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.

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

“…Even ruthenium as the central atom can be replaced, although the formation of the corresponding osmium complex 6 was much less efficient for reasons that are not entirely clear at this point [30] . Despite the poor productivity, this example is important from the conceptual viewpoint in that it shows for the first time that discrete carbene complexes involving metals other than ruthenium can be generated by gem hydrogenation [1–8] . In view of the importance of metal carbenes in general as catalysts or reactive intermediates, this finding is deemed to provide an encouraging outlook.…”

“…In a recent Communication, we reported that ruthenium complexes of the general type [(NHC)(η 6 ‐cymene)RuCl 2 ] ( 1 ) are able to effect the geminal hydrogenation of internal alkynes, provided that the reaction mixture is irradiated with UV‐A light ( λ =370±40 nm) [1] . Delivery of both H‐atoms of H 2 to the same C‐atom of the triple bond (“ gem hydrogenation”) entails concomitant formation of a discrete metal carbene at the adjacent position (Scheme 1).…”

“…Delivery of both H‐atoms of H 2 to the same C‐atom of the triple bond (“ gem hydrogenation”) entails concomitant formation of a discrete metal carbene at the adjacent position (Scheme 1). Despite the immense literature on catalytic hydrogenation at large, this reactivity mode had been unknown until it was discovered during recent investigations into the mechanism of trans hydrogenation catalyzed by [Cp*RuCl] 4 or related piano‐stool complexes; [2 – 8] the light‐driven catalyst system represents only the second known example [1] …”

“…As a result of the chelate structure, this substituent clashes into one of the mesityl groups of the NHC ligand resulting in severe congestion. As manifested in the X‐ray structure, this unfavorable disposition distorts the ligand sphere of 3 a and may therefore account, at least in part, for the low yield with which the complex is formed [1] . This handicap could be avoided by turning the ruthenacyclic ring by 90 °C such that it lies parallel rather than orthogonal to the mesityl group; such an arrangement requires the chloride ligands to be cis ‐oriented.…”

“… Concept of alkyne gem hydrogenation; first application to the direct formation of a Grubbs‐Hoveyda type catalyst; [1] IMes=1,3‐dimesitylimidazol‐2‐ylidene.…”