Gold cycloisomerization

SCHEME 7.26 Platinum and gold cycloisomerization of 1,5-enynes with subsequent (a) migration of an acetyl group or (b) and (c) oxidation of a hydroxy group. 

The first gold catalyzed C-S bond formation was demonstrated in a route to the 2,5-dihydrothiophene 16 via cycloisomerization of the allene 17 which occurred with high chirality transfer (d.r. > 95 5) <06AG(E)1897>. 

An interesting example of a gold-catalyzed cycloisomerization of P-aminoallene 168 to tetrahydropyridine 169 is depicted below <06OL4485>. Patil et al. report a similar gold-catalized hydroamination of allenes to produce 2-vinyl piperidine 170 in good yield <06TL4749>. 

Disubstituted furans 62 are obtained from a gold-catalyzed cycloisomerization/dimerization pathway involving terminal allenyl ketones and a,(3-unsaturated ketones <00AG(E)2285>. 

The Diels-Alder reaction outlined above is a typical example of the utilization of axially chiral allenes, accessible through 1,6-addition or other methods, to generate selectively new stereogenic centers. This transfer of chirality is also possible via in-termolecular Diels-Alder reactions of vinylallenes [57], aldol reactions of allenyl eno-lates [19f] and Ireland-Claisen rearrangements of silyl allenylketene acetals [58]. Furthermore, it has been utilized recently in the diastereoselective oxidation of titanium allenyl enolates (formed by deprotonation of /3-allenecarboxylates of type 65 and transmetalation with titanocene dichloride) with dimethyl dioxirane (DMDO) [25, 59] and in subsequent acid- or gold-catalyzed cycloisomerization reactions of a-hydroxyallenes into 2,5-dihydrofurans (cf. Chapter 15) [25, 59, 60],... 

In a recent report, Toste and Shen developed a gold(I)-catalyzed cyclization of alkynes using silyl ketene amides that, by means of prior hydrolysis, provided 1,6-enyne (285) or 1,5-enyne systems (287) activated for the intramolecular cycloisomerization [160]. 

Gold(I)-catalysed cycloisomerizations of enynes (38) afford gold-carbene complexes (39) that have been trapped in an intramolecular fashion yielding highly strained tetracycles (40).38... 

The key step in the gold-catalysed cycloisomerization of enynes (75) is believed to be the nucleophilic attack at the Au-coordinated C=C triple bond to form a vinylic gold intermediate (76) that is eventually converted into the final product (77).119... 

Aminoalkenes, oxidative cyclization, 10, 710-711 Aminoalkoxides, on zinc compounds, 2, 371 a-Aminoalkylallenes, cycloisomerizations, 10, 720 a-Aminoalkylcuprates, preparation, 9, 519-520 -Aminoalkylidynes, diiron carbonyl complexes with cyclopentadienyl ligands, 6, 248 Aminoalkynes, hydroamination, 10, 717 a-Aminoallenes, activation by gold, 9, 574 Amino r]5-amides, in Ru and Os half-sandwich rf3-arenes,... 

Cyclohexyldienyl complexes, with Ti(IV), 4, 327 Cyclohexylisocyanides, with gold(I) halides, 2, 281 Cyclohexylphosphine, for semiconductor growth, 12, 9 Cyclohexyl selenides, preparation, 9, 480 Cyclohydrocarbonylation alkenes, 11, 515 alkynes, 11, 522 dienes, 11, 522 overview, 11, 511-555 for ring expansion, 11, 527 Cycloisomerizations, via silver catalysts, 9, 558 Cyclomanganation, product types, 5, 777-778 Cyclometallated azobenzenes, liquid crystals, 12, 251 Cyclometallated complexes for OLEDs... 

Moreover, following the cycloisomerization reaction, a tandem dimerization reaction is also possible on the same substrates under Pd11, Ag1, and Aura catalysis, leading to different substituted furans (4 or 6) depending on the nature of the catalyst used (Scheme 5.5). Indeed, from compound 3 (Scheme 5.5), palladium(II) catalysis led to a 59% yield of 4, whereas silver(I) and gold(III) catalysis led to furans 5 and 6.41... 

The cycloisomerization of allenyl ketones was initially described as being catalyzed by rhodium(I) or silver(I) by Marshall et al.21 The activity of copper, silver, and gold for this reaction was first compared in two papers published later (Scheme 12.7).22 In the case of copper and silver, only a cycloisomerization was observed (Table 12.4, entries 1 and 2) with gold, a dimer is obtained as well (entry 3). 

The synthesis of dihydrofurans from propargyl esters is another reaction in which both gold and silver have been employed. In early work, Shigemasa et al.37 reported the efficient chirality transfer in the cycloisomerization of monoesters of butynediols. They used 8-15 mol% of AgBF4 as catalyst in benzene at 80°C in the dark. 

The same products are accessible by silver-catalyzed cycloisomerization of allenic ketones. Marshall and Bartley327 used AgNOs/silica gel in hexane to convert the allenic ketones 384 into the furans 386 with excellent yields (Scheme 112). Deuterium labeling experiments were interpreted in terms of the intermediate 385 which seems to arise from the coordination of silver catalyst to the allenic double bond distal to the carbonyl group. Again, gold precatalysts can be used with much lower catalyst loadings than their silver counterparts (see Section 9.12.4.3). 

The activation of allenes is a rather new, but particularly promising area of gold catalysis.381,400 The first example for such a transformation is the cycloisomerization of allenic ketones 480 to furans 482 which probably occurs via intermediate 481 (Scheme 147). Hashmi et /.401,401a showed that this reaction proceeds much faster when gold(m) chloride in acetonitrile is employed as the precatalyst instead of the traditionally used silver salts (cf. Section 9.12.3.2). The products are usually contaminated by substituted furans originating from a Michael addition of aurated 482 to the substrates 480, thereby indicating that the gold catalyst is also capable to activate C-H bonds of furans. 

Zinc chloride was reported to be a suitable catalyst as compared to gold in cycloisomerization of homopropargylic ketones. Thus, reaction of 1,4-disubstituted and 1,2,4-trisubstituted but-3-yn-l-ones in the presence of zinc chloride gave the corresponding 2,5-di- and 2,3,5-trisubstituted furans in high yields <07OLl 175>. 

Electrophilic Au(I) complexes or their halide AuX analogoues typically cyclize enynes (I, Scheme 58) (475) by a 5-exo-dig pathway to give a variety of cycloisomerization and addition derivatives. The mechanism is proposed to involve formation of a cyclopropyl gold-carbene intermediate... 

Scheme 59. l,8-Dien-3-yne substrate cycloisomerization and ring expansion via a cyclopiopyl gold-carbene intermediate. 

Cycloisomerization. Gold salts and complexes are popular catalysts for organic transformations because it is found that the metal has high affinity to allenes and alkynes. A gold ion usually requires stabilization of a phosphine. As shown by the cyclization of 1,2,7-aIkatrienes, BINAP and its congners are adequate ligands. 

From the 5-silyl derivatives of o-alkynylarylthiols, cycloisomerization leads to 3-silyl-benzothiophenes. The gold salt is found to be highly effective in promoting Meyer-Schuster rearrangement. ... 

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