Direct metathesis of carbon-carbon σ-bonds at a versatile macrocycle-supported diiron platform

The metathesis of C-C σ-bonds holds potential as an atom-economical strategy for molecular engineering. While this reaction has been achieved in industrial and research settings, the former is energy-intensive and the latter has only been reported to operate either indirectly, proceeding through well-known π-bond metathesis chemistry, or through the use of structural/electronic factors that assist in C-C bond activation (e.g. anchimeric effects, directing groups, photochemical effects, etc.). Herein, we report the first example of a direct, thermodynamically favored C-C σ-bond metathesis reaction in which the C(sp)-C(sp²) bonds of diarylacetylenes are selectively cleaved before their fragments are metathesized and C(sp)-C(sp²) linkages are re-formed. Building on a known macrocycle-supported diiron system that undergoes selective C-C σ-bond oxidative addition, Lewis acid catalysts are shown to reversibly abstract Fe-bound aryl groups and perform aryl group exchange between diiron sites. Oxidatively induced reductive elimination reactions then re-form the C(sp)-C(sp²) σ-bonds to generate metathesis products. These studies culminated in a stoichiometric process for C-C σ-bond metathesis between diphenylacetylene and various diarylacetylenes.