Synthesis of Medium-Sized Carbocycles by Gallium-Catalyzed Tandem Carbonyl-Olefin Metathesis/Transfer Hydrogenation

We report a catalytic tandem process involving a ring-closing carbonyl-olefin metathesis and a transfer hydrogenation. 1,4-Cyclohexadiene has been used as H2 surrogate to reduce the cyclic alkenes formed after the metathesis step. The same NHC-stabilized cationic gallium(III) complex, [IPr˙GaCl2][SbF6], performs the two steps with functional group tolerance. This reaction is stereoselective, leading either to 1,2-cisdisubstituted cyclopentanes or to a variety of cyclohexanes. DFT computations support an unexpected mechanism involving the activation of 1,4-cyclohexadiene by superelectrophilic gallium(III) dimers.