A Conjugated Polymer from 2-Benzylidene-4,5-dicyano-1,3-dithiole

Abstract

Dicyanoarenes and -alkenes typically react with alkali metal alkoxides in alcohols to give molecular cyclotetramers, the best known example of these being the phthalocyanines. 1 In marked contrast to the prior literature, 1 we now report that 2-benzylidene-4,5-dicyano-1,3-dithiole (1) reacts with lithium n-butoxide in n-butanol at reflux to give a new polymer with a conjugated backbone structure.Previously available as a minor reaction byproduct, 2 1 was synthesized in the present work from the known diester 3 by conversion to a bis-amide, mp 248-249 °C (dec), using aqueous NH 3 . The amide was reacted with POCl 3 in pyridine to give 1, a red-orange solid, mp 146-147 °C, completely characterized by infrared, 1 H, and 13 C NMR spectra, mass spectrum, and elemental analysis. In tetrahydrofuran (THF) solution 1 exhibits the following electronic spectrum: λ max , 460 nm (log ) 3.24). The fluorescence spectrum of 1 in the same solvent exhibits a maximum at 530 nm when excited with 460 nm light.In refluxing n-butanol, 1 and n-BuOLi react over 2-3 h to give a dark blue solution from which a blue-black amorphous (by X-ray powder diffraction) solid 2 is isolated, after hydrolytic workup in chloroform and 10% HCl, in 53% yield; mp (capillary) 170 °C (dec). In the absence of butoxide, 1 is recovered unchanged from refluxing n-butanol. From the broad resonances of 1 H NMR and molecular weight estimation (weight-average MW 14 000-60 000 (GPC, versus polystyrene standards), M w /M n ) 2.01), 2 is found to be polymeric, and its color and electronic spectrum in THF solution (λ max 530-550 nm, tailing past 1000 nm) indicate a conjugated backbone. The absorption spectrum of 2 in THF is shown in Figure 1. Absorption spectra of 2 in dimethylformamide or dimethyl sulfoxide give broad maxima at 575-580 nm, with shoulders at 630 and 850 nm, with tailing past 1000 nm. It is not likely that the long absorption tail is due to scattering from insoluble polymer. Solutions were filtered before spectra were recorded, and in the solutions used for spectra, no precipitates formed on standing for times in excess of a month. Similar polymers were isolated from reaction of 1 with sodium pentoxide in refluxing n-pentanol and in refluxing (dimethylamino)-2-propanol.An approximate structure for 2, depicted in Figure 2, is proposed on the basis of elemental analysis, IR, and 1 H and 13 C NMR spectra. The atomic ratio of S:N is unity and indicates the preservation of the 1,3-dithiole ring during the polymerization. The absence of a -CN stretch in the IR spectrum and lack of a -CN resonance near 115 ppm in the 13 C NMR spectrum, along with the intense visible and near-infrared electronic spectrum, suggest a conjugated backbone structure formed by cyclopolymerization across the dicyanoalkene moiety.

Reactions of phthalonitriles with carbohydrates and alkali: phthalocyanine and oligomer formation

“…We have also observed polymerization at one of two cyano groups in the polymer obtained from reaction of 1,2-dicyanoacenaphthylene with dimethylaminoethanol and its sodium salt [17,18]. Reaction involving one of two cyano groups in a phthalonitrile or dicyanoalkene clearly competes with the better known cyclotetramerization leading to Pcs [1][2][3][4][5][6] and cyclopolymerizations [7,19,20] leading to polyiminopyrrolines or polyisoindolines.…”

“…Further, we have found that 2 reacts with unprotected sugars, sucrose in methanol or lactose in tetrahydrofuran (THF) in the presence of alkali [8] to give a polymer generally similar to that from 2 and Li n-butoxide in refluxing butanol in yields up to 75%. Further, the polymer of 2 prepared using the sugar reagent has higher molecular weight and improved thermal properties compared to those reported [7]. Since there is interest at present [9] in the synthesis of Pc under mild conditions, we deemed it of interest to react 1 and several of its derivatives with a variety of carbohydrates in the presence of alkali.…”

“…We have recently reported [7] that 2-benzylidene-4,5-dicyano-1,3-dithiole (2), in marked contrast to other dicyanoalkenes and -arenes, reacts with Li n-butoxide in refluxing butanol to give a linear polymer with a conjugated electronic structure. Further, we have found that 2 reacts with unprotected sugars, sucrose in methanol or lactose in tetrahydrofuran (THF) in the presence of alkali [8] to give a polymer generally similar to that from 2 and Li n-butoxide in refluxing butanol in yields up to 75%.…”

Structure, polymorphism and thermal properties of phenyliminoisoindolines

“…They are also of interest 3 as intermediates for the synthesis of hemiporphyrazines. The linear cyclopolymerization of 2-benzylidene-4, 5-dicyano-1,3-dithiole to a related structure has been reported recently 4 and Sn(II) reagents 5 and unmodified sugars 6 bring about the linear oligomerization and polymerization of phthalonitriles and certain dicyanoalkenes to structures related to polyisoindolines.…”

“…TGA: a 28.9% weight loss at 170 C, an additional 31.6% weight loss at 260 C and a further 20.3% weight loss at 321 C. Observed x-ray powder pattern, d (Å ): 9.0, 5. 4…”