Routes to organogold compounds and gold catalysed A3-coupling reactions

• Main Author: Price, Gregory Arthur
• Other Authors: Brisdon, Alan; Quayle, Peter
• Published: University of Manchester 2013
• Subjects: 547
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.607025

A new route to the synthesis of cyclometallated gold(III) complexes containing functionalised (2-dimethylaminomethyl)phenyl ligands is described. The synthesis of 2-Me2NCH2-C6H4AuCl2 (8a), 2-Me2NCH2-5-CH3-C6H3AuCl2 (49), 2-Me2NCH2-5-CF3-C6H3AuCl2 (50) and 2-Me2NCH2-5-OMe-C6H3AuCl2 (51) has been accomplished via transmetallation from the corresponding boroxines to sodium tetrachloroaurate in aqueous acetonitrile. This protocol gives comparable yields to those obtained via transmetallation from an organomercurial. However this route is advantageous as it avoids the use of extremely toxic mercury compounds. The novel bimetallic tin(IV) complexes 1,4-(SnPh3)2-2,5-(NMe2CH2)2-C6H2 (66),1,4-(SnMe3)2-2,5-(NMe2CH2)2-C6H2 (67) and 1,4-(SnMe2Cl)2-2,5-(NMe2CH2)2-C6H2 (68) have been synthesised by in-situ quenching of 1,4-(Li)2-2,5-(NMe2CH2)2-C6H2 (57) with two equivalents of ClSnPh3, ClSnMe3 or Cl2SnMe2 respectively. Transmetallation from 1,4-(SnMe2Cl)2-2,5-(NMe2CH2)2-C6H2 (68) with two equivalents of sodium tetrachloroaurate in refluxing acetonitrile resulted in the formation of the relatively insoluble di-gold(III) complex 1,4-(AuCl2)2-2,5-(NMe2CH2)2-C6H2 (71). The novel bisphosphine 1,4-(PPh2)2-2,5-(NMe2CH2)2-C6H2 (69) has also been prepared by in-situ quenching of 1,4-(Li)2-2,5-(NMe2CH2)2-C6H2 (57) with two equivalents of ClPPh2. 69 was reacted with ClAu(THT) to form the novel di-gold(I) phosphine complex 1,4-(ClAuPPh2)2-2,5-(NMe2CH2)2-C6H2 (70) in excellent yield. The catalytic activity of a variety of gold(I) and gold(III) complexes in the A3-coupling of aldehydes, amines and alkynes has been investigated. 2-Me2NCH2-C6H4AuCl2 (8a) was found to be an effective catalyst for A3-couplings, with H2O as solvent, giving essentially quantitative conversions after 24 h. The couplings were tolerant to a diverse range of substrates. A range of chiral Au(III) and Au(I) complexes were prepared and screened in the reaction of benzaldehyde, dibenzylamine and phenylacetylene. In all cases no discernible ee was observed. A series of A3-coupling reactions catalysed by 8a, Na[AuCl4] .2H2O and [AuCCPh]n (139) were also monitored by 1H NMR spectroscopy and this data suggests a common, catalytically active species, is formed during these reactions. The addition of PPh3 to the gold catalysed couplings was found to severely retard the reactions. In contrast the use of chloroform as the solvent resulted in significant improvements to the rate of reaction. Overall the data collected provides a mechanistic insight into the development of asymmetric A3-coupling reactions. The evidence suggests that a single universal catalytically active acetylide species is formed from both Au(I) and Au(III) precursors.