Issue in Honor of Prof. Rainer Beckert ARKIVOC 2012 (iii) 356-370
On the Vilsmeier formylation of N-aryl-substituted
2-aminothiophenes -a simple route to new thieno[2,3-b]quinolinium
salts Horst Hartmann*
Department Chemie, Technische Universität Dresden, Dresden, Germany E-mail:
[email protected]
Dedicated to Prof. Dr. Rainer Beckert on the occasion his 60th birthday
Abstract
The Vilsmeier reaction of N-substituted 2-arylamino-thiophenes-5-carboxylic acid or their alkyl derivatives gives rise, depending on the substitution pattern at the thiophene moiety, to the formation of either N-substituted 2-arylamino-thiophene-5-carbaldehydes, corresponding imminium salt precursors, or novel thieno[2,3-b]quinolinium salts. These salts are highly reactive towards nucleophiles and can be easily transformed by reduction into the corresponding 4,9-dihydro derivatives.
Keywords: aromatic substitution, heterocycles, Vilsmeier formylation, reduction, 2aminothiophenes, cations
Introduction
Similarly to N,N-disubstituted anilines the heterocyclic N,N-disubstituted 2-aminothiophene analogues 1 are highly reactive towards a large variety of electrophilic reagents. As far as the 5position in these electron-rich heterocyclic compounds 1 is unsubstituted, 5-substituted 2aminothiophene derivatives 2 are formed by reaction with these reagents. For instance, with the Vilsmeier reagent (VR),1 with nitrous acid and its derivatives,2 or with aryldiazonium salts3 the corresponding substitution products 2a, 2c and 2d, respectively, are formed (Scheme 1). All these compounds received a lot of interest as versatile starting materials for deeply coloured methine, azamethine and azo dyes, some of which exhibit interested non-linear optical properties which enable to use these compounds for opto-electronic applications, eg. as NLO materials,4 as NIR dyes,5 or as sensors for estimating the solvent polarity.6
Furthermore, N,N-disubstituted 2-aminothiophenes 1 have been used, as far as they are Naryl- substituted, as building blocks for a variety of oligomeric compounds of the general
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