Gold complexes substitution reactions

Another general procedure to prepare gold(II) complexes consists of substitution reactions on gold(II) derivatives. Halide ligands can be substituted by neutral donor ligands2025-2032 such as... 

Schwarz s group (154, 176) studied substitution reactions of gold complexes (AuL+), where L is a neutral ligand), and discussed the relative Au(I) affinity for ligands. 

Substitution reactions on cationic gold(I) derivatives afford the corresponding complexes containing two isocyanides (Figures 7.30 and 7.31) [29]. A structure—... 

Gold(III), like Ptn, displays predominantly square coordination. Although there is little evidence for any persisting 5-coordinate complexes, there is no doubt that, as with Pt11 species, substitution reactions proceed via 5-coordinate intermediates. There is no evidence for a simple aqua ion, [Au(H20)4]3+, but mixed chloro-aqua and chloro-hydroxo complexes are formed by hydrolysis of [AuCI4]" Distorted octahedral coordination occurs rarely, [Au(en)2Cl2]+ being an example. 

Alkenes act as nucleophiles with alkynes in the presence of gold catalysts. In the most simple version of the reaction, enynes are converted with gold complexes or salts, and in the absence of nucleophiles, into rearranged dienes, cyclopropanated carbocycles, and/or bicyclic cyclobutenes. Depending on the length of the tether and the nature of the substituents, the olefin attack to the alkyne occurs in an endo or an exo fashion (equation 33). Besides, substitution at the alkene plays an important role on the regioselectivity of the nucleophilic attack. ... 

In the same manner, dimethylsulfide substitution proved to be a practical procedure for the preparation of NHC-gold complexes. Good yields were obtained for the synthesis of monocarbene Au(I) complexes (104) from [(Me2S)AuCl] and one equivalent of free NHC (Scheme 17) iHios jjjg steric hindrance of NHCs was found to be of critical importance, since NHCs with smaller nitrogen substituents such as Mes, wo-propyl or n-butyl led to a lower yield and formation of a byproduct identified as the biscarbene complex [(NHC)2Au]+ Cl. Fortunately, the silver-mediated transmetallation was an efficient alternative route for these NHCs (see Section 2.6). When the same reaction was performed with two equivalents of in situ generated NHCs, the cationic biscarbene complex (105) [(NHC)2Au]X (X = Cl, Br, orPFe) was obtained as the only product. ... 

Hydroarylation can also be mediated by Au(I) and Au(III) (Scheme 33) (384). In the case of aryl substituted alkynes, the Au(III) Ji complex undergoes electrophilic aromatic substitution with the electron-rich arene to give aLkenyl-Au(III) complex, which is immediately protonated by the H generated upon C C bond formation. For the Au(I)-catalyzed hydroarylation, the cationic gold complex k coordinates the alkyne, with subsequent nucleophilic attack by the arene from the opposite face leading to an alkenyl-gold complex, which is protonated to the desired products. The nature of the reaction causes the regioselectivity of this reaction to be sensitive to electronic rather than steric factors. 

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