Cyclotrimerization and Cocyclization

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]. 

The cyclization of arynes can be applied to the synthesis of various polycyclic aromatics. Hexabenzo[a,c,g,/,w,o]triphenylene (260) was obtained as a single product in 39 % yield from 9,10-didehydrophenanthrene generated from 259 [80]. Cyclization of 3,4-didehydrophenanthrene 261 gave rise to the polycyclic compound 262, which has a double helicene structure, in 26 % yield [81]. 

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]. 

The polycyclic arynes derived from phenanthrenes 267,269, and 271 underwent cocyclization with two molecules of DMAD, giving the tetraesters 268, 270, and 272, respectively, in good yield [84,85]. 

Yamamoto et al. found that benzyne and methallyl chloride 273 underwent an interesting 2 1 cycloaddition to afford the phenanthrenes 274 and 275 using phosphine-free Pd2(dba)3 [86], This novel reaction is explained by the following mechanism. First, insertion of benzyne to jr-allylpalladium generates 276, which undergoes the insertion of benzyne again to give 277. Subsequent intramolecular alkene insertion, and jS-H elimination provide 278, and then 279. 

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