Cycloaddition of arynes

The highly conjugated nature of polyaromatics makes them attractive materials with conduction properties. The demand for polyaromatics can be fulfilled easily by [2 + 2 + 2] cycloaddition reaction with benzyne. This affords a fused aromatic framework in a single operation. Undoubtedly, benzynes are highly reactive substances that may easily lead to decomposition. However, with the choice of a suitable catalyst, benzyne chemistry can be harnessed successfully. 

Hsieh and Cheng reported that naphthalene-based aromatics can be accessed by a nickel-catalyzed cycloaddition route [26]. In this reaction, diiodoarenes serve as 

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Cycloaddition of arynes to enamines gives aminobenzocyclobutenes, which undergo amine elimination or ring expansion on thermolysis of the amine oxide269 (Scheme 116). 

The susceptibility of a benzenoid ring to dienophilic attack may be enhanced through metallation. For example, 225 was one of two products obtained from the reaction of hexabromobenzene with mesitylmagnesium bromide This product clearly arises from cycloaddition of arynes derived from 222 to a mesitylene ring (a small amount of the C2v... 

Benzazetidines are conceivable products from the [2+2] cycloaddition of arynes across the C=N bond. Evidence for the transient formation of these intermediates has been presented However, yields are very low and the azetidines have never been isolated, only... 

I have selected a number of important topics in newly developed organopalladium chemistry, and have asked researchers who have made important contributions to these fields to review them. I am pleased that most of them have kindly accepted my request. For this book I have selected recent advances (covering mainly the last five years), most of which have not previously been the object of reviews. The book I am editing will cover Pd-catalyzed reactions that are novel, and entirely different from the more standard ones. Considerable patience will be required by readers when they face and try to understand topics such as (3-carbon elimination, palladacycles, Pd/norbornene-catalyzed aromatic functionalizations, arylation of aromatics, three-component cycliza-tions of allenes, and cycloaddition of arynes, for example. I believe their efforts will be well rewarded. 

A regioselective synthesis of 3-(2-hydroxyaryl)pyridines 197 initiated by the [3+2] cycloaddition of arynes with pyridine N-oxides has been reported by Larock and coworkers (Scheme 12.56) [105]. The initial cycloadduct 198 readily rearranges to form intermediate 199, which affords the observed 3-arylpyridines rather than... 

In an efficient procedure, indazoles 12 are obtained under mild conditions by 1,3-dipolar cycloaddition of arynes and diazoalkanes [450] ... 

Sato and co-workers described the nickel-catalyzed [2 + 2 + 2] cycloaddition of arynes and an unactivated alkene allowing for the original preparation of... 

T. Jin, Y. Yamamoto, An efficient, facile, and general synthesis of IH-indazoles by 1,3-dipolar cycloaddition of arynes with diazomethane derivatives, Angew. Chem. Int. Ed. 46 (2007) 3323-3325. 

F. Shi, R. Mancuso, R.C. Larock, 1,3-Dipolar cycloaddition of arynes with azomethine imines synthesis of l,2-dihydropyrazolo[l,2-a]inda-... 

In palladium-catalyzed [2 - - 2 - - 2] cycloaddition, arynes can be used as alkyne components. In 1994, Pena, Romero, and co-workers reported palladium-catalyzed homo-[2- -2- -2] cycloaddition of arynes (Scheme 6.9) [12]. The reactions of 2-silylaryl trifluoromethanesulfonates 26 with CsF generated the corresponding arynes 27, which were trimerized in the presence of a catalytic amount of Pd(PPh3)4 or Pd2(dba)3 to afford substituted triphenylenes 28. In the reactions of unsymmetric arynes, moderate to high regioselectivities were observed. A mechanism via pallada-cycle intermediates, generated through the oxidative cyclization of two molecules of arynes, was proposed. 

The same group reported the efficient synthesis of polycyclic aromatic compounds by the palladium-catalyzed [2 - - 2 - - 2] cycloaddition of arynes with alkynes (Scheme 6.10) [12d,13]. The reactions of aryne precursor 29 and alkyne 30 afforded phenanthrenes 31 (aryne/alkyne = 2 1) as a major product. The use of electron-rich... 

Allyl chlorides can be employed in place of alkynes for the palladium-catalyzed [2-I-2-I-2] cycloaddition of arynes, as reported by Yamamoto et al. in 2000 [16]. Substituted allyl chlorides 40 reacted with aryne from 39 accompanying the elimination of HCl in the presence of a paUadium(0) catalyst and CsF to produce phenanthrenes 41 and 42 in good yields, although the regioselectivity was moderate (Scheme 6.13). A mechanism that has been proposed involves a cascade-type addition of -ir-allylpalladium complex onto two benzynes. In the presence of additional alkynes 44, three-component cycloaddition product 45 was obtained in good yields with complete regioselectivity (Scheme 6.14). 

Guitiin, P6rez, and co-worlcers reported the palladium(0)/(/ )-BINAP complex-catalyzed enantioselective complete intermolecular cross-[2 + 2 + 2] cycloaddition of arynes with two dimethyl acetylenedicarboxylates for the preparation of a functionalized [5]helicene (Scheme 10.25) [24], Although the [5]helicene desired was obtained in low yield as a result of the formation of several isomers, relatively high enantioselectivity was observed (67% ee). 

Catalytic transformation of arynes [1] and o-quinodimethanes (o-QDMs) [2] has proven in the past decade to be a potent and unique method for constructing aromatic skeletons, since Pena, Romero, Alonso, and co-workers reported the direct synthesis of triphenylene derivatives by the palladium-catalyzed [2 -b 2 -t- 2] cycloaddition of arynes in 1998 [3], demonstrating that their high reactivity was controllable precisely under transition-metal catalysis (Scheme 28.1). 

This chapter deals with advances in the synthetic application of the transition-metal-catalyzed reactions of arynes or o-QDMs, except for the cycloaddition of arynes, which was described in earlier chapters. Because arynes and o-QDMs, which can be regarded as dearomatized reactive intermediates, are utilized for the construction of aromatic skeletons via the reactions discussed herein, this chapter is entitled Dearomatization-Aromatization Sequence. ... 

An extensive review of recent advances in transition-metal-ffee carbon-carbon and carbon-heteroatom cycloadditions of arynes, generated from 2-(trimethylsilyl)aryl triflates, has been presented. The Diels-Alder reaction of arynes, generated from... 

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