Competing mechanisms for the reaction of dichloropropynylborane with 2-tert-butylbutadiene. Diels–Alder reaction versus alkynylboration

Abstract

Density functional theory and the quantum theory of atoms in molecules approach were used to study two competing process: the Diels–Alder reaction (DA) and the 1,4-alkynylboration (AB) between dichloropropynylborane (1) and 2-tert-butylbutadiene (2) in dichloromethane. We analyzed several reaction pathways related with such reactions for both orientations (meta and para). The stepwise mechanisms for the two competitive reactions share the first step that leads to an intermediate zwitterionic structure. The second step is more favorable for the reaction occurring via TSC-m that leads to the meta enyne product 5, which is the kinetic product. The formation of the meta DA product cannot be explained through a direct cycloaddition, due to the higher activation free energy of the associated transition structure (TSB-m). An alternative transition structure with [4 + 3] character (TSD-m) that connects the meta enyne 5 with the meta cycloadduct 3 was found. We propose that at longer reaction times, 5 rearranges to the thermodynamic product 3 via TSD-m passing by a six-membered ring structure and a seven-membered ring structure. The topological analysis of the charge density along the selected reaction coordinates provided some understanding on the intriguing competitive reactions.