Synthesis of Medium-Sized Carbocycles by Gallium-Catalyzed Tandem 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.