Alkenes gold catalysts

Into the previously reported transition-metal catalyzed intermolecular hydroalk-oxylations of alkenes, gold catalysts showed to be the most active catalysts, with divergent reactivity with respect to PtCl2, leading to the formation of aUylethers [156]. Opposite to monosubstituted allenes, disubstituted aUenes yield no aliphatic acetals. Deuteration studies support the hypothesis of a zwitterionic Pt carbene as... 

Despite its apparent noble character, gold catalysts have been recently found to be active in many homogeneous and heterogeneous catalytic processes such as oxidation reactions, nucleophilic additions, cross-coupling reactions, and alkene and imine hydrogenations (69—71). Corma and co-workers showed that Au... 

Some gold catalyst species proved to be better than platinum in the intramolecular reactions of unactivated alkenes, as studied by Widenhoefer et al. [61-63]. Gold was allowed to work under mild conditions and the scope of the reaction was also broader than with other late-transition-metal catalyst systems, leading to the formation of five-and six-membered rings. 

It has recently been found that NEt3 is a gas-phase promoter for propene epoxidation by supported gold catalysts [245]. In more recent studies, Hughes et al. reported that catalytic amounts of peroxides could initiate the oxidation of alkenes with 02, without the need for sacrificial H2 [243]. The process worked for a range of substrates (cyclohexene, ds-stilbene, styrene and so on) and even in the absence of solvent hence, we may refer to this as green technology. 

A remarkable and potentially very useful property of gold catalysts is that they are essentially fully selective for alkene formation.20 This is because alkynes and alkadienes are much more strongly adsorbed than the derived alkenes, which are therefore displaced from the surface and unable to return, at least until the first stage of the reaction is complete. The 2-butenes were however found to isomerise after butadiene had completely disappeared.38... 

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. ... 

Alkene Oxidation over Copper, Silver, and Gold Catalysts... 

Improvement of the activity of homogeneous gold catalysts that will make it possible to activate alkenes and other less reactive substrates at ambient temperatures... 

Iwasima and colleagues engineered the Pt-catalyzed [3h-2] cycloaddition of A-(<9-alkynylphenyl)imines with activated alkenes into a synthesis of the mitosene skeleton (Scheme 2, equation 1) [6]. The gold catalyst, AuBrj, is equally effective. Nakamura s group described a novel Pt-catalyzed dehydroalkoxylation/cyclization cascade process... 

In an effort to extend tins reactivity to alkenes from allenes, the Toste group [300] performed the cyclohydroamination reaction with their preferred gold catalyst systems using diene-substituted suhbnylamides. Such substrates are prone to proton-mediated reactivity profiles, as discussed earlier, and generation of an acidic species has been postulated for catalytic turnover in Au(I)-catalyzed reactions [300],... 

Reactions with conjugated enynes as dienes in Diels-Alder reactions yield cyclohexadiene or benzene products on reaction with alkene or alkyne dienophiles, respectively. These reactions proceed via a stepwise mechanism to avoid formation of a cyclic allene and are referred to as dehydro-Diels-Alder reactions. In 2008, Barluenga and Aguilar demonstrated that gold catalysts promote intermolecular hetero-dehydro-Diels-Alder reactions between dienynes and nitriles. Dienyne 99 combines with phenyinitrile (100) to afford substituted pyridine 101. ... 

An intramolecular version of this reaction, employing a furan as the nucleophile with an acetylenic ketone as the acceptor was employed in a short synthesis of crassifolone 6.200 (Scheme 6.91). The cyclization substrate 6.198 was synthesized using standard chemistry and cyclized on treatment with a cationic gold catalyst. The product 6.199 could be converted to the natural product by conjugate addition of a Grignard reagent to form the quaternary centre, followed by oxidative introduction of an alkene. 

In the hydroamination of unsaturated carbon-carbon bonds, gold catalysts play an important role. Intermolecular hydroamination of alkenes [177], 1,3-dienes [204], terminal and internal alkynes [205], and allenes [206] are known to proceed smoothly in the presence of PhsP AufI) or AuCls catalyst. In addition, amino olefins also efficiently undergo intramolecular hydroamination using similar gold catalysts. He and coworkers have developed the catalytic cycloaddition of tosylated amino olefins [207], A representative example is shown in Scheme 18.35. When N-tosylated y-amino olefin (97) is exposed to a mixture of PhsP AuCl and AgOTf (5 mol% each) in toluene at 85 °C, pyrrolidine (98) is obtained in 96% yield. The gold(I)-catalyzed intramolecular hydroamination is applicable to N-alkenyl carbamates [208], N-alkenyl carboxamides [209], and N-alkenyl ureas [210], The use of microwave irradiation results in completing the hydroamination in a much shorter time than that required under thermal reaction conditions [211], The... 

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