Disilaruthena- and ferra-cyclic complexes containing isocyanide lig-ands as effective catalysts for hydrogenation of unfunctionalized ste-rically hindered alkenes

Disilaferra- and disilaruthenacyclic complexes containing mesityl isocyanide as a ligand, 3' and 4', were synthesized and character-ized by spectroscopy and crystallography. Both 3' and 4' showed good catalytic activity for the hydrogenation of alkenes. Com-pared with the iron and ruthenium carbonyl analogs, 1' and 2', the isocyanide complexes 3' and 4' were more robust under the hydrogenation conditions, and were useful even at higher temperatures (~80 °C) under hydrogen pressure (~20 atm). The iron complex 3' possessed the highest catalytic activity toward hydrogenation of mono-, di-, tri-, and tetrasubstituted alkenes among iron catalysts reported. Ruthenium complex 4' catalyzed hydrogenation under very mild conditions, such as room temperature and 1 atm of H2. The remarkably high catalytic activity of 4' for hydrogenation of unfunctionalized tetrasubstituted alkenes was espe-cially notable, because it was comparable to the activity of iridium complexes reported by Crabtree and Pfaltz, which are catalysts with the highest activity in the literature. DFT calculations suggested two plausible catalytic cycles, both of which involved activa-tion of H2 assisted by the metal-silicon bond through σ-bond metathesis of late transition metals (oxidative hydrogen migration). The linear structure of M-C≡N-C (ipso carbon of the mesityl group) played an essential role in the efficient hydrogenation of steri-cally hindered tetrasubstituted alkenes.