Field Effects Govern the Decarboxylation of Copper(II)-Benzoates: Kinetic and Mechanistic Studies

An experimental mechanistic study of the decarboxylation reactivity of well-defined 1,10-phenanthroline-ligated copper(I)- and copper(II)-benzoate complexes is reported. This work demonstrates decarboxylation to occur from both copper(I)- and copper(II)-benzoates with the (phen)Cu^II(2-nitrobenzoate)₂ (2) proceeding through an oxidative decarboxylation pathway to generate the homocoupled biaryl product while decarboxylation from (phen)Cu^I(2-nitrobenzoate) (1) generates primarily the arene via redox-neutral protodecarboxylation (phen = 1,10-phenanthroline). The time course kinetics and additive experiments demonstrate the decarboxylation of copper(II)-benzoates to both generate and be catalyzed by copper(I). We also disclose the characterization and reactivity of an off-pathway bis(phenanthroline)copper(I)-benzoate species [(phen)₂Cu^I][2-nitrobenzoate] (3) as well as the interconversion of these species under typical reaction conditions. These studies reveal an unexpected inhibitory effect of phen on the decarboxylation of copper(I) species with the unligated species Cu^I(2-nitrobenzoate) (4) showing rapid decarboxylation. Finally, synthesis, characterization and reaction kinetics of the decarboxylation of a series of differently substituted copper(II)-benzoate complexes reveal the field effect (F) to be the dominant factor governing the rate of decarboxylation in these systems.