Does the Ruthenium Nitrato Catalyst Work Differently in <i>Z</i>‑Selective Olefin Metathesis? A DFT Study

In the new class of N-heterocyclic carbene (NHC) chelated ruthenium catalysts for <i>Z</i>-selective olefin metathesis, the nitrato-supported complex <b>3cat</b> appears distinct from all the other carboxylato-supported analogues. We have performed DFT calculations (B3LYP and M06) to elucidate the mechanism of <b>3cat</b>-catalyzed metathesis homodimerization of 3-phenyl-1-propene. The six-coordinate <b>3cat</b> transforms via initial dissociation and isomerization into a trigonal-bipyramidal intermediate (<b>5</b>), from which two consecutive metathesis reactions via the side-bound mechanism lead to (<i>Z</i>)-PhCH<sub>2</sub>CHCHCH<sub>2</sub>Ph (major) and (<i>E</i>)-PhCH<sub>2</sub>CHCHCH<sub>2</sub>Ph (minor). In the overall mechanism, <b>3cat</b> functions similarly to the pivalate-supported analogue <b>1cat</b>. The substitution of a smaller nitrato group does not change the side-bound olefin attack mechanism for either the initiation or homocoupling metathesis. The chelation of the NHC ligand causes this class of Ru catalysts to favor the side-bound over the bottom-bound mechanism. The calculated energetics corroborate the experimental observation that <b>3cat</b> is somewhat more active than <b>1cat</b> in catalyzing the homodimerization of 3-phenyl-1-propene.