Transition metal catalysis with briphos ligands

Abstract

1. The reactivity and selectivity of the transition metal catalysis can be controlled by various ligands that bind to the central metals. Prof. Hyunwoo Kim’s research group have recently reported 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 π-acceptor ability. In this thesis, I found that the enhanced π-acceptor ability of briphos promoted the ligand acceleration effect (LAE) in Ir-catalyzed allylic amination. Furthermore, briphos ligands leads to new selectivity in Ir-catalyzed allylic amination compared to previous reports. It was fully described that the linear selectivity is caused by branched-to-linear isomerization. Mechanistic studies revealed that the isomerization is caused by the facile activation of branched allyl amines to -allyl intermediates with briphos ligands.

2. Allenes are a class of versatile synthetic building blocks used for the production of natural products, pharmaceuticals, and organic compounds. It is important to develop efficient synthetic methods to obtain allenes. Although there are several recent reports for the synthesis of allenes, the synthesis of tetrasubstituted allenes has been a challenge. In order to access this challenging synthetic target, a new class of briphos-olefin ligands have been developed in this study. Briphos-olefin ligands have two different coordinating sites, providing an unsymmertrical environment around the central metal. It is proposed that the unsymmertrical environment may enhance the selectivity by providing a restricted substrate coordination to the metal center. It is found that Briphos-olefin ligands with two modulating sites is useful for Rh-catalyzed propargylic substitution to prepare various tetrasubstituted allenes. It is also demonstrated that briphos-olefin ligands act as bidentate ligands, as supported by NMR and crystallographic analysis.