Gold catalysis Alkyne activation

The proposed reaction mechanism is shown in Scheme 6.75. The nitroalkene moiety of bifunctional ortAo-alkyne-substituted nitrostyrenes 159 is activated through hydrogen bonding with catalyst 160 to incorporate the stereoehemieal information in the first AFC reaction. Then the alkyne is activated under gold catalysis to affect the seeond AFC/ring expansion cascade. 

O-Vinyl oximes prepared from oximes and activated alkynes rearrange with gold catalysis to substituted pyrroles (Scheme 126). °... 

Until 1998, only gold(III) was believed to be effective for catalyzing these processes because, as mentioned previously, only the gold(I) compound K[Au (CN)2] was tested and it was inert to catalysis. Fortunately, Teles et al. reported very strong activity in the addition of alcohols to alkynes when they used cationic gold( I) -phosphane complexes [14]. In this study, the aforementioned authors tested for the first time the suitability of nucleophilic carbenes that displayed even greater activity than other gold complexes, but they were unable to synthesize the subsequent cationic derivatives. 

In the Au(I) catalysis of electron-poor alkynes such as 4, the catalytically active species is likely to be a cationic ligand-stabilized gold(I) Jt-complex, as in previously reported additions of oxygen nucleophiles to alkynes [5], Gold catalysts are very soft and thus carbophilic rather than oxophilic. On the basis of this assumption a plausible mechanism can be formulated as shown in Scheme 6. The cationic or strongly polarized neutral Au(I)-catalyst coordinates to the alkyne, and nucleophilic attack of the electron-rich arene from the opposite side leads to the formation of a vinyl-gold intermediate 7 which is stereospecifically protonated with final formation of the Z-olefm 8 [2, 4]. Regioselectivity is dominated by elec-... 

For alkynes (and in part, allenes), synthetically useful protocols for Markovnikov and anti-Markovnikov selective hydrations, hydroalkoxylations (mainly intramolecular), and hydrocarboxylations are available and find increasing applications in organic synthesis. In the past decade, the research focus on cationic gold(l) complexes has led to new additions to the catalysis toolbox. It can be predicted that a further refining of such tools for alkyne functionalization with respect to catalytic activity and functional group tolerance will take place. 

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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