Triazene decomposition reaction, aromatic

Aryltriazenes can also be decomposed by hydrogen fluoride in organic solution after extraction from their aqueous mother phase. In this case, hydrogen fluoride can be used in small excess but the nature of the solvent is crucial for example, tetrahydrofuran gives complex mixtures, dichloromethane promotes radical reactions (dimerizations, reductions) and acetic acid favors triazene decomposition before fluorination. Aromatic and haloaromatic compounds seem to be the best solvents.283 Such a technique, especially suited for the rapid introduction of an 18F atom, has been employed to produce [ 8F]haloperidol (3), the specific receptors of which have been localized in the brain by positron emission transaxial tomography.298... 

The 3,3-dialkyl-l-aryltriazene (0.1 mmol) was treated with TBAF or CsF (0,2 mmol) in the presence of MsOH or TFA (5 mol equiv), or cation-exchange resin (BioRaD AG 50W-X12-hydrogen form), in anhyd MeCN, trichloroacelonitrilc. THF, DMF, DIVlSO. PhBr, toluene, CCI4 or 1,2-dichlorobenzene (3 ml,). The optimum reaction temperature for aromatic triazene decomposition in most solvents was 70-80 C. 

Since dialkyltriazenes were first used in aromatic fluorination by Wallach, ° many fluoroaro-matic compounds have been obtained in high yield by the decomposition of 3,3-dialkyl-l-aryl-triazenes with various fluorides in acidic media (see Vol. ElOa, p 725fT). Aryltriazenes are a potential source of aryldiazonium salts under controlled, mild, acid conditions. Therefore, this replacement (Route A) can be considered as a type of Balz-Schiemann reaction (see Section... 

The modem methods for performing the GBH reaction are based on the thermal decomposition of certain aryltriazenes in the presence of trifluoroacetic acid [66,67], or on the phase transfer catalysed reaction of aryldiazonium tetrafluoroborate in the presence of potassium acetate [61], both methods in an excess of arene as the reactant and reaction solvent. The first method requires previous preparation of the aiyltriazene (VII) by diazotation of an aromatic amine in hydrochloric acid followed by addition of piperidine [66], other dialkylamine or 5-aminotetrazole [67]. For example, from the diazotized aniline solution 43, by addition of an equimolar amount of piperidine, the yellow-orange 1-phenyl-3,3-(pentanediyl)triazene (48) readily crystallize in 81% overall yield [66], Scheme 17. 

The third possible mechanism we believe, accounts for all our observations, and in addition, it is generally applicable to the triazene and nitrous acid methods of deamination. Based on the structures of aromatic diazonium salts, the formation of ion pair 42 seems to be a logical first step in the reaction. By analogy to other ion pairs, the N—O bond in 42 could have some covalent character and the nitrogen could be bonded to one of the oxygens in particular . However, for the decomposition in a solvent as polar as acetic acid (Table 8), it appears that the C—and N—O bonds (34) are either broken simultaneously or in rapid succession in view of the independence of the scrambling with respect to the carbonium ion character of R. 

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