Diazonium ions, aromatic fluorides

The controlled thermal decomposition of dry aromatic diazonium fluoborates to yield an aromatic fluoride, boron trifluoride and nitrogen is known as the Schiemann reaction. Most diazonium fluoborates have definite decomposition temperatures and the rates of decomposition, with few exceptions, are easily controlled. Another procedure for preparing the diazonium fluoborate is to diazotise in the presence of the fluoborate ion. Fluoboric acid may be the only acid present, thus acting as acid and source of fluoborate ion. The insoluble fluoborate separates as it is formed side reactions, such as phenol formation and coupling, are held at a minimum temperature control is not usually critical and the temperature may rise to about 20° without ill effect efficient stirring is, however, necessary since a continuously thickening precipitate is formed as the reaction proceeds. The modified procedure is illustrated by the preparation of -fluoroanisole ... 

These reactions are convenient methods for putting sulfur-containing groups onto an aromatic ring. With Ar S", diazosulfides Ar—N=N—S—Ar are intermediates, which can in some cases be isolated. Thiophenols can be made as shown above, but more often the diazonium ion is treated with EtO—CSS" or S2", which give the expected products, and these are easily convertible to thiophenols. Aryl triflates have been converted to the aryl thiol using NaST(P5) and a palladium catalyst, followed by treatment with tetrabutylammonium fluoride. See also 14-27. 

Aromatic fluorination A new method of aromatic fluorination involves treatment of aryltriazenes, readily prepared from aryldiazonium ions and dialkyl-amines, with 70% hydrogen fluoride in pyridine. The yields of product from this reaction are usually higher than those obtained by the reaction of HF-pyridine with a diazonium ion (6, 285) o-methoxy, iodo-, bromo- and nitro-substituted aryltriazenes generally give unsatisfactory yields. This method may be useful for the synthesis of " F-labeled compounds. 

Fluorine can also be introduced onto aromatic rings via diazonium ions. One procedure is to isolate aryl diazonium tetrafluoroborates. These thermally decompose to give aryl fluorides presumably by formation of the aryl cation which abstracts fluoride from the anion.Hexafluorophosphate salts behave similarly.The... 

Heavily fluonnated aminobenzenes, pyridines, and pyrimidines are diazotized in strong-acid media Solid sodium nitrite added directly to the fluonnated amine dissolved in 80% hydrofluonc acid, anhydrous hydrogen fluoride, or (1 1 wt/wt) 98% sulfuric acid in (86 14 wt/wt) acetic and propionic acids affords the electrophilic fluoroarenediazonium ion Addition of an electron rich aromatic to the resultant diazonium solution gives the fluoroareneazo compound [10 II] (equa tions 9 and 10)... 

One of the best methods for the introduction of iodine into aromatic rings is the reaction of diazonium salts with iodide ions. Analogous reactions with chloride, bromide, and fluoride ions give poorer results, and 14-25 and 13-20 are preferred for the preparation of aryl chlorides, bromides, and fluorides. However, when other diazonium reactions are carried out in the presence of these ions, halides are usually side products. Aniline has also been converted to fluorobenzene by treatment with t-BuONO and Sip4 followed by heating. A related reaction between PhN=N—N C4Hg and iodine gave iodobenzene. ... 

The reactions of picryl fluoride with (2,3-dihydroxypropyl)-ammonium or -phosphonium compounds have been found120 to yield zwitterionic spiro-cr-adducts such as (31). Aromatic diazonium cations have been shown to add to the 9-position of cr-adducts of 9-nitroanthracene to yield neutral derivatives.121 A study has been reported of the effects of ion pairing and change of solvent on the kinetics of decomposition of some anionic cr-adducts.122... 

---