Diethyl 2,3-dihydrothieno[3,4-b]-1,4-dioxine-5,7-dicarboxylate

The title compound, C12H14O6S, is a dicarboxylic acid diethyl ester of 3,4-ethylenedioxythiophene, which is a component of electrically conductive poly(3,4-ethylenedioxythiophene) (PEDOT). The ethylene group is disordered over two sites with occupancy factors 0.64 and 0.36. Both the carbonyl groups are coplanar with the thiophene ring. The molecules form centrosymmetric dimers with an R 2 2(12) coupling by intermolecular C—H⋯O hydrogen bonds [3.333 (5) Å] at the ethoxycarbonyl groups. The dimer units are arranged to form a ribbon-like molecular sheet.

Furthermore, the thin films of oxidized PEDOT are almost transparent. Therefore, these are used for organic electrodes in the study of electronic devices (Groenendaal et al., 2000;Pei et al., 1994). With regard to the hole-transporting abilities, the arrangement of 3,4-ethylenedioxythiophene units in film has attracted considerable attention. A few crystal structures including a 3,4-ethylenedioxythiophene ring system were reported (Sotzing et al., 1996;Abboud et al., 1998;Kumar et al., 1998). In this paper, we report the crystal structure of compound (I) that is a dicarboxylic acid diethyl ester of 3,4-ethylenedioxythiophene.
The compound (I) crystallizes in the P1 space group. The molecular structure is shown in Fig. 1

Experimental
The title compound (I) was prepared as follows: A solution of diethyl 3,4-dihydroxythiophene-2,5-dicarboxylate (3.12 g, 12 mmol) and caesium fluoride (7.26 g, 48 mmol) in dry acetonitrile (200 ml) was stirred for 1 h under nitrogen. After addition of a solution of ethylene di(p-toluenesulfonate) (5.55 g, 15 mmol) in dry acetonitrile (100 ml), the reaction mixture was refluxed for 48 h. The reaction mixture was filtered and the precipitate was washed with acetonitrile. The filtrate was concentrated and the residue was chromatographed on alumina gel (CH 2 Cl 2 ) and silica gel (CH 2 Cl 2 ) to afford the compound of (I) (2.38 g, 69%) as colorless needles. Physical data for (I) Colorless crystals of (I) suitable for X-ray analysis were obtained from a methanol solution.

Refinement
All the H atoms were placed in geometrically calculated positions, with C-H = 0.97 (methylene) and 0.96 (methyl) Å and U iso (H) = 1.2U eq (C) (methylene) and 1.5U eq (C) (methyl), and refined using a riding model.  Fig. 1. The molecular structure of (I), with atom labels and 50% probability displacement ellipsoids for non-H atoms and H atoms are shown as small spheres of arbitrary radii. The disordered atoms (C5B and C6B) are omitted for clarity.   as large as those based on F, and R-factors based on ALL data will be even larger.
The methylene carbon atoms and the associated hydrogen atoms of the dioxine ring are disordered over two sites