Triazines, Sandmeyer reaction

SnCl2 reduction produced the 4-hydrazinoisoxazole (243). In ethanol the diazonium salt reacted with the 4-aminoisoxazole to produce the linear triazine (244) (Scheme 85). Diazoisoxazoles can also be treated with KI or H20/urea to produce the 4-iodo or 4-hydroxy derivatives (63AHC(2)365). These Sandmeyer reactions have been extended to a variety of isoxazole systems (77JMC934, 63AHC(2)365). 

The stabilities of pyridine-2- and -4-diazonium ions resemble those of aliphatic rather than benzenoid diazonium cations. Benzenediazonium ions are stabilized by mesomerism which involves electron donation from the ring, but such electron donation is unfavorable in 2- and 4-substituted pyridines. On formation, pyridine diazonium cations normally immediately react with the aqueous solvent to form pyridones. However, by carrying out the diazotization in concentrated HC1 or HBr, useful yields of chloro- and bromopyridines 752 can be obtained. Iodinated pyridines can be obtained in good yield using the Sandmeyer reaction. Aminopyridazines and -pyrazines, 2- and 4-aminopyrimidines, and amino-1,2,4-triazines behave similarly. Nucleophilic fluorination via the BalzSchiemann reaction of diazonium fluoroborates yields fluoropyridines, including 2-fluoropyridines. Fluoroborates can also be converted into fluoro compounds by ultraviolet irradiation. 

Perchlorination of triazines and related nitrogen compounds 346 Hoffmann degradation of perchloroaromatic amines 347 Other oxidations of 2,3,5,6-tetrachloro-p-phenylenediamine 348 An exceptional Sandmeyer-Schiemann-type reaction 350 Charge-transfer complexes 351... 

There have been many reports of reactions that are broadly classified as Sandmeyer-like, yet they do not strictly belong to this specific category. Several groups have reported that triazines of type 12 can be used to mask the diazo, then allow for direct Sandmeyer-type reaction to form the substituted products The iodination of diazoarenes is commonly... 

---