Dediazoniations via Aryne Intermediates

Similar correlations of orbital interactions with substituent effects were also found in additions of alkenes to substituted carbenes and of N2 to transition metal complexes (see Zollinger, 1983 b, 1990). 

Arynes in general are the subject of a chapter in a volume of Patai and Rappoport s series The Chemistry of Functional Groups (Gilchrist, 1983). The formation of arynes from diazonium salts is treated in three chapters of two volumes of the same series (Wulfman, 1978 Hegarty, 1978 Zollinger, 1983 a). 

The formation of arynes (8.26) as metastable intermediates in aromatic dediazo-niations was postulated by Stiles and Miller (1960) for the case of the 2-carboxy-benzenediazonium zwitterion (8.25) and by Cadogan and Hibbert (1964) for unsubstituted benzenediazonium salts.  

The unsubstituted benzyne (8.26) from 2-diazoniobenzenecarboxylate can be trapped in the absence of a Bronsted acid as biphenylene, i.e., dimeric benzyne (8.30 Ford, 1971), or by cycloaddition to a 4-71 donor such as furan to give 8.31 (Stiles et al., 1963). 

Gompper et al. (1968) determined the ratio of the two final products obtained from 2-diazoniobenzenecarboxylate in the presence of furan, namely the cycloaddition product 8.31 and 2-hydroxybenzoic acid (salicylic acid). This ratio depends on the water concentration but is independent of the concentration of added furan. This observation is consistent with the stepwise formation of benzyne (route A) involving 

---

Via aryne intermediates The aryne is formed in two steps, the first being a het-erolytic dediazoniation, followed by a heterolytic dissociation of a substituent (H, COO etc.) in one of the o-positions relative to the diazonio group (I)n + DN/). The (metastable) aryne reacts rapidly by addition of a Brpnsted acid (H2O, HC1 etc.). 

---