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

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

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


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. This observation can be explained via a polar mechanism involving a direct addition of
BBr[3 ]under the participation of Br^– anion into the anti-arrangement [1]. The second possibility
is a radical mechanism involving an addition of BBr[3] on the vinyl bromide radical. The second
mechanism involves a very interesting intermediate. Both these possibilities are 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 and the interaction of BBr[3 ]and
vinyl bromide radical. 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 the
presence of CH[2]Cl[2] using SCRF model of implicit solvent.


[1] 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.