Structures of some 2-aminobenzothiazoles and their sites of protonation

Abstract

Extensive physical data, viz. dissociation constants and UV, IR and NMR data have been gathered on 2-amino- (I) and 2-methylamino- (II) benzothiazoles, 2-dimethylaminobenzothia-zole (III) and 2-imino- (IV) and 2-methylimino- (V) 3-methylbenzthiazolines, to arrive at the structures of I and II, since these are capable of existing in tautomeric forms la, lb and Ha, lib; compounds III, IV and V have been studied as the frozen models of the amino and imino forms respectively. The dissociation constants of the compounds I-V in methanol and methyl cellosolve indicate preponderance of la and Ha. This is further supported by UV and IR data; no conclusion could be drawn from NMR data. The site of protonation in compounds I-III is Na (nuclear nitrogen) whereas in IV and V it is at Nb (extra-annular nitrogen), as revealed by UV data and NMR data in trifluoroacetic acid.