Synthesis and solid-state structures of dimethyl 2,2′-bithiophenedicarboxylates

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-crystal X-ray structural analysis of these compounds shows that the thiophene rings in 3 and 4 are nearly coplanar (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 electrostatic stabilization of the planar structure due to attraction of the 3-carbonyl oxygen to the sulfur of the distal 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 atoms in 5 are completely syn. This is the first instance where a bithiophene has been shown to adopt a conformation 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 these molecules