The Influence
of Anionic Ligands on Stereoisomerism
of Ru Carbenes and Their Importance to Efficiency and Selectivity
of Catalytic Olefin Metathesis Reactions
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Abstract
Investigations detailed herein provide
insight regarding the mechanism
of stereochemical inversion of stereogenic-at-Ru carbene complexes
through a nonolefin metathesis-based polytopal rearrangement pathway.
Computational analyses (DFT) reveal that there are two key factors
that generate sufficient energy barriers that are responsible for
the possibility of isolation and characterization of high-energy,
but kinetically stable, intermediates: (1) donor–donor interactions
that involve the anionic ligands and the strongly electron donating
carbene groups and (2) dipolar effects arising from the syn relationship
between the anionic groups (iodide and phenoxide). We demonstrate
that a Brønsted acid lowers barriers to facilitate isomerization,
and that the positive influence of a proton source is the result of
its ability to diminish the repulsive electronic interactions originating
from the anionic ligands. The implications of the present studies
regarding a more sophisticated knowledge of the role of anionic units
on the efficiency of Ru-catalyzed olefin metathesis reactions are
discussed. The electronic basis for the increased facility with which
allylic alcohols participate in olefin metathesis processes will be
presented as well. Finally, we illustrate how a better understanding
of the role of anionic ligands has served as the basis for successful
design of Ru-based <i>Z</i>-selective catalysts for alkene
metathesis
