DFT Study on the Relative Stabilities of Substituted Ruthenacyclobutane Intermediates Involved in Olefin Cross-Metathesis Reactions and Their Interconversion Pathways

DFT
(M06-L) calculations have been used to determine the relative
stabilities of the metallacyclobutane intermediates arising from the
cross-metathesis reactions of terminal olefins as well as to get insights
into the origin of the nondetection of the α,β-substituted
species. For that, we discuss the structures, NMR signatures, stabilities
with respect to separated reactants, and experimentally proposed interconversion
pathways of all potential metallacyclobutane intermediates arising
from propene and styrene homocoupling. For the case of propene, the
unsubstituted and mono- and disubstituted metallacycles are lower
in Gibbs energy than the separated reactants under the NMR experimental
conditions. Moreover, for the same number of substituents, regardless
of their nature, the metallacycles presenting substituents at the
Cα carbons are always lower in energy than those
presenting substituents at Cβ, the energy difference
being between 1.7 and 8.8 kcal mol–1. The computed
energy barriers associated with the olefin and carbene rotation processes,
two of the experimentally proposed pathways for the metallacycle interconversion,
are low and are in excellent agreement with the values previously
determined through NMR studies. Cycloaddition and cycloreversion energy
barriers are also low, and in fact, there is not a significant difference
between the barrier heights of the processes leading to observed or
nonobserved intermediates. Therefore, the nondetection of metallacyclobutane
intermediates with substituents in Cβ seems to arise
from their lower stability in comparison with the isomers with substituents
in Cα, which makes their detection not feasible under
thermodynamic equilibrium conditions. That is, for cross-metathesis
processes involving small terminal alkenes and activated carbenes,
the nature of the observed metallacycles is based on thermodynamic
control. The preference of having the substituents in Cα is attributed to the formation of stronger M–C and C–C
bonds during the cycloaddition when the substituents are in an α
position due to higher charge transfer from the original alkene fragment
to the metal carbene.