Selenazoles sulfonation

Alkyl selenazoles have been sulfonated. Thus 2,4-dimethylselenazole with fuming sulfuric acid (5% SO3) at 100°C gives 2.4-dimethy -5-sulfonylselenazole [m.p. 238 C (decomp.) (26)], which is relatively unstable. decomposing to give metallic selenium. 

Selenazole, 2-acetamido-nitration, 6, 341 Selenazole, 2-alkyl-nitration, 6, 340 Selenazole, 2-amino-bromination, 6, 341 sulfonation, 6, 341 synthesis, 6, 342... 

The halogenation of selenazoles goes less smoothly than the nitration and sulfonation. For example the bromination of 2,4-dimethyl-selenazoie with cold bromine first gives an unstable monobromo derivative (mp 168°C). This is transformed easily into a compound of mp 205°C (decomp.) which Haginiwa assumes is the hydrogen bromide salt of 2,4-dimethyl-5-bromoselenazole. 

The quaternary salts of selenium-nitrogen heterocycles are labile to nucleophiles and can be converted to other heterocyclic systems by ring expansion [82, 103], An example is conversion of 1,2,4-selenadiazolium trifluoromethane sulfonate (67) into 1,3,5-selenadiazine (68) (Scheme 17) [104], 2,3-Dimethyl-1-benzo-l, 3-selenazolium tetrafluoroborate is readily condensed with aromatic aldehydes to 2-styrylselenazole [105] or treated with sodium hydride to give 3-methyl-2-methylene-2,3 -dihydro-1 -benzo-1,3 -selenazole [106],... 

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