Quaternization of benzothiadiazoles and related compounds.

The review of the chemistry of the benzodiazoles is largely confined to considerations of the four parent heterocyclic systems under investigation, namely, 1.2.3-benzothiadiazole and 1,2,3-benzoselenadiazole, 2.1.3-beazothiadiazole and 2,l,3-benzoselenadiazole. The benzothiadiazoles and their selenium analogues only combine with one molecular proportion of quaternizing agent; while some bis- quaternaries have been prepared, di-quaternary salts are never obtained. The merits of various quaternizing agents, viz., alkyl iodides, dialkyl sulphates, alkyl p-toluenesulphonates, and alkyl 2,4-dinitrobenzenesulphonates for the preparation of methyl and ethyl quaternary salts of these very weak bases are discussed, and the anions of such compounds are exchanged to form chlorides, bromides, iodides, perchlorates, thiocyanates and cyanides, with interesting properties. Though 1,2,3-benzothiadiazole is an unsymmetrical compound, in practice only one of the three possible isomers of alkyl-1,2,3- benzothiadiazolium salts is obtained. The position of alkylation is considered in the light of evidence from reductive cleavage (to give N-alkyl-o-aminothiophenols), reaction with alkali and spectroscopic measurements; N(3) appears the most likely site. There is little doubt about the structure of alkyl-2,l,3-benzothiadiazolium and -benzoselenadiazolium salts; their decomposition in water yields sulphurous or selenious acid and the N-alkyl-o-phenylenediamine. The alkyl 2,4-dinitrobenzenesulphonates afford the best scope for obtaining higher alkyl quaternaries. The benzothiadiazoles and their selenium analogues undergo quaternization with n-propyl, n-butyl, n-amyl, iso-butyl and iso-amyl 2,4-dinitrobenzenesulphorates, though thermal decomposition of the iso-alkyl esters is a serious problem. Secondary or tertiary alkyl 2,4-dinitrobenzenesulphonates are unobtainable but very small quantities of sec.-alkyl quaternary iodides of l,2,3-benzothiadiazole and 2,1,3-benzoselenadiazole result from the use of iso-propyl iodide. Some polymethylene bis-quaternaries are prepared, especially by means of a convenient one-stage process involving heterocyclic base, ?,w-di-iodoalkane and silver 2,4-dinitrobenzenesulphonate. The work done establishes that the effectiveness of the alkyl 2,4- dinitrobenzenesulphonates as quaternizing agents decreases as the series is ascended. Also, the reactivity of the parent bases decreases in the order.. 1,2,3-benzoselenadiazole > 2,1,3-benzoselenadiazole > 1,2,3-benzothiadiazole > 2,1,3-benzothiadiazole and follows closely the magnitude of their pKa values, the determination of which gave important information concerning protonation.