Ruthenium-based catalysts bearing unsymmetrical NHCs for specific olefin metathesis applications

Novel ruthenium complexes for olefin metathesis incorporating unsymmetrical, backbone-substituted N-heterocyclic carbenes (uNHCs) were synthesised and fully characterised. The effect of NHC backbone configuration (syn or anti), in combination with a pendant N-benzyl substituent and increasingly sterically demanding N'-aryl groups, was systematically evaluated in representative metathesis reactions and in transformations targeting the valorisation of renewable substrates into industrially relevant products. The NHC substitution pattern significantly affected both the stability and catalytic behaviour of the resulting complexes. In comparison to the commercial benchmark Hoveyda-Grubbs second-generation catalyst (HGII), which remains generally superior in standard metathesis reactions, the newly developed complexes proved to be more efficient in selected sustainable processes such as the self-metathesis of eugenol acetate and the ethenolysis of ethyl oleate. Notably, the N-benzyl, N'-2,6-diisopropylphenyl catalyst featuring an anti NHC backbone achieved more than 90% selectivity towards the desired terminal olefins and a TON up to 12 900 in the ethenolysis reaction. Moreover, in the synthesis of degradable polymers via alternating ring-opening metathesis polymerisation of bio-based monomers (an exo-norbornene derivative and a cyclic enol ether), the analogous catalyst bearing a syn NHC backbone displayed higher Z-selectivity compared to HGII (Z double bond content 78% vs. 21%).