Light‐Driven gem‐Hydrogenation: An Orthogonal Entry into “Second Generation” Ruthenium Carbene Catalysts for Olefin Metathesis

The newly discovered light‐driven gem‐hydrogenation of alkynes opens an unconventional yet efficient entry into five‐coordinate Grubbs‐type ruthenium carbene complexes with cis‐disposed chloride ligands. Representatives of this class featuring a chelate substructure formed by an iodo‐substituted benzylidene unit react with (substituted) 2‐isopropoxystyrene to give prototypical “second generation” Grubbs‐Hoveyda complexes for olefin metathesis. The new approach to this venerable catalyst family is safe and versatile for it uses a triple bond rather than phenyldiazomethane as the ultimate carbene source and does not require any sacrificial phosphines