Mechanism of Base-Catalyzed Halogen Dance Reactions and the Role of Bromo-Bridged Transition States

Base-catalyzed halogen dance reaction, has puzzled chemists ever since, to come up with new ideas regarding the mechanism of this beautiful chemical transformation. Series of fast metal-halogen exchanges are among the most recent mechanisms. Using DFT-Cam-B3LYP/ land2dz computations and focused on Halogen Dance (HD) reactions of bromobenzene derivatives, new bromo-bridged Transition States (TS) are proposed. These are then used to lay out 8 possible isomerization and 18 possible disproportionation paths. Mechanistic pathways were then analyzed based on TS’s energy and protonation/deprotonation steps to find the most suitable pathways. Overall pattern of these mechanisms draws for the first-time a cascade-like pattern for base-catalyzed halogen dance in its entirety. The present work shed light on a dynamic domino mechanism which dominates the realm of nonaqueous strongly basic medium reactions. Trends like these may well be a shared feature in base digestion of wood for its precious poly aromatic component like lignin.