| Abstract
| - The syntheses of dimethyl 2,2‘-bithiophene-4,4‘-dicarboxylate (3), dimethyl 2,2‘-bithiophene-3,4‘-dicarboxylate (4), and dimethyl 2,2‘-bithiophene-3,3‘-dicarboxylate (5) are described. Single-crystalX-ray structural analysis of these compounds shows that the thiophene rings in 3 and 4 are nearlycoplanar (dihedral angle close to 0°) and they adopt the anti sulfur conformation in the solid state.Further, the structure of 4 is in agreement with our previous suggestion that there is an electrostaticstabilization of the planar structure due to attraction of the 3-carbonyl oxygen to the sulfur of thedistal ring. In 5, however, the thiophene rings are nearly perpendicular (dihedral angle 75°),indicating considerable steric hindrance between the two large ester groups at the 3- and 3‘-positions.Unlike compounds 3 and 4, where the thiophene rings have the sulfur atoms anti, the sulfur atomsin 5 are completely syn. This is the first instance where a bithiophene has been shown to adopt aconformation where the sulfur atoms are completely syn. The solid-state conformations of 3, 4,and 5 are in agreement with ab initio theoretical calculations on these compounds; particularly,the planar conformations of 3 and 4 reflect the previously calculated low rotation barriers of thesemolecules.
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