Mechanistic Analysis of Alkyne Haloboration: A DFT, MP2, and DLPNO-CCSD(T) Study

Stereocontrol of the alkyne haloboration reaction has received attention in many experimental but few theoretical studies. Here we present a detailed quantum-chemical study of mechanisms leading to Z versus E isomers of haloboration products, considering acetylene and propyne combined with BCl₃, BBr₃, and BI₃. Calculations using B3LYP-D3, MP2, and DLPNO-CCSD(T) methods are used to study polar reactions between the alkyne and BX₃ in the absence and presence of an additional halide anion whose content in the reaction mixture can be controlled experimentally. The formation of anti-haloboration products via radical mechanisms is also explored, namely, by adding BX₃ to (Z)-halovinyl radical. For the anti-haloboration of propyne, the radical route is prohibited by the regiochemistry of the initiating halopropenyl radical, while the polar route is unlikely due to a competitive allene generation. In contrast, energetically accessible routes exist for both syn- and anti-bromoboration of acetylene; hence, careful control of reaction conditions is necessary to steer the stereochemical outcome. Methodologically, MP2 results correspond better to the DLPNO-CCSD(T) energies than the B3LYP-D3 results in terms of both reaction barrier heights and relative ordering of energetically close stationary points.