Direct π-Activation vs. O-Activation in Halogen-Bonding Catalysis

Halogen bond donors had an increasing impact on catalysis in recent years, and their development is highly active. In particular, numerous iodine-based halogen bond donors have been developed and used to promote various reactions through coordinating carbonyl groups, a ubiquitous mode of activation in catalysis. We now report computational data which strongly suggests that an alternative activation mode, through direct π-complexation, is operative for unsaturated carbonyl substrates. Calculations also suggest that solvent polarity could have a clear impact on the preference of the mode of activation, raising the possibility of a mechanistic manifold switch through solvent variation. These findings could profoundly impact the development of the next generation of halogen-bond donor catalysts.