A Quantum Chemical Study of Isomerization Step 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)-alkene is formed by the direct addition of boron tribromide to acetylene which undregoes a subsequent isomerization into (£>dibromo(2-bromovinyl)borane cf. Figure 1.
Figure 1: Z/E isomerization of (Z)-dibromo(2-bromovinyl)borane.
Wang and Uchyiama have proposed a polar isomerization mechanism catalyzed by another BBr; molecule with a barrier of ca. 26 kcal mol"1.2 This is in contrast with recent experimental findings obtained in our department.3 The present study concentrates on an alternative mechanism initiated by a reaction of Z-alkene with the HBr molecule and with the radical Br., whose presence is expected due to the interaction with residual air humidity.
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[1] Suzuki, A. Heterocycles 80 (2010) 15.
[2] Wang, C, Uchyiama, M. Eur. J. Org. Chem. (2012) 6548.
[3] Polášek, J., Bachelor's thesis, MUBrno (2014).