Experimental and quantum chemical studies of alkyne bromoboration

An interesting feature of alkyne bromoboration is that the reaction of BBr 3 with acetylene provides an trans-adduct whereas reactions of BBr3 with all other alkynes tested provide cis-adducts. Recent investigations [1] of the reaction do not correspond with a generally accepted assumption that such outcome is a result of a consecutive isomerization of an initially formed cis-(2-bromovinyl)dibromoborane in presence of BBr3. It has been actually proven that the cis-adduct that was indeed found besides the trans-one in the crude reaction mixtures is stable even in neat BBr3. In addition, it was also noticed that in some conditions, the trans-adduct can be obtained almost exclusively in a much faster reaction that would correspond to its formation by the consecutive isomerization of the cis-adduct. To rationalize these observations, these possibilities were investigated throughout our study: BBr3 addition on acetylene, both polar and radical, as well as a cis-to-trans isomerization [2], both polar and radical. The radical mechanism of BBr3 addition on acetylene involves a very interesting solvent-stabilized radical intermediate involving bridging bromine atom. All these mechanisms could compete with the traditional concerted mechanism of a syn-addition of BBr3 to acetylene with consecutive cis-to-trans isomerization. By means of ab initio calculations, we model the interaction between BBr3 and acetylene in the presence as well as absence of an aditional Br-anion, the interaction of BBr3 and vinyl bromide or vinyl bromide radical. We model as well the interaction of cis-adduct with HBr and the interaction of cis-adduct with bromine radical in the presence as well as absence of BBr3. Calculations for BBr3 addition on acetylene and on vinyl bromide radical 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 or in the presence of CH2Cl2 using SCRF model of implicit solvent. References [1] Polasek, J. Diploma Thesis. Masaryk University, 2017, Brno. [2] Semrad, H; Stosek, J.; Munzarova, M. L. Ab initio studies of the acetylene bromoboration mechanism.