DFT and ab initio studies of the addition step of alkyne bromoboration

                            Jakub Stošek^1 and Markéta L. Munzarová^1.

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


Even though alkyne bromoboration reaction is used by synthetic organic chemists since 1963, a
theoretical study of Wang and Uchyama from 2012 [1] is the only theoretical analysis of its
mechanism. An interesting feature of alkyne bromoboration is that the reaction of BBr[3] with
acetylene provides an anti-adduct whereas reactions of BBr[3] with all other alkynes tested provide
syn-adducts. Wang and Uchyama explain this observation via a subsequent isomerization of the
syn-adduct promoted by another BBr[3 ]molecule. Their theoretical hypothesis is, however, in
contradiction with experimental mechanistic studies of Jan Polášek and Ctibor Mazal from 2014 [2].
At the same time, the latter experimental study suggested a possibility of an alternative
mechanism, involving a direct addition of BBr[3 ]under the participation of Br^– anion into the
anti-arrangement. This possibility is explored throughout our study. By means of ab initio
calculations, we model the interaction between BBr[3 ]and acetylene in the presence as well as
absence of Br^– anion. Similar studies are performed for the cases of acetylene bromo-, chloro- and
iodoboration. The guesses of all transition states are estimated using the single coordinate
driving method. These guesses are then optimized and followed by the frequency analysis to verify
the optimized transition states. All calculations are carried out at the B3LYP or MP2 level of
theory as implemented in the Gaussian09 quantum chemical software. The mechanism is studied in
vacuum as well as in the presence of CH[2]Cl[2]. The calculations for molecules in CH[2]Cl[2 ]are
provided using SCRF model of implicit solvent [3].


[1] Wang, Ch.; Uchyiama, M. Mechanistic Understanding of Alkyne Haloboration: An Ab initio study.
Eur. J. Org. Chem. 2012, 33, 6548-6554.

[2] Polášek, J. Reinvestigation of acetylene bromoboration reaction. Bachelor thesis, Masaryk
Univerzity, Brno, Czech Republic, 2014.

[3] Semrád, H.; Stošek, J.; Munzarová, L. M. Ab initio studies of the acetylene bromoboration
mechanism. Conference abstract, 52^nd Symposium on Theoretical Chemistry – Chemistry in Solution,
Bochum, Germany, 2016.