Grubbs Metathesis Enabled by a Light‐Driven gem‐Hydrogenation of Internal Alkynes

Abstract [(NHC)(cymene)RuCl 2 ] (NHC=N‐heterocyclic carbene) complexes instigate a light‐driven gem ‐hydrogenation of internal alkynes with concomitant formation of discrete Grubbs‐type ruthenium carbene species. This unorthodox reactivity mode is harnessed in the form of a “hydrogenative metathesis” reaction, which converts an enyne substrate into a cyclic alkene. The intervention of ruthenium carbenes formed in the actual gem ‐hydrogenation step was proven by the isolation and crystallographic characterization of a rather unusual representative of this series carrying an unconfined alkyl group on a disubstituted carbene center.