclopentanes and various cyclohexanes. DFT computations support an unexpected mechanism involving activation of 1,4-cyclohexadiene by superelectrophilic gallium(III) dimers.