Quantum Chemical Analysis of Isomerization Mechanisms

                                  within Acetylene Bromoboration

                                 Hugo Semrád and Markéta Munzarová

      Department of Chemistry, Faculty of Science, Kamenice 5, CZ-625 00 Brno, Czech Republic


Haloboration reactions of alkynes represent key elements in many organic synthetic routes, as
evidenced in a series of 21 papers by A. Suzuki through 1983-1992.^1 During an acetylene
bromoboration reaction, (Z)-dibromo(2-bromovinyl)borane [(Z)-1] is formed by the direct addition of
boron tribromide to acetylene which undergoes a subsequent isomerization into (E)-1 by means of
hydrogen bromide and/or bromine radical (Fig. 1).^2

Figure 1: Z/E isomerization of (Z)-dibromo(2-bromovinyl)borane caused by a bromine radical.


The results published in 2012 by Wang and Uchyiama^3 are in contrast with recent experimental
findings obtained in our department.^2 The present study concentrates on alternative mechanisms
initiated by a reaction of Z-alkene with the HBr molecule and with the bromine radical, whose
presence is expected due to the interaction with residual air humidity. The calculations show the
reaction pathway caused by HBr molecule provides high energy barriers at addition–elimination step
whereas the radical isomerization reactions seem more probable because of much lower energy
barriers.


[1] Suzuki, A. Heterocycles 80 (2010) 15.

[2] Polášek, J.; Paciorek, J.; Stošek, J.; Semrád, H.; Munzarová, M. and Mazal, C. Stereoselective
Bromoboration of Acetylene with Boron Tribromide. A Preparation of (Z)-Bromovinylboronates. Journal
of Organic Chemistry. 2020. Articles ASAP (Article) Publication Date (Web): May 21, 2020.

[3] Wang, C., Uchyiama, M.  Eur. J. Org. Chem. (2012) 6548.