Cyclic (Alkyl)(amino)carbenes (CAACs) in Ruthenium Olefin Metathesis

Discovered in 2005, cyclic (alkyl)(amino)carbenes (CAACs) have led to numerous discoveries in the field of ruthenium olefin metathesis, until then largely dominated by the well-known N - heterocyclic carbenes (NHCs). In comparison to the latter, CAACs are simultaneously more nucleophilic (σ-donating) and electrophilic (π-accepting), which leads to very strong metal–carbene bonds. Consequently, (CAAC)ruthenium complexes are very robust, which leads to enhanced catalytic activities, as exemplified by the industrially relevant ethenolysis of unsaturated fatty acids (TONs of up to 390000). Herein, we provide a comprehensive overview of the effect of CAAC ligands in olefin metathesis, including results which are described in patents.