Towards (C,C)‐cyclometalated N‐(9‐alkylfluorenyl)NHC Ruthenium Complexes for Z‐selective Olefin Metathesis

Three Hoveyda-Grubbs complexes supported by N-(9-alkylfluorenyl)imidazol-2-ylidene ligands (alkyl=methyl, ethyl or benzyl) have been synthesized. With the aim to generate chelating, cyclometalated (C,CNHC) ruthenium complexes for Z-selective olefin metathesis, the C(sp3)-H activation of the dangling alkyl group has been studied. While the methyl derivative leads to the expected cyclometalated complex, a C(sp2)-H activation of the fluorenyl moiety and no evolution/transformation are observed for the ethyl and benzyl derivatives, respectively. Although highly fragile under catalytic conditions, the cyclometalated complex leads to Z/E stereoselectivity up to 94/6. An unprecedented insertion of the alkylidene moiety into the Ru-CNHC bond leading to ruthenium N-heterocyclic olefin complexes is also observed and supported by calculations of the corresponding reaction pathway.