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Mechanism of Enyne Metathesis Catalyzed by Grubbs Ruthenium−Carbene Complexes: A DFT Study
journal contribution
posted on 2005-05-25, 00:00 authored by Jörg J. Lippstreu, Bernd F. StraubThe complete catalytic cycle of the reaction of alkenes and alkynes to dienes by Grubbs ruthenium
carbene complexes has been modeled at the B3LYP/LACV3P**+//B3LYP/LACVP* level of theory. The
core structures of the substrates and the catalyst were used as models, namely, ethene, ethyne, hept-1-en-6-yne, (Me3P)2Cl2RuCH2, and [C2H4(NMe)2C](Me3P)Cl2RuCH2. Insight into the electronically most
preferred mechanistic pathways was gained for both intermolecular as well as for intramolecular enyne
metathesis. Alkene metathesis is predicted to proceed fast and reversible, while the insertion of the alkyne
substrate is slower, irreversible, and kinetically regioselectivity determining. Ruthenacyclobut-2-ene structures
do not exist as local minima in the catalytic cycle. Instead, vinylcarbene complexes are formed directly.
The alkyne insertion step and the cycloreversion of 2-vinyl ruthenacyclobutanes feature comparable predicted
overall barriers in intermolecular enyne metathesis. For intramolecular enyne metathesis, a noncyclic alkene
fragment of the enyne substrate is first incorporated into the Grubbs catalyst by an alkene metathesis
reaction. The subsequent insertion of the alkyne fragment then proceeds intramolecularly. Alkene
association, cycloaddition, and cycloreversion to the diene product complex close the catalytic cycle. Rate
enhancement by an ethene atmosphere (Mori's conditions) originates from a constantly higher overall alkene
concentration that is necessary for the rate-limiting [2 + 2] cycloreversion step to the diene product complex.