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The role of halogen bonding (XB) in chemical
catalysis has largely involved using XB donors as Lewis acid activators to modulate
the reactivity of partner Lewis bases. We explore a more uncommon scenario,
where a Lewis base modulates reactivity via a spectator halogen bond
interaction. Our computational studies reveal that spectator halogen bonds may
play an important role in modulating the rate of SN2 reactions. Most
acceptors such as PF3 significantly decrease the barrier to
subsitution by decreasing electron density in the very electron rich transition
state. Such π-backbonding
represents an example of a heretofor unexplored situation in halogen bonding:
the combination of both s-donation
in this “non-covalent” interaction.