NHC ruthenium complexes as second generation Grubbs catalysts

Abstract

Complex 4a is able to catalyse RCM to give di-, tri- or tetra-substituted cycloolefins while complex 1 failed in the case of tri- and tetrasubstituted ones. The stability and tolerance towards different functional groups of the second generation Grubbs' catalyst has also been demonstrated. En-yne CM between ethylene and alkynes substituted with a heteroatom in the propargylic or homopropargylic position failed when 1 was employed - except in the case where the heteroatom was protected. Nevertheless, even when a protection protocol is employed alcohols, ethers or silyl ethers do not react or react very poorly. Chelation at the Ru centre is thought to be responsible for decelerating or shutting down catalysis. Complex 4a is successful in this reaction without resorting to a protection/deprotection sequence and gives the desired products with good/excellent yields. Catalyst 4a also performs well in ROMP reactions, thus showing its ability to build polymeric structures while tolerating the presence of a huge variety of polar functional groups. Different low-strain cyclic olefins, like cyclooctene or norbornene [using 1,4-bis(acetoxy)-cis-2-butene as a chain transfer agent (CTA)], have also been studied.