Mechanistic understanding of alkyne haloboration: An Ab initio study

Abstract

Ab initio calculations have been performed with second-order Møller-Plesset perturbation theory (MP2) to understand the mechanisms of alkyne haloboration. The present study throws light on three important aspects of this reaction: (1) reaction routes, (2) stereoconversion pathways, and (3) physicochemical uniqueness. The cis-Markovnikov reaction route is controlled by a characteristic four-centered transition state, reflecting the roles of alkynes and boron halides, and also the stability of the product. For the stereoconversion pathways, a boron-halide-mediated addition-elimination process appears to be most favorable. Nevertheless, the high activation barrier suggests that prolonged heating with excess boron halide (commonly used in synthesis) would favor cis/trans isomerization. Finally, comparison with other halometalations showed clear thermodynamic and kinetic advantages of haloboration as a unique, catalyst-free protocol among many elemento-metalation reactions. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.