Abstract
In the last two decades, the activation of C–C multiple bonds by gold catalysts has received considerable attention [1]. Most of the applications have concerned the addition of carbon-, nitrogen- and oxygen-nucleophiles to the multiple bond of alkynes [2] while the activation of the alkene bonds for the addition of these nucleophiles has been far less undertaken [3-5]. Indeed, activation of alkenes is a more difficult task [6] and the C≡C bond of alkynes is activated by gold templates in the presence of alkenes [7]. Such a preference of gold templates for alkynes, named alkynophilicity, has been exploited in a number of transformations of enynes where the alkene functionality acts as a nucleophile toward the activated alkyne within the same molecule [8]. Gold-alkene adducts have been suggested to be key intermediates in the C–E bond formation (E = C, N, O) catalyzed by both gold(I) and gold(III) species [3-5], in the gold(III) catalyzed hydrogenation [9] and in the gold(I) catalyzed oxidative cleavage of C–C double bonds in water [10]. In a few cases the proposed intermediates have been detected spectroscopically and their successive reactions monitored [4a, 10]. The first report on a gold-alkene complex dates back to 1964, when A. J. Chalk published the synthesis of an analytically well-defined AuCl adduct of 1,5-cyclooctadiene, namely (1,5-cyclooctadiene)(AuCl)2[11]. But it is in the last few years that a variety of gold alkene complexes have been fully characterized, also by X-ray crystallography, and the nature of the gold-alkene bond theoretically investigated. An excellent review dealing with gold-alkene and gold-alkyne complexes appeared very recently [12]. This chapter covers the literature up to mid-2010 and it is arranged according to the oxidation state of gold. Preparative aspects as well as spectroscopic and structural features are presented. A brief description of the M-alkene bond has been given in Section 1.1.2.
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