Arynes from Other Sources

The first two precursors in this category to be discovered (see Table 5), 112 and 113, contain the relatively weak C-I and C-Hg bonds which undergo either photolysis or thermolysis to yield benzyne (1). The latter process is noteworthy as one of the first generations of this intermediate in the gas phase. Similar o-iodoorganometallic intermediates may be involved in the formation of arynes from o-diiodobenzene derivatives in the presence of zinc or copper. Both direct thermolysis and photolysis of 114 also lead to arynes, presumably via a stepwise mechanism involving o-iodophenyl radicals (47). The extent to which the latter go on to arynes depends not only on the nature of the aryl residue but, as mentioned in Section ILl.C, on the source of the radical center. Pyrolysis of dibromo (115), iodonitro (116), nitrobromo (117), and dinitro (118) aromatic compounds in a mass spectrometer also produces the corresponding arynes and didehydro compounds. 

The thermolysis of heavy metal acid salts also leads to arynes. Silver o-halobenzoates decompose by a different, apparently free radical, pathway than their alkali metal counterparts (32d). While the o- 

TABLE 5. Arynes from Other Sources 0-C5H4XY 

Pyrolysis of silver phthalates (122) also leads to products which have suggested the intermediacy of either arynes or the related diradicals (le). Mercuric phthalates (123) lose COj in a stepwise manner to give the trimeric phenylmercury carboxylates (124) n = 3) and the o-phenylenemercury trimers (125) n = 3) before going on to aryne-type products. Although it has been claimed that the monomeric form of 124 decomposes to benzyne (1), both the trimer and hexamer of 125 apparently do not. 

Finally, benzyne has been postulated to be formed in small amounts during the pyrolysis of chlorobenzene, nitrobenzene, benzotrifluoride, and even benzene itself by the m ramolecular elimination of HCl, HNOj, HCF3, and respectively. Plasmolysis of halobenzenes, benzene, and phenyl-acetylene also provides arynes by a similar process. /n ermolecular dehydrogenation either with other arynes [equation (11)] or nitrenes [equa- 

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Photolysis of the mercury compound 286 in benzene leads to 3-phenyl-thiophene (291) and 3-iodothiophene (287). The former could arise from an insertion reaction of the aryne (4) with benzene if it is assumed that the expected but missing 2-phenylthiophene 292 completely isomerizes to the 3-isomer (291) under the reaction conditions. The argument is not convincing, however, since the 3-iodothiophene (287) formed as a coproduct, presumably by homolysis of the C-Hg bond, also yields 3-phenylthiophene (291) on photolysis in benzene, albeit in very low yield. Furthermore, by analogy with the reaction of benzyne the major products should be those of cycloaddition, which, in the case of 4, might lead, after loss of acetylene, to thianaphthene (293). No such products were reported from the photolysis of the mercury compound (286), but they have been observed from other possible sources of the aryne (4) (Section III.3.A.f). 

The palladium-catalyzed cyclotrimerization of arynes can also be used to construct flat hydrocarbons with extended conjugation, such as hexabenzotri-naphthylene (supertriphenylene, 87). In this case the required aryne 86 was generated in the presence of palladium by treatment of triflate 85 with tetra-butylammonium fluoride, and afforded trimer 87 in 20% isolated yield from 85 (Scheme 18) [51]. As in other cases (see below), the use of a soluble fluoride source was necessary because the extended planar reaction product is extremely... 

Biehl and Kamila synthesized benzothiopyrano[2,3-b] indoles from the reaction between arynes and indole-2-thiones (Scheme 7, equation 1) [28]. 4,5-Dimethoxybenzyne was generated from 2-diazonio 4,5-dimethoxybenzenecar-boxylate hydrochloride. Other aryne sources were less efficient or failed. Lin, Wang, and colleagues discovered that benzynes react with 2-azidoaaylates in the presence... 

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