Theoretical Insight into PtCl₂-Catalyzed Isomerization of Cyclopropenes to Allenes

Abstract

To understand the mechanism of allene formation through the rearrangement of cyclopropenes catalyzed by PtCl₂, we have performed a detailed density functional theory calculation study on a representative substrate, 1-(trimethylsilyl)-2-(phenylethyl)­cyclopropene. Three reaction pathways proposed in the original study have been examined; however the calculated results seem not to completely rationalize the experimental findings. Alternatively, by performing an exhaustive search on the potential energy surface, we present a novel mechanism of PtCl₂, which is fixed appropriately on the cyclopropene/allene to form the linear Cl–Pt–Cl disposition, a vital configuration for catalyzing the rearrangement of cyclopropene. The newly proposed mechanism involves an S_N2-type C–C bond activation of the cyclopropene by PtCl₂ fixed on a cyclopropene molecule via the d−π interaction between the metal center and the substrate to form the product precursor PtCl₂-allene with the metal center coordinated to the external CC bond in the allene framework. Once formed, the PtCl₂-allene immediately serves as a new active center to catalyze the rearrangement reaction rather than directly dissociating into the allene product and the PtCl₂ catalyst due to its high stability. During the catalytic cycle, an allene-PtCl₂-allene sandwich compound is identified as the most stable structure on the potential energy surface, and its direct dissociation results in the formation of the product allene and the regeneration of the catalytically active center PtCl₂-allene with an energy demand of 24.4 kcal/mol. This process is found to be the rate-determining step of the catalytic cycle. In addition, to understand the experimental finding that the H-substituted cyclopropenes do not provide any allenes, we have also performed calculations on the H-substituted cyclopropene system and found that the highest barrier to be overcome during the catalytic cycle amounts to 35.2 kcal/mol. This high energy barrier can be attributed to the fact that the C–H bond activation is more difficult than the C–Si bond activation. The theoretical results not only rationalize well the experimental observations but provide new insight into the mechanism of the important rearrangement reaction