To “Rollover” or Not? Stereoelectronically Guided C–H Functionalization Pathways from Rhodium–Abnormal NHC Intermediates

Rollover C–H activation with transition-metal complexes has been found to be a difficult but viable pathway to functionalize potentially chelating molecules, which are otherwise reluctant to react further. However, selective rollover or nonrollover C–H activation pathway depends on the stereoelectronic demand of the associated organometallic intermediate(s). The presented work addresses the above issue on abnormal N-heterocyclic carbene (NHC) platform. Catalytic reactions of pyridine-imidazolium substrates with internal alkynes have been selectively guided toward either rollover or nonrollover C–H functionalization route via fulfilling the steric and electronic demands of the relevant rhodium­(III)–abnormal NHC metallacyclic intermediates.