Gold catalysis intramolecular

Thanks to a recent renaissance in gold catalysis, new gold-mediated transformations are being discovered with ever-increasing frequency. Some of these discoveries have been attributed to the action of gold vinylidenes. Fiirstner and coworkers uncovered one such example while screening catalysts for intramolecular alkyne hydroarylation (Scheme 9.23) [46]. 

Che et al. recently applied gold catalysis to the synthesis of lactams by the intramolecular addition of P-ketoamide to unactivated alkenes, as shown in Equation8.27 [83]. 

The reaction worked with both internal and terminal alkynes (except silylated alkynes) and in many solvents, even in the neat alcohol added [105]. The mechanism proposed involved two catalytic cycles first, gold catalysis would lead to dihydro-furan by a fast intramolecular reaction then, the subsequent slower intermolecular reaction would be produced by the addition of alcohol to the enol ether to deliver a ketal (Scheme 8.18). 

Apart from these two examples, in the rest of the olefin activations, gold coordinates to the olefin, turning it susceptible to nucleophilic attack. The early examples of alkene functionalization by gold catalysis (equation 144) focused on the intermolecular addition of 1,3-diketones to styrenes. " An intramolecular version with ketoamides to yield pyrrolidinones was later developed and followed by the intermolecular addition of phenols (equation 145) and carboxylic acids to double bonds,a work that included an example of intramolecular addition of an aliphatic alcohol to an olefin. 

The intramolecular reaction between furans and alkynes often does not yield the expected hydroarylation products, but, instead, gives phenols 6.220 in an efficient and selective manner (Scheme 6.96). This reaction was probably the first surprising transformation from gold catalysis, indicating that gold could be more than just an electrophilic trigger. 

The synthetic potential of gold catalysis, more especially for the generation of structural complexity, is particularly well illustrated by the total syntheses of Englerin A and B, reported independently in 2010 by Echavarren et al. and Ma et al. [31] (Scheme 16.26). A very similar approach was used by these two groups to produce, via a gold-catalyzed [2+ 2-1-2] intramolecular cycloaddition between an alkyne, an alkene, and a ketone, the core structure of the Englerins. Notably, this complex gold-catalyzed sequence, which operates with an absolute control of the stereoselectivity, allowed the creation of three new asymmetric centers and three new bonds (2 C-C and 1 C-O bonds). 

In recent years, several groups have developed enantioselective tandem reactions based on the combination of gold catalysis and organocatalysis. Among them, Gong et al. reported that an achiral gold complex compatibly worked with a chiral phosphoric acid to promote a domino intramolecular hydroamination-reduction reaction, readily transforming... 

Scheme 7.30 Domino intramolecular hydroamination-reduction reaction catalysed by chiral phosphoric acid catalysis and gold catalysis.

Scheme 7.36 Domino intramolecular hydrosiloxylation-Diels-Alder reaction catalysed by chiral phosphoramide catalysis and gold catalysis.

Soon thereafter in 2011, Toste and coworkers reported a synthesis that differed from the biosynthetic proposals laid out by the May and Dethe groups (Scheme 4). The Toste group s experience with cationic gold catalysis inspired an intramolecular Friedel-Crafts allylation reaction to form the flin-deroles pyrrolidine ring (see 24) from 77-functionalized indole 23. AUene 23 was readily available from commercially available 3-hydroxyethyIindole 22, which could either be purchased or synthesized via reduction of... 

Cyclic amino-carbenes, in molybdenum carbonyls, 5, 457 Cyclic bis(phosphine) dichlorides, with iron carbonyls, 6, 48 Cyclic carbenes, as gold atom ligands, 2, 289 Cyclic carbometallation, zirconium complexes, 10, 276 Cyclic carbozirconation characteristics, 10, 276 intermolecular reactions, 10, 278 intramolecular reactions, 10, 278 Cyclic dinuclear ylides, and gold , 2, 276 Cyclic 1,2-diols, intramolecular coupling to, 11, 51 Cyclic enones, diastereoselective cuprate additions, 9, 515 Cyclic esters, ring-opening polymerization, via lanthanide catalysis, 4, 145 Cyclic ethers... 

Dake s group84 published an interesting report in which AgOTf and cationic gold (I) complexes were compared for their use in the synthesis of the pyrrole scaffold. From (3-alkynyl ketones 66 (Scheme 5.29) and various primary amines 67, the imine intermediates JJ were formed in situ and the intramolecular cyclization produced various pyrroles 68. Both catalysts AgOTf (5 mol%) or Au(PPh3)OTf (5 mol%) were efficient, but the reaction proceeded more rapidly with silver catalysis. 

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. 

The attractive combination of transition metals and iodobenzene diacetate as strong oxidant allowed the subsequent development of additional diamination reactions. For example, in the presence of a conventional ttiphenylphosphino-gold(l)chloride complex, intramolecular diamination using ureas as nitrogen sources proceeds within a gold(l/lll) cycle with results that are comparable to the mentioned paUadium(II/IV) catalysis [99]. However, as the initial step consists of an anti-aminoauration, this reaction provides the opposite stereochemistry with respect to the overall product configuration. 

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. 

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