Kinetic and stereoselectivity effects of phosphite ligands in dirhodium catalysis

A phosphite additive that can act as an axial ligand for a dirhodium tetracarboxylate catalyst improves the enantioselectivity of silane insertion of a diazo substrate. A kinetic study enables measurement of the catalytic rate constant for the catalyst bound to an axial ligand. Although a single axial ligand has an inhibitory effect on reactivity at the distal rhodium center, axially-bound catalysts are the predominant active species in solution for phosphite concentrations above 6 mol % under our reaction conditions. We examine changes in product enantioselectivity as a function of ligand to shed light on the structure and kinetics of product formation steps.