The chemistry of 2,1-benzisothiazoles. Part VI. Quaternary salts and their decomposition by base to o-aminobenzaldehydes

Abstract

2.1 -Benzisothiazoles react readily with dialkyl sulphates, alkyl bromides, and alkyl toluene-p-sulphonates at 11 5-1 20" to yield quaternary salts. Such salts are decomposed by aqueous acid or base, affording N-substituted o-aminobenzaldahydes ; the parent o-aminobenzaldehyde can be obtained from the product of the reaction between 2.1 -benzisothiazole and ethyl chloroformate. Bridged binuclear o-aminobenzaldehydes may also be prepared.UNTIL recently, reports of quaternary salts of 2,lbenzisothiazoles were of 3-amino-or 3-azo-substituted derivatives only.24 However, a recent paper 5, gives details of the preparation and ring-opening reactions of simple 2,l-benzisothiazole quaternary salts. The work we now describe, completed before the appearance of this paper: extends the scope of such reactions.2,l-Benzisothiazole itself (I; R1 = H) and its simple derivatives afford quaternary salts (11) in good yield on direct heating with dialkyl sulphates, alkyl bromides, or alkyl toluene-P-sulphonates. The salts (11) are usually obtained as colourless crystalline solids by this procedure; in some cases, however, the salt is obtained asan oil, and is best isolated as the orange crystalline iodide by treatment with aqueous sodium iodide solution.

Nucleophilic attack on 2,1-benzisothiazolium salts

“…Indeed when l e was treated with sodium bicarbonate, Zmethylaminobenzophenone was isolated. This is comparable to earlier reactions of simpler 2,1-benzisothiazoles (5). Two other unstable products were formed in lesser amounts which may represent side reactions.…”

“…Reaction of 1-methyl-2,1-benzisothiazolium iodide (la) with sodium bicarbonate This was performed as described (5). Examination of the product obtained by tlc afforded 2-methylaminobenzaldehyde (2a) (34%) and I-methyl-2,l-benzisothiazoline-3-thione (5a) (22%) identical with an authentic sample (6)…”