Pseudo-C₃-Symmetric Titanium Complexes For Asymmetric Catalysis

Abstract

The use of C2-symmetric chiral ligands to promote selectivity in transition metal catalysed asymmetric transformations has been well documented over the last few decades. In recent years, more and more interest has been focused on C3-symmetric chiral ligands and complexes and their applications in this field.2 It has been proposed, for a number of reasons, that transition metal complexes derived from C3 symmetric ligands have even greater potential for asymmetric catalysis than their C2-symmetric counterparts.3This project is focused on the development of a new, chiral family of amine tris(phenolate) ligands, such as 1. Despite ligand 1 being achiral, it has been shown that it forms a chiral (but racemic) monomeric complex with titanium such as 2 (Scheme 1).4 The chirality is due to its propeller-like structure, leading to both the P and M isomers. Complexes of this type have been shown to catalyse a number of organic transformations.5In light of this, the design and synthesis of chiral pseudo-C3-symmetric ligands (R) 3 and (R)-4 was completed via the protocol depicted in Scheme 2 using a chiral auxiliary controlled addition to an imine, followed by oxidative cleavage and bisalkylation of the primary amine and subsequent hydrogenolytic deprotection of the benzyl protecting groups. Upon coordination to titanium, the chirality of the ligands effectively locked the conformation of the propeller-like complex, such that the α-methyl group occupied its predicted pseudoaxial orientation (Scheme 3).In the screening of these titanium complexes in a number of organic transformations (R,M)-6 showed moderate selectivity in the oxidation of aryl alkyl sulfides, delivering the (R)-sulfoxide in enantioselectivities of up to 47% ee (Scheme 4).

Date of Award	1 Sept 2008
Original language	English
Awarding Institution	University of Bath
Supervisor	Steven D Bull (Supervisor)