A thorough DFT study of the mechanism of homodimerization of terminal olefins through metathesis with a chelated ruthenium catalyst: From initiation to Z selectivity to regeneration

Density functional theory (DFT) calculations (B3LYP, M06, and M06-L) have been performed to investigate the mechanism and origins of Z selectivity of the metathesis homodimerization of terminal olefins catalyzed by chelated ruthenium complexes. The chosen system is, without any simplification, the experimentally performed homocoupling reaction of 3-phenyl-1-propene with 1cat, a pivalate and N-heterocyclic carbene (NHC) chelated Ru precatalyst. The six-coordinate 1cat converts to a trigonal-bipyramidal intermediate (3) through initial dissociation and isomerization. The metathesis reaction of complex 3 with 3-phenyl-1-propene occurs in a side-bound mechanism and generates the trigonal-bipyramidal Ru-benzylidene complex 6. Complex 6 is the active catalyst for the subsequent side-bound metathesis with 3-phenyl-1-propene, which forms metallacyclobutanes that lead to the (Z)- and (E)-olefin homodimers. The transition states of cycloreversion leading to the (Z)- and (E)-olefins differ in energy by 2.2 kcal/mol, which gives rise to a calculated Z selectivity that agrees with experimental results. The Z selectivity stems from reduced steric repulsion in the transition state. The regeneration of complex 6 occurs along with the formation of the gaseous byproduct ethylene, whose evolution drives the overall reaction. As our results indicate, the chelating ligands are crucial for this new class of Ru catalysts to achieve Z-selective olefin metathesis, because they direct olefin attack, differentiate energies of the transition states and intermediates, and support the complexes in certain coordination geometries.