The [3+2] cycloaddition reaction in CpRu(allyl)(acetylene)

Abstract

The transition metal-mediated coupling of allyl and alkyne ligands to yield cyclopentadienyl and pentadienyl compounds has been studied theoretically using M06 DFT calculations using CpRu(allyl)(acetylene) as a representative example. We find that the first step in the mechanism is the formation of a 16-electron α, π-vinyl olefin intermediate, whereas the alternative, least-motion 3+2 cycloaddition via a concerted process was calculated to lie at a prohibitively high energy and has the characteristics of a symmetry forbidden reaction. The 3+2 concerted path is shown to be unlikely for η3-allyl complexes in general. There are two competing mechanisms, which lead to Cp2Ru + H2 or CpRu(η5-pentadienyl). The pentadienyl product is predicted to form by either the rearrangement of the α, π-vinyl olefin complex or an electrocyclic ring opening of a cyclopentadiene intermediate. The alternative mechanism involves a ring closure of the α, π-vinyl olefin intermediate to yield CpRu(H)(cyclopentadiene). Four mechanistic scenarios were investigated for the reductive elimination of H2 to give Cp2Ru. The lowest energy process occurs via CpRu(H)2(η3-Cp). The other three possibilities are symmetry forbidden. It is shown that the partitioning of pentadienyl versus cyclopentadienyl products depends on the electronic characteristics of the metal. As the transition metal becomes more electron rich, the pathway to the pentadienyl product becomes more favored. Calculations on CpCo(π-allyl)(acetylene)+ and CpTc(π-allyl)(acetylene)− are in agreement with this hypothesis.