Development of chiral cyclometallated gold(III) complexes for asymmetric catalysis

Abstract

In the past two decades, the rapid growth of homogenous gold catalysis has been witnessed. A series of catalysts have been developed for diversified organic transformations. In asymmetric gold catalysis, the use of chiral gold catalysts is limited to the adoption of chiral gold(I) complexes. Yet, asymmetric gold(III) catalysis remains largely unexplored. Chiral gold(I) complexes having a linear geometry with two coordination sites, causing a long distance between chiral ligands and substrates, which poses a significant challenge for asymmetric catalysis. Gold(III) complexes have a square-planar geometry with four coordination sites. Therefore, the chiral environment around the gold(III) center can be easily controlled by rational ligand design in a modular approach. This thesis demonstrated the development of a series of chiral gold(III) complexes for asymmetric catalysis. In Chapter 2, the catalytic properties of C,O-chelated BINOL/cyclometallated oxazoline gold(III) complexes in asymmetric carboalkoxylation have been studied in details. The chiral induction was found to be mainly depended on the substituent group of the chiral oxazoline ligands. Circular dichroism study indicated that acid activation of the chiral C,O-BINOL/oxazoline gold(III) complexes in catalysis were possibly based on protonation of O or C atom on the tautomerized BINOL ligand. In Chapter 3, a new type of O,O-chelated cyclometallated oxazoline gold(III) complexes have been successfully developed and applied in the asymmetric carboalkoxylation reactions. The O,O-chelated cyclometallated oxazoline gold(III) complexes with different structures were tested to establish a structure-activity relationship between the chiral environment of the catalysts and enantioselectivity in asymmetric catalysis. The enantioselectivity of the catalyzed reactions was improved to 90% ee by increasing the steric size of the substituent on the chiral oxazoline ligand. Catalytically active species and mechanism of chiral induction were studied based on ESI-MS. In Chapter 4, based on our previous work on the modular synthesis of gold(III) complexes, a novel water-soluble amphiphilic cyclometallated gold(III) complex was designed. The amphiphilic gold(III) complex had an aliphatic hydrocarbon chain with a hydrophilic tertiary ammonium terminus in its structure. Transmission electron microscopy indicated a self-assembled nanofiber structure of the amphiphilic cyclometallated oxazoline gold(III) complex.