Abstract
A new method for the preparation of solvatochromic 5-(5-X-6-hydroxynaphthyl-2)-7H-indolo[1,2-a]quinolinium merocyanines A new method for the preparation of 7H-indolo[1,2-a]quinolinium solvatochromic dye precursors has been investigated. 5-(6-Hydroxynaphthyl-2)-7,7-dimethyl-7H-indolo[1,2-a]quinolinium perchlorate, 5-(5-bromo-6-hydroxynaphthyl-2)-7,7-dimethyl-7H-indolo[1,2-a]quinolinium perchlorate and 5-(5-nitro-6-hydroxynaphthyl-2)-7,7-dimethyl-7H-indolo[1,2-a]quinolinium perchlorate were obtained via demethylation of the corresponding 5-(5-X-6-methoxynaphthyl-2)-7,7-dimethyl-7H-indolo[1,2-a]quinolinium salts. The application of pyridinium hydrochloride in the demethylation of the methoxy perchlorates allowed to obtain the products with the almost quantitative yield in a very short time, especially when the reaction was carried out using microwave irradiation.
Highlights
The demethylation of 5-(5-X-6-methoxynaphthyl-2)-7,7dimethyl-7H-indolo[1,2-a]quinolinium perchlorates proceeds according to Scheme 2
The new procedure was applied to the preparation of three compounds: 5-(6-hydroxynaphthyl-2)-7,7-dimethyl7H-indolo[1,2-a]quinolinium perchlorate (2a), 5-(5bromo-6-hydroxynaphthyl-2)-7,7-dimethyl-7H-indolo[1,2a]quinolinium perchlorate (2b), and 5-(5-nitro-6hydroxynaphthyl-2)-7,7-dimethyl-7H-indolo[1,2a]quinolinium perchlorate (2c). 5-(6-Methoxynaphthyl-2)7,7-dimethyl-7H-indolo[1,2-a]quinolinium perchlorate (1a)[11], was used as a starting material in the case of 2a
In order to obtain 2b and 2c an additional step was performed before demethylation i.e. bromination or nitration of 1a to 5-(5-bromo-6-methoxynaphthyl-2)-7,7-dimethyl7H-indolo[1,2-a]quinolinium perchlorate (1b) or 5-(5nitro-6-methoxynaphthyl-2)-7,7-dimethyl-7H-indolo[1,2a]quinolinium perchlorate (1c), respectively (Scheme 3)
Summary
Continuous interest in solvatochromic dyes results from their applicability in different fields, e.g. for the determination of the polarity of solvents[1] or solid surfaces employed as stationary phases in chromatography[2], for the characterisation of polymers[3], organized media[4] and supercritical fluids[5] as well as in the analysis of solventdependent kinetic and thermodynamic processes occurring in mixed solvents[6], to monitor the presence of some metal ions (i.e. cobalt, copper and iron)[7], and in quantitative analysis of binary and ternary solvent mixtures[8]. Among numerous compounds exhibiting solvatochromic properties, derivatives of hydroxyaryl-7H-indolo[1,2a]quinolinium cation[9,10,11,12] play an important role. These merocyanine-like dyes are generated in situ treating corresponding hydroxy perchlorates with anhydrous potassium carbonate. The high polarity of pyridinium hydrochloride allows to replace conventional heating with microwave irradiation[14] which in various organic synthesis can distinctly accelerate a reaction and influence the yield, selectivity and direction of the reaction[15]. The new procedure consists in the application of pyridinium hydrochloride as an effective demethylating agent of the corresponding 5-(5-X-6-methoxynaphthyl-2)-7,7-dimethyl7H-indolo[1,2-a]quinolinium salts. The experiments are conducted under conventional heating or microwave irradiation
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