A Thorough DFT Study of
the Mechanism of Homodimerization of Terminal Olefins through Metathesis
with a Chelated Ruthenium Catalyst: From Initiation to <i>Z</i> Selectivity to Regeneration
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Abstract
Density functional theory (DFT) calculations (B3LYP,
M06, and M06-L) have been performed to investigate the mechanism and
origins of <i>Z</i> 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 <b>1cat</b>, a pivalate and N-heterocyclic carbene (NHC) chelated Ru precatalyst.
The six-coordinate <b>1cat</b> converts to a trigonal-bipyramidal
intermediate (<b>3</b>) through initial dissociation and isomerization.
The metathesis reaction of complex <b>3</b> with 3-phenyl-1-propene
occurs in a side-bound mechanism and generates the trigonal-bipyramidal
Ru–benzylidene complex <b>6</b>. Complex <b>6</b> is the active catalyst for the subsequent side-bound metathesis
with 3-phenyl-1-propene, which forms metallacyclobutanes that lead
to the (<i>Z</i>)- and (<i>E</i>)-olefin homodimers.
The transition states of cycloreversion leading to the (<i>Z</i>)- and (<i>E</i>)-olefins differ in energy by 2.2 kcal/mol,
which gives rise to a calculated <i>Z</i> selectivity that
agrees with experimental results. The <i>Z</i> selectivity
stems from reduced steric repulsion in the transition state. The regeneration
of complex <b>6</b> 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 <i>Z</i>-selective olefin
metathesis, because they direct olefin attack, differentiate energies
of the transition states and intermediates, and support the complexes
in certain coordination geometries