Influence of syn and anti Configurations of NHC Backbone on Ru-Catalyzed Olefin Metathesis

The synthesis and characterization of two ruthenium-based olefin metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands with syn (12) and anti (13) methyl groups on the backbone and o-tolyl N-substituents are reported. The catalytic performance of both 12 and 13 has been evaluated in ring-closing metathesis, ring-opening metathesis polymerization, and cross-meta thesis reactions. The results have been compared to those obtained in the presence of the Grubbs second-generation catalyst 3a, lacking substituents on the NHC backbone. Of particular interest, compound 12 outperformed both 3a and 13 in the ring-closing metathesis of hindered olefins, demonstrating to be the most active monophosphine catalyst in this class of metathesis reactions up to now. The influence of syn and anti stereochemical relationship of methyl groups on the NHC backbone of 12 and 13 in the formation of sterically encumbered olefins has been discussed. The X-ray structure of novel ruthenium complex 15, generated during an attempt to prepare pyridine-based complex 14 and identified as a decomposition product derived from C-H activation of N-tolyl substituents, is also reported.