Regiodivergent Conjugate Addition Controlled by Rhodium(I) and Palladium(II) Catalysts: A Combined Computational and Experimental Study

Abstract

A new divergent catalytic method for the 1,4- and 1,6-selective conjugate additions of arylboronic acids to alpha,beta,gamma,delta-unsaturated imino esters mediated by two different transition metal systems was developed. The rhodium complex carrying briphos, a new bicyclic, bridgehead phosphoramidite ligand, showed excellent regioselectivity towards 1,4-addition with product ratios of >50:1, whereas the palladium catalyst Pd(OPiv)(2) reversed the regiochemistry to afford the 1,6-product with mild levels of selectivity of approximate to 1:7. Both theoretical and experimental studies were utilized to understand the catalytic systems and to elucidate the molecular level mechanism leading to the selective reactions. Since Rh(I) and Pd(II) are both d(8)-complexes, their electronic structures and binding behaviours can be compared. Our calculations suggest that the outcome of the catalysis in the Rh(I) system is under kinetic control, where the migratory insertion of the alkene is the step that determines the regiochemistry. When the Pd(II) catalyst is used, the regiochemistry is under thermodynamic control where the relative stability of the eta(3)-pi-allyl-over the eta(3)-aza-pi-allyl-intermediate dictates the selectivity at the 1,6-migratory insertion step.