Gold-catalysed transformations

A gold-catalysed transformation of cw-l-oxiranyl-l-alkynylcyclopropanes into halo-genated products occurs in the presence of A -halosuccinimide (Scheme 122). ... 

Gold-triflimide complexes have proven to be very versatile catalysts. However, they are ineffective for some gold-catalysed transformations. A good alternative to such complexes is the use of stable gold solvate species, which offer a more flexible choice of counterion. 

In this section, we will focus on highly efficient gold-catalysed transformations, i.e. catalyst loadings <0.1 mol%, and will pay special attention to protocols that afford high turnover numbers (TON) and frequencies (TOP). 

Scheme 16.4 Silver-free catalysts for dual gold-catalysed transformations.

Addition of alcohols to unsaturated carbon molecules is one of the most straightforward and environmentally friendly procedures to form C-O bonds. Several species are known to catalyse such transformations. Among them, gold complexes have proven to be extremely active to catalyse the addition of oxygen nucleophiles to all nes, in order to produce the corresponding ketone, ketal or vinylether species. Moreover, such reactions are among the most efficient gold-catalysed transformations reported to date. 

Nolan and coworkers have recently reported the gold-catalysed transformation of allqmes and allylic alcohols to homoallylic ketones. This reaction involved the hydroalkojqrlation of the allq ne followed by Claisen rearrangement. It proceeded in solvent-free conditions, at veiy low catalyst loading. Internal and terminal allq nes were successfully used as well as different allylic alcohols. The recyclability of the catalyst was studied by conducting the reaction with model substrates (diphenyalcetylene and allyl... 

Another attractive green alternative is the use of ionic liquids (IL) as solvents/ IL present several advantages over conventional solvents. The final product of the reaction can he typically separated hy simple extraction with an organic solvent and the IL phase might he reused in catalysis. IL have heen found to he adequate solvents to conduct several gold-catalysed transformations. 

One of the principles of Green Chemistiy is the reduction of the energy requirements to perform a reaction, that is, reactions that can be conducted at room temperature are preferred. For this reason, in this section we will discuss relevant examples of gold-catalysed transformations at room temperature. The selected protocols are grouped by transformations. 

A gold-catalysed transformation of a-(2-indolyl) propargylic alcohols with imines in the presence of the oxidant 8-isopropylquinoline A-oxide gives dihydro-y-carbolines by intermolecular trapping of an a-carbonyl gold carbenoid intermediate, followed by cyclization and a gold-assisted 1,2-acyl migration (Scheme 129). ... 

Scheme 13.4 Catalysed transformations of polycyclic hydrocarbons in a gold-lined reactor (1) 433 K (2) 543 K.

With another change of scene, we note the ability of gold to catalyse transformation of polycyclic hydrocarbons having fascinating structures (Scheme 13.4) 19 the reactions depicted there were catalysed either in the vapour phase using a gold-lined reactor at temperatures between 373 and 543 K and contact times of 10-20 s they were also effected by the complex Au(DCP)2C1 (DCP = dicyclopentadiene) in solution over 1 day at room temperature. 

The Hashmi phenol synthesis is another example of a perfect atom-economical reaction. Hashmi and coworkers reported this transformation for the first time in 2000, and have been actively working on improving and developing more efficient catalytic processes ever since. As a result of this work, they have reported several extremely active catalytic systems for this transformation. In 2011, they reported TONs of 1900 and 1860 using catalysts VI and VII, respectively, in a comparative study between the use of KITPhos and SPhos ligands in the gold-catalysed phenol synthesis (Scheme 16.14). ... 

As has been discussed in Section 16.4.3, Marinelli reported the synthesis of indoles by means of gold-catalysed eyelisation of 2-allqTiylanilines in ionic liquids at 50 °C. Prior to this report, the authors showed that the transformation could also be performed at room temperature in ethanol or ethanol/water mixtures using the same catalyst. In this case the catalyst loading was 4 mol% (Scheme 16.56). 

In 2006, Gagosz reported the gold-mediated stereoselective synthesis of 2,5-dihydrofurans from butynediol monobenzoates at room temperature. The transformation involved two sequential gold-catalysed reactions the formation of an allene derivative followed by a cyclisation. The reaction was... 

Ma and coworkers reported the gold-catalysed cyclisation of l-(indol-2-yl)-3-allg n-l-ol derivatives to afford carbazoles at room temperature. The transformation was promoted by 5 mol% of gold(in) chloride and the corresponding carbazoles were isolated in moderate to veiy good yields (Scheme 16.63). In an earlier report the authors also showed two examples of the gold-catalysed formation of carbazoles using l-(indol-2-yl)-2-allenyl-l-ols as precursors. 

The intramolecular alkyne coupling of diyne 11.281 was used in a synthesis of bryostatin 11.283 to close the macrocycle 11.282 (Scheme 11.94). This reaction was followed by a gold(l)-catalysed 6-endo ring closure of 11.282 and some routine transformations to complete the synthesis. As so often in macrocyclizations, a low concentration (0.002 M) was found to be important. 

In 2000, Hashmi reported one of the first applications of gold(m) chloride to homogeneous catalysis. In this report, several examples of intramolecular and intermolecular cyclisations involving, carbon-carbon and carbon-oxygen bond formation, were efficiently promoted by gold at room temperature and at low catalyst loading. The authors reported a cyclo-isomerisation/dimerisation of allenyl ketones and a,(3-unsaturated ketones (Scheme 16.50). This one-pot transformation was catalysed by 1 mol % of gold(m) chloride and proceeded at room temperature. 

Genet, Michelet and coworkers reported the gold-promoted cyclisation of bis homopropargylic diols to afford bicyclic ketals at room temperature. The transformation was catalysed by 2 mol% of gold(i)- or gold(iii)-chloride in methanol and the final products were isolated in very good to excellent yields after short reaction times (Scheme 16.58). 

Echavarren reported the cycloisomerisation of 1,7-enynes catalysed hy a cationic gold(i) species at room temperature. The reaction proceeded using 2 mol% of XXXI in dichloromethane. Remarkably, most of the transformations required only 5 min (Scheme 16.68). 

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