Cyclometallated gold(III) complexes for catalysis and bioconjugation

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Cyclometallated gold(III) complexes for catalysis and bioconjugation

 

Author: Ko, Hok Ming
Title: Cyclometallated gold(III) complexes for catalysis and bioconjugation
Degree: M.Phil.
Year: 2015
Subject: Gold compounds
Catalysis
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Biology and Chemical Technology
Pages: ix, 219 leaves : illustrations ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2816353
URI: http://theses.lib.polyu.edu.hk/handle/200/8083
Abstract: Square planar gold(III) complexes have four coordination sites, and hence the diverse ligand design can fine - tune the reactivity of the gold(III) centre. In addition, the four coordination sites allow a distinctive advantage in the design of chiral ligands for asymmetric catalysis that is severely limited in the linear gold(I) catalysts having only two coordination sites. However, poor catalytic activity of the coordinatively saturated gold(III) complexes hindered the development of gold(III) catalysis. We envision that novel strategies of ligand design and substrate activation are the key to open up a new direction for gold(III) catalysis. We report the development of stable bis-cyclometallated gold(III) complexes [Au(C^N)₂BF₄-] (HC^N = 2-phenylquinoline and 3-phenylisoquinoline) as efficient catalysts for organic synthesis by employing two novel strategies: (1) distorted square planar complex design and (2) gold-silver dual catalysis for substrate activation. X-ray crystallography study on the bis-cyclometallated gold(III) complex with bulky 2-phenylquinoline ligands revealed the distorted square planar geometry and significant elongation of Au-N bonds (up to 0.141 A ) compared to other cyclometallated gold(III) complexes in literatures. Secondly, the significantly higher catalytic activity (83% vs 8% isolated yields) in propargylamine synthesis could be attributed to this unique distorted complex geometry. The bis-cyclometallated gold(III) complex could also catalyse the stereoselective propargylamine synthesis (up to 90% isolated yield and dr >99:1) and oligosaccharide modification with high aldehyde conversion. Thirdly, the catalysts were found to be active in indole alkylation by using a novel gold-silver dual catalysis (up to 80% isolated yield). Alkylated indoles with different substituents could also be obtained (up to 94% isolated yield). Finally, recyclability experiments of the catalyst in the propargylamine and dual metal-catalysed alkylated indole synthesis were conducted, demonstrating the exceptionally higher recyclability of bis-cyclometallated gold(III) complexes in catalysis over KAuCl4.Given the four cordination sites of gold(III) centre, we envisage that the reactivity of gold(III) reductive elimination could be fine-tuned by modular ligand assembly. In this work, we are exploring the novel application of meticulously designed gold(III) complexes for modification of cysteine by C-S bond formation. A ligand controlled C-S bond formation reaction from gold-peptide adducts for chemoselective cysteine modification has been developed. Cyclometallated gold(III) complexes with bidentate msen as an ancillary ligand exhibited excellent cysteine chemoselectivity to give gold-peptide adducts up to 99% conversion in aqueous medium under mild conditions and various pH values. The structures of gold-peptide adducts and S-arylated peptides were supported by model reactions between the gold(III) complexes and N-acetyl-L-cysteine benzyl amide. C-S bond formation from gold-peptide adducts could be controlled by the corresponding arylpyridine ligands to give S-arylated peptides up to 99% conversion at 40 ℃. A dansyl functionalized gold(III) complex was synthesized for chemoselective cysteine modification with a biophysical probe under mild reaction conditions.

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