Synthesis and UV/Vis spectroscopic properties of new [2-(N,N-dialkylamino)-1,3-dithiolium-4-yl]phenolates Mihail Lucian Birsa 1 * and Dumitru Ganju 2 1 Department of Organic Chemistry, Faculty of Chemistry, `Al.I. Cuza' University of Iasi, 11 Bd. Carol I, 6600 Iasi-6, Romania 2 Department of Inorganic Chemistry, Faculty of Chemistry, `Al.I. Cuza' University of Iasi, 11 Bd. Carol I, 6600 Iasi-6, Romania Received 13 May 2002; revised 18 November 2002; accepted 5 December 2002 ABSTRACT: In weak alkaline aqueous medium, 4-(hydroxyphenyl)-2-(N,N-dialkylamino)-1,3-dithiolium salts led to the corresponding dipolar betaines. These compounds were described as zwitterionic phenolates with an intramolecular charge-transfer UV/Vis absorption and represent useful precursors for the preparation of the first 1,3- dithiolium chlorides. Copyright  2003 John Wiley & Sons, Ltd. KEYWORDS: 1,3-dithiolium salts; mesoionic compounds; zwitterions; solvatochromism INTRODUCTION Compounds that exhibit intramolecular charge-transfer UV/Vis absorptions are of interest as chromophores for dyes, non-linear optics, synthetic light-harvesting sys- tems and theoretical aspects of charge transport at the molecular level. 1–3 There is special interest in systems where the electron-donor moiety is linked through a p- or s-bonded bridge to the electron-acceptor moiety. 4 Although tetrathiafulvalenes are well-known p-electron- donor systems, a variety of acceptor units have been investigated in this context. Special attention has been devoted to cationic systems such as pyridinium and bipyridium cations. 5–7 The charge-transfer properties of linked phenothiazine–bipyridinium systems have also been investigated. 8–12 In this context, it is presumed that 1,3-dithiolium ions can also serve as acceptor moieties in zwitterionic compounds with intramolecular charge- transfer absorptions. It should be noted that 1,3- dithiolium ions are also useful precursors for the synthesis of symmetrical and non-symmetrical tetrathia- fulvalenes. 13–15 The synthesis of some 4-(3',5'-dichloro- 2'-hydroxyphenyl)-2-(N,N-dialkylamino)-1,3-dithiolium perchlorates by the Vilsmeier–Haack reagent, under Harnisch reaction conditions, on the corresponding N,N-dialkylaminocarbodithioates has been reported by Cascaval. 16 Instead of the expected 1,3-dithiols, the sodium tetrahydroborate reduction products of these 1,3- dithiolium salts have been proved to be 4,6-dichloro-2- [2-(N,N-dialkylamino)-1,3-dithiolium-4-yl]phenolates, a new class of mesoionic compounds named by the author ‘iasinone.’ Although the failure of the reduction reactions is not clear, it is obvious that the basic conditions induced by sodium tetrahydroborate are responsible for the formation of dehydrohalogenated compounds. This unexpected result prompted us to undertake a study of the behaviour of 4-(2'-, 3'- and 4'-hydroxyphenyl)-2- (N,N-dialkylamino)-1,3-dithiolium salts in weak alkaline median, such as aqueous alkali metal hydrogencarbo- nates, and to investigate the p-acceptor properties of the 1,3-dithiolium ion unit. RESULTS AND DISCUSSION 4-(2'-Hydroxyphenyl)-2-(pyrrolidin-1-yl)-1,3-dithiolium perchlorate (2a) and 4-(4'-hydroxyphenyl)-2-(pyrrolidin- 1-yl)-1,3-dithiolium perchlorate (2b) were synthesized as colourless crystals by cyclization of 1-(hydroxyphenyl)- 2-(pyrrolidin-1-ylthiocarbonylthio)-1-ethanones (1a and 1b), in the presence of H 2 SO 4 –CH 3 CO 2 H (1:3, v/v), followed by addition of 70% perchloric acid to the reaction medium (Scheme 1) (for other variants, see Ref. 17). The latter are easily available from reaction of the corresponding hydroxyphenacyl halides and pyrrolidi- nium pyrrolidin-1-ylcarbodithioate. Treatment of per- chlorates 2a and 2b, under heterogeneous conditions, with saturated aqueous potassium hydrogencarbonate solution affords [2-(pyrrolidin-1-yl)-1,3-dithiolium-4- yl]phenolates (3a and 3b), in quantitative yields. The molecular structure of the new compounds was proved by analytical and spectral data and by the following chemical transformation: treatment of an acetone suspen- sion of the zwitterionic compounds 3a and 3b with 70% JOURNAL OF PHYSICAL ORGANIC CHEMISTRY J. Phys. Org. Chem. 2003; 16: 207–212 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.599 Copyright  2003 John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2003; 16: 207–212 *Correspondence to: M. L. Birsa, Department of Organic Chemistry, Faculty of Chemistry, ‘Al.I. Cuza’ University of Iasi, 11 Bd. Carol I, 6600 Iasi-6, Romania. E-mail: lbirsa@uaic.ro