“Inverted” cyclic(alkyl)(amino)carbene ligands allow olefin metathesis with ethylene at parts-per-billion catalyst loading

Sterically tuned cyclic(alkyl)(amino)carbene (CAAC) ligands lead to development of a new class of catalysts for industrially relevant ethenolysis of methyl oleate that operates at as low as 500 parts-per-billion (ppb) loading and gives turnover numbers (TONs) up to 740,000, even when a lower-grade (3.5) ethylene is used. X-ray diffraction analysis of these complexes revealed that—unlike all other reported solid-state structures of CAAC-based Ru catalysts—in these complexes, the CAAC ligand is inverted, which means that N-aryl substituent is placed in the unoccupied space over chloride ligands, while the substituents at C(2) carbon of CAAC are located over the benzylidene part of the complex. It seems that such a unique spatial arrangement facilitates the coordination of the olefinic substrate to the metal center and in addition may prevent the decomposition of the methylidene intermediate via C–H insertion, which can explain the observed unprecedented productivity of these catalysts.