The reductive cleavage of carbon-halogen bond represents a critical step in the transformations of polyhalogenated compounds in the chemical systems and in environment. Yet, in contrast to the monohalogenated molecules, only a limited number of publications addressed the mechanism and thermodynamics of the dissociative electron transfer (DET) with participation of polyhalogenated species. Thus, we report the results of the experimental and computational studies of such processes with participation of polybromosubstituted aliphatic molecules R-Br, (R-Br = C3Br2F6, CBr3NO2, CBr3CN, CBr3CONH2, CBr3CO2H, CHBr3, CFBr3, CBr4, CBr3COCBr3). Cyclic voltammograms of these electrophiles showed completely irreversible reduction waves. The widths of the waves and the dependences of reduction peak potentials on the scan rates were consistent with the concerted DET mechanism. In view of irreversibility of the CV waves, the redox-potential of the R-Br electrophiles and the corresponding radicals R● resulting from the reductive cleavage of the R-Br molecules were calculated in acetonitrile using the appropriate thermodynamic cycles. These calculations showed that the R● radicals are stronger oxidants than their parents, which explained two-electron transfer reduction peaks observed in the cyclic voltammograms of the R-Br molecules. Furthermore, the quantum-mechanical computations revealed that the lowest unoccupied molecular orbitals (LUMO) of the R-Br molecules (and the singly-occupied molecular orbitals in the corresponding vertically-excited anion radicals) are delocalized mostly over their bromine-containing fragments. All these orbitals show nodes in the middle of the C-Br bonds confirming their anti-bonding character. The injections of an electron into these LUMOs result in the barrierless dissociation of the anion-radical species leading to the formation of the loosely-bonded {R●…Br-} associates. Thus, electrochemical and computational analysis consistently pointed out the concerted mechanism of the reductive cleavage of the C-Br bonds (despite the presence of the electron withdrawing and /or π-acceptor substituents) in the R-Br molecules. These mechanistic and thermodynamic data allowed to quantify the kinetics of reduction of R-Br electrophiles by sterically hindered and accessible electron donors in the frameworks of the outer- and inner-sphere DET processes.

Dissociative electron-transfer reactions of polybrominated aliphatic electrophiles

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.