Supramolecular catalysis of unimolecular rearrangements: Substrate scope and mechanistic insights

A cavity-containing metal-ligand assembly is employed as a catalytic host for the 3-aza Cope rearrangement of allyl enammonium cations. Upon binding, the rates of rearrangement are accelerated for all substrates studied, up to 850-fold. Activation parameters were measured for three enammonium cations in order to understand the origins of acceleration. Those parameters reveal that the supramolecular structure is able to reduce both the entropic and enthalpic barriers for rearrangement and is highly sensitive to small structural changes of the substrate. The space-restrictive cavity preferentially binds closely packed, preorganized substrate conformations, which resemble the conformations of the transition states. This hypothesis is also supported by quantitative NOE studies of two encapsulated substrates, which place the two reacting carbon atoms in close proximity. The capsule can act as a true catalyst, since release and hydrolysis facilitate catalytic turnover. The question of product hydrolysis was addressed through detailed kinetic studies. We conclude that the iminium product must dissociate from the cavity interior and the assembly exterior before hydroxide-mediated hydrolysis, and propose the intermediacy of a tight ion pair of the polyanionic host with the exiting product.