The uranyl unit as electrophilic catalyst of acyl transfer reactions

The kinetics of the base induced alcoholysis of a series of metallomacrocycles in which the salophen-uranyl unit is connected by polymethylene bridges of variable length to a pyrogallol monoacetate unit have been investigated in the MeO(-)Me(4)N(+)/MeOH and EtO(-)Me(4)N(+)/EtOH base-solvent systems. Analysis of rate data collected over a wide range of base concentrations, coupled with equilibrium measurements for complex formation between the alkoxide ion and the uranyl centre, showed that the ester cleavage processes are the results of two competing pathways involving alkoxide ion attack on uncomplexed and alkoxide-complexed substrates. Rate data related to the former pathway show that the proximal uranyl centre activates the ester carbonyl towards nucleophilic addition, thus behaving as an immobilized Lewis acid catalyst. The size of the acceleration critically depends on both solvent nature and distance of the ester function from the metal electrophile.