Mechanistic analysis of alkyne trans-haloboration: A DFT and MP2 study

                     Jakub Stošek^1, Ctibor Mazal^1 and Markéta L. Munzarová^1

  ^1Department of Chemistry, Faculty of Science, Masaryk University. Kotlářská 2, CZ-611 37 Brno


The application of alkyne haloboration with boron trihalides is known to result in various rations
of cis/trans isomers, depending on alkyne identity and reaction time. While the currently accepted
mechanism of trans adduct formation assumes exlusively a cis/trans isomerization, in our recent
communication^ [1] we discussed an alternative route suggested by experiment and supported by
theoretical data. Here we report a thorough quantum chemical study of direct acetylene
trans-haloboration considering both polar (X  {Cl, Br, I}) and radical (X = Br) mechanisms. In
particular, we model the interaction between BBr[3 ]and acetylene in the presence as well as
absence of an additional Br^– anion as well as the interaction of BBr[3 ]and vinyl bromide or vinyl
bromide radical. Bromine-bridged free radicals with Br attached either to boron or to the
neighboring carbon are established as key intermediates.


[1] Polášek, J.; Paciorek, J.; Stošek, J.; Semrád, H.; Munzarová, M. L.; Mazal, C. Stereoselective
Bromoboration of Acetylene with Boron Tribromide. A Simple Preparation of (Z)-Bromovinylboronates.
to be submitted to  J. Am. Chem. Soc. 2020.