Gold-Alkyne Complexes

Since the discovery of the exceptional catalytic activity of gold, alkynes have played a lead rôle in a wide range of organic reactions and transformations [1]. Outstanding examples of the gold catalyzed transformations of alkynes are: the hydrochlorination of ethyne to vinyl chloride [2], the intramolecular hydroamination for the construction of nitrogen heterocyclic frameworks [3], the addition of water and alcohols to afford carbonyl compounds [4, 5], the partial hydrogenation [6] and the intramolecular cyclization of alkynyl furans to phenols [7]. Following these first examples, the potential of gold to catalyse synthetic transformations of these versatile substrates has been exploited in a variety of reactions. A wide array of nucleophiles were found that could be added to alkynes, either in an intramolecular or intermolecular fashion, to afford a variety of organic products, including structurally complex molecules. What makes alkynes so special is the high reactivity, accompanied by the high selectivity with which they are activated by gold catalysts, both homogeneous and heterogeneous [8]. The key step in most of these gold-catalyzed transformations is η2-coordination of the active gold species to the C-C triple bond [9]. This leads to a significant activation of the triple bond for nucleophilic attack (Scheme 1). A number of π-complexes 1 have been spectroscopically detected and in some cases isolated and structurally characterized, so that valuable spectroscopic and structural data are now available. Experimental work has been complemented by theoretical calculations which, on the whole, provide useful information on the nature of the gold-alkyne interaction and shed some light on the reaction mechanisms [8, 10, 11]. This chapter covers the literature up to mid-2010 and it is arranged according to the oxidation state of gold and, within a given oxidation state, to the charge of the complex. Preparative aspects as well as spectroscopic and structural features are presented. In the first section a brief description of the M-unsaturated ligand bond is given. In the last section an overview of the theorethical work is given.