Computational study of mechanistic pathway and effect of zero valent metals on reductive debromination of some polybrominated biphenyls

Complete debromination of three polybrominated biphenyls (PBBs), namely 2,2′,3,3′,4,4′-hexabromobiphenyl (PBB-128), 2,2′,3,3′,4,5- hexabromobiphenyl (PBB-129) and 2,2′,3,3′,4,5′- hexabromobiphenyl (PBB-130) aided by zero valent magnesium (Mg⁰) and zinc (Zn⁰) in the presence of methanol was studied computationally using dispersion corrected ωB97XD functional with cc-pVTZ basis set. All possible debromination pathways were explored for each parent PBB and the most energetically favoured pathway was identified. Mono-debromination of each PBB afforded a two-step mechanism, viz, formation of a highly charged covalent intermediate that involves a phenyl-metal-bromine connectivity and protonation by methanol molecule. Total activation energy for the step-wise removal of one bromine substituent increased along the debromination pathway. Using substituent positions, favourable sequence of stepwise debromination was found to follow the order 3 or 3′ 2 or 2′ 4 or 4′, while Mg⁰ catalyzed the debromination reaction better than Zn⁰. The order of ease of debromination was found to be PBB-128PBB-130 PBB-129.