Theoretical study of rhodium- and cobalt-catalyzed decarboxylative transformations of isoxazolones: origin of product selectivity

Divergent catalytic reactions provide access to diverse nitrogen-containing heterocycles through controlled catalysts. This study presents a computational study of rhodium- and cobalt-catalyzed decarboxylative transformations of isoxazolones. The calculations clarified the mechanistic details of the reaction and the origins of the catalyst controlled product selectivity. We identified a mechanism in which alkene insertion precedes CO2 elimination in rhodium- and cobalt-catalyzed decarboxylative transformations of isoxazolones. The kinetic feasibility of this mechanism is attributed to the avoidance of formation of a highly unstable four-membered rhodacycle intermediate. The reason for the formation of different products with the two metals lies in the different geometries of the metal centers in the key transition states. The energy barrier of C–N-bond-forming reductive elimination via the triplet state for the cobalt system is small because the distorted trigonal–bipyramidal coordination geometry of the transition state maximizes the overlap between the π* orbital of the N atom and the σ* orbital of the C atom.