Development of bicyclic bridgehead phosphoramidite ligands and their application to catalysis

Abstract

Development of highly active and selective catalysts is a continuing goal in the area of transition-metal catalysis for pharmaceutical and fine chemical industries. Controlling various ligand effects is a major research direction to improve the catalytic activity and selectivity of given transition metals. Among the reported ligands, the phosphorus (P) ligand is a logical first choice to study such ligand effects due to its wide availability and diversity. The reported methods in this regard include chelate control as well as steric and electronic control, which were systematically demonstrated in terms of the bite angle, the Tolman cone angle and electronic parameter. Indeed, the concept of tuning the P ligands has successfully led to the development of highly efficient transition metal catalysts. We have developed a new class of bicyclic bridgehead phosphoramidite (briphos) ligands based on the bicyclo[3.3.1]nonane structure. The geometrical constraints in briphos with respect to its monocyclic analogs enhance $\pi$ -acceptor ability. The enhanced $\pi$ -acceptor ability of briphos gives dramatic ligand acceleration effect (LAE) in low-valent transition metal-catalyzed reactions. Furthermore, facile tuning of briphos leads to new catalytic reactivity as well as asymmetric induction and control of regioselectivity in Rh(I)-catalyzed conjugate additions.