Ruthenium Olefin Metathesis Catalysts with Frozen NHC Ligand Conformations

The catalytic behavior of Grubbs and Hoveyda–Grubbs II type ruthenium complexes bearing N-heterocyclic carbene (NHC) ligands with syn-phenyl groups on the backbone and syn- or anti-oriented o-tolyl N-substituents was evaluated in a series of olefin metathesis transformations. Further advance in the synthesis of the best-performing syn catalysts and a deeper investigation into the solution-state structure of the Hoveyda–Grubbs type II complex with anti N-tolyl groups by 2D-NMR and DFT studies are also reported. Of particular interest, syn complexes emerged among the best-performing catalysts in all of the explored metathesis reactions, especially in the ring-closing metathesis (RCM) of hindered olefins, allowing also for the difficult formation of macrocyclic trisubstituted alkenes. An important unexpected result was obtained in the RCM of linalool, where both syn and anti catalysts appeared to be involved in the dehydration reaction of the cyclization product (1-methylcylopent-2-en-1-ol). This process allowed for the formation of well-defined mixtures of methylcyclopentadiene isomers, which represent valuable precursors for the synthesis of renewable high-density fuels.