Hoveyda-Grubbs catalyst analogues bearing the derivatives of N-phenylpyrrol in the carbene ligand - structure, stability, activity and unique ruthenium-phenyl interactions

We have synthesized a series of N-phenylpyrrole and N-phenylindole carbenes and used them as ruthenium-ligating moieties in the synthesis of Hoveyda-Grubbs catalyst derivatives. We show that most of these complexes are difficult to synthesize and unstable apart from the N-phenylpyrrole-2,6-diisopropylphenyl ruthenium complex and its perbrominated derivative. These two systems are almost completely inactive in ring-closing metathesis at room temperature and become active only at 80 °C. DFT, SAPT0 and DLPNO-CCSD(T) calculations suggest that the rarely occurring phenyl-ruthenium interactions are responsible for the very slow initiation of these precatalysts at low temperatures.