Cocyclization arynes with alkynes

Pd-catalyzed cocyclization of arynes with alkynes also proceeds smoothly. Yamamoto obtained phenanthrene derivatives exelusively in good yield, regardless of the electronie nature of the alkynes using Pd(OAe)2 and P(o-Tol)3. The reaction of 251 with 4-octyne gave rise to the phenanthrene 263 [82], Similarly, Perez carried out the reaction of benzyne with electron-dehcient alkynes such as dimethyl acetylenedi-carboxylate (DMAD) (264), and obtained the phenanthrene 265 as the main product using Pd(PPh3)4. On the other hand, when Pd2(dba)3 was used, the naphthalene derivative 266 was the main produet [83]. 

Finally, transition metal-catalyzed reactions of arynes have been explored as a useful method for the construction of a wide variety of carbo- and heteocycles. These reactions include cyclotrimerization of arynes, cocyclization of arynes with alkynes or alkenes, and carbopaUadation of arynes with Pd-complexes. Moreover, some insertion reactions of arynes into a-bonds are also catalyzed by metal complexes. 

Recently, the Yamamoto group and the Guitian group as pioneers in this field have developed efficient Pd-catalyzed reactions of arynes, particularly cyclotrimerization and cocyclization with alkynes, offering useful synthetic methods for polycyclic aromatic compounds [78]. The Spanish group reported that the cyclotrimerization of benzyne, generated from 251, occurred in situ at room temperature in the presence of Pd(PPh3)4 in MeCN, and obtained triphenylene (253) in 83 % yield [79]. 

Alkynes have been reported to participate in cocyclization with arynes under the same reaction conditions, affording a mixture of phenanthrenes 218, naphthalenes 219 and triphenylenes (Scheme 12.62) [115]. The chemoselectivity depends on both the palladium catalyst and the alkyne substrate used. For example, when dimethyl acetylenedicarboxylate (DMAD) is employed as the alkyne, a remarkable chemoselectivity is achieved by appropriate selection of the... 

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