Alkyne-gold rhodium

The NHCs have been used as ligands of different metal catalysts (i.e. copper, nickel, gold, cobalt, palladium, rhodium) in a wide range of cycloaddition reactions such as [4-1-2] (see Section 5.6), [3h-2], [2h-2h-2] and others. These NHC-metal catalysts have allowed reactions to occur at lower temperature and pressure. Furthermore, some NHC-TM catalysts even promote previously unknown reactions. One of the most popular reactions to generate 1,2,3-triazoles is the 1,3-dipolar Huisgen cycloaddition (reaction between azides and alkynes) [8]. Lately, this [3h-2] cycloaddition reaction has been aided by different [Cu(NHC)JX complexes [9]. The reactions between electron-rich, electron-poor and/or hindered alkynes 16 and azides 17 in the presence of low NHC-copper 18-20 loadings (in some cases even ppm amounts were used) afforded the 1,2,3-triazoles 21 regioselectively (Scheme 5.5 Table 5.2). 

In the field of homogeneous catalysis, electrophilic metals [palladium(II), plati-num(II), rhodium(II), iridium (I), ruthenium(II), cobalt(I), titanium(II) and gold(I)] activate alkynes under mild conditions [2-8]. When an alkyne behaves as a ligand, there are four orbitals that can participate in the bonding (Fig. 1.1) [4]. The in-plane orbitals, Try and n, are responsible for a donor interaction (M <- L donation) and a 7r-acceptor interaction (M L back-donation) respectively. The orthogonal, out-of-plane orbitals, and n , are engaged in the M <- L 7t donation and the d symmetry M L back-donation respectively. This latter interaction can be neglected, due to the weak overlap of the orbitals. 

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