Development of Chiral Gold(III)-complexes and Studies of Their Catalytic Activity

Abstract

The purpose of this master project was to synthesise chiral ligands and coordinate these to gold(III), thereby developing novel chiral gold(III)-complexes. These complexes were then used as gold-catalysts in cyclopropanation reactions in which their catalytic activity and their enantioselectivity could be measured.

Six different squaramide ligands were synthesised by the addition of different amines to squaric ethyl ester, resulting in yields varying from 7% to 68%. Gold(III)-coordination of the squaramides was unsuccessful. After the failure of creating gold(III)-complexes of squaramides, an iminoisoindoline was synthesised (94%) and subsequent gold(III)-coordination resulted in iminoisoindoline-Au(III)-complex (94%).

Gold(III)-coordination to several commercially available oxazoline-based ligands were also attempted. Gold(III)-complex 10 was successfully coordinated and crystallized (63%), and the structure was confirmed by X-ray crystallography. Gold(III)-complex 12 had already been developed during the pre-master project and was repeated here (22%). Gold(III)-coordination of oxazoline-based ligand 13 resulted in a mixture of gold(III)-complex 14 and several other products. Complex 14 was attempted isolated from the mixture, without success. Gold(III)-coordination of oxazoline-based ligand 15 resulted in a complex mixture which did not contain the wanted gold(III)-complex.

The catalytic ability of the successful gold(III)-complexes 8, 10 and 12, the mixture containing gold(III)-complex 14 and the squaramide 4[AuCl4]-salt were tested in cyclopropanation reaction of propargyl esters with vinyl substrates, affording the cyclopropanation products in 63-89% yield, with different cis:trans ratios.

Enantioselectivity of the cyclopropanation products tested by chiral HPLC, but none of the products showed any enantiomeric excess.