Ruthenium-based complexes bearing saturated chiral N-heterocyclic carbene ligands: Dynamic behavior and catalysis

The synthesis of Ru-based catalysts, presenting saturated chiral C(2) symmetric (3, 5) and C, symmetric (4) N-heterocyclic carbene (NHC) ligands bearing N-(S)-phenylethyl groups, was carried out. Variable-temperature NMR studies were conducted to investigate the interconversion of atropisomers in solution. The complex behaviors were rationalized evaluating the rotation barrier of alkylidene and NHC groups around the C-Ru bonds, by DFT calculations. Comparison between NMR data and DFT calculations suggested that interconversion between different atropisomers, which occurs at room temperature, is due to the free rotation of the benzylidene group around the Ru = C bond. The activity and stereoselectivity of 3-5 were investigated in ring-closing metathesis (RCM), asymmetric ring-closing metathesis (ARCM), cross-metathesis (CM), and ring-opening metathesis polymerization (ROMP). 4 showed the highest activity in all reactions and gave a significantly low E:Z ratio in the CM reaction. Modest enantioselectivity in the ARCM of an achiral triene was observed in the presence of C(2) symmetric catalyst 5.