Molecular assembly using gold catalysis

EPSRC Grant YEP098: Molecular assembly using gold catalysis

In 2010 we developed the first Gold-hydroxide species, [Au(IPr)OH] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). The isolation, characterisation and reactivity studies of this complex opened new doors in gold chemistry. As a result, we have created a library of [Au(NHC)OH] (NHC = N-heterocyclic carbene) species and have studied their application in the synthesis of new gold complexes and catalysis. The use of silver salts to activate gold complexes is one of the biggest issues in gold chemistry due to the non-innocent nature of such species. The use of gold-hydroxides activated by equimolar amounts of acid has allowed us to overcome this problem and develop silver-free protocols for gold catalysis in a number of important transformations including but not limited to nitrile and alkyne hydration, Beckmann type rearrangement, alkoxycyclisation, hydroamination, allylic acetate rearrangements and hydrophosphoryloxylation.
In addition, during these studies a new gold species was discovered: a digold-hydroxide, [{Au(IPr)}2(µ-OH)][BF4]. Reactivity studies on this new complex revealed that it may be a key intermediate in gold-catalysed water-inclusive reactions. In addition, a synergistic effect due to use of the digold-hydroxide could be observed in the hydration of nitriles.
The application of both, mono- and digold-hydroxides in catalysis has allowed us to access a wide range of organic transformations, including polymer and macrocycle syntheses, as well as access to complex heteroaromatic compounds, synthesis of prostaglandins, etc. In addition, these gold complexes have been used for the trapping and utilization of greenhouse gases such as CO2 as a carbon source in the form of carboxylation reactions. Due to the importance of such a reaction, a copper-hydroxide species was studied as a cheaper alternative to the gold congener and proved very successful. Further to this research, the reverse process, i.e. decarboxylation of carboxylic acids was used to prepare aryl-gold complexes, synthetically important intermediates in gold catalysis.
As part of our studies we have also investigated the effect that different ancillary ligands have on the selectivity of gold-catalysed reactions. This has lead us to synthesise a number of new gold(I) and gold(III) complexes bearing different NHC ligands and we have studied the effects that structural differences between ancilliary ligands have on the metal centre and hence, the activity and selectivity of such complexes in reactions , e.g. in allene synthesis.