Selenazoles quaternary salts

Alkyl and aryl selenazoles are weakly basic, and their quaternary salts are easily hydrolvzed in aqueous solution. 

Because of the basic nitrogen atom, alkyl-selenazoles form quaternary salts. 2,4-Dimethyl-3-ethylselenazolium iodide (mp 157-158 0) was prepared by Brooker et al in 87% yield as colorless crystals by heating of 2,4-dimethylselenazole in excess ethyl iodide for 2 days. By reaction with the corresponding quaternary salts, the following cyanine dyes (8) were prepared" l 3-diethyl-4-methylselenazolo-2 -... 

Quaternary salts are obtained by alkylation of selenazole bases, the heterocyclic nitrogen atom playing the role of nucleophile with regard to the electrophilic carbon of the alkylating agent. 

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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