Arylation Reactions Involving the Formation of Arynes

The first benzazetidine (243) was isolated from the photolysis of 3-phenyldihydroben-zotriazine (242) (66JA1580). Another route to benzazetidines involving formation of the N to aryl-C bond utilizes intramolecular nucleophilic substitution via aryne (287). It is not general, however, and is only satisfactory when R and/or are alkoxy groups. The reaction also fails for iV-alkylamines (78LA608). 

Finally, the same group carried out a mono and a double annulation of arynes to synthesize fluoren-2-ones 60 and polycyclic aromatic systems 62 respectively [40]. These methods involve the in situ formation of highly reactive arynes by reaction of 2-(trimethylsilyl)aryl triflates 58 and CsF. In the presence of Pd(dba)2 (dba = dibenzylideneacetone), a crosscoupling and cyclization with a 2-halobenzaldehyde occur in the former case and C—H activation and cyclization occur in the latter case (Scheme 8.13). 

The second example of a cine-substitution of a thiophene involves the reaction of arylthiolates with 3,4-dinitrothiophene (331) or 3-nitro-4-phenylsul-fonylthiophene (332) to give the 2,4-substituted products 333. Both an elimination-addition mechanism via the aryne (334) or an abnormal addition-elimination (AEa) mechanism (Section II.2.A.e) via the Meisenheimer complex (335) have been considered for these reactions. The former is unlikely for several reasons including the lack of precedence for aryne formation from aryl nitro compounds (Section II. 1) under these reaction conditions and the fact that addition of the nucleophile ArS" to the aryne (334) would have to proceed via the 3-thienyl anion (336) rather than via a more stable 2-thienyl anion such as 320 as would be expected. Contrariwise, cine-substitution by the AEa mechanism is favored by the ability of the complex (335) to stabilize the negative charge by delocalization to both the NO2 group and the a position of the thiophene ring." As in the pyrrole series (Section III.2.B) the actual mechanism appears to be more complex, however, involving several addition and elimination steps via 337, which was recently isolated from the reaction (X = NO2) and shown to go to the product 333 under the reaction conditions. It therefore appears that neither of the cine-substitutions of thiophene described in this Section proceeds via an aryne intermediate. 

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