POLYHYDROXYOLIGOTHIOPHENES .2. HYDROGEN-BONDING-ORIENTED SOLID-STATE CONFORMATION OF 3,3'-BIS(2-HYDROXYETHYL)-2,2'-BITHIOPHENE AND REGIOSELECTIVE SYNTHESIS OF THE CORRESPONDING HEAD-TO-HEAD TAIL-TO-TAIL QUATERTHIOPHENE AND SEXITHIOPHENE/

This paper reports the X-ray structure of 3,3'-bis(2-hydroxyethyl)-2,2 '-bithiophene (1), which is the building block for the synthesis of he ad-to-head/tail-to-tail 2-hydroxyethyl-substituted oligothiophenes. Co ntrary to all the bithiophenes reported so far, 1 exhibits a noncoplan ar anti conformation and an inter-ring twist angle (67.5 degrees) whic h is the largest ever measured for adjacent rings of a-conjugated olig othiophenes. This unusual conformation appears to be dictated by inter molecular hydrogen-bonding interactions involving the OH groups, which bind the molecule in close packed layers. The paper also describes th e regioselective synthesis of the dimer and the trimer of 1, namely of 3,3',4 '',3 triple prime-tetrakis(2-hydroxyethyl)-2,2':5',2 '':5 '',2 triple prime-quaterthiophene (3c) and of 3,3',4 '',3 triple prime,4'' '':3''''-hexakis(2-hydroxyethyl)-2':5',2 '': 5 '',2'''':5'''',2'''': 5 '''',2''''-sexithiophene (4b). 3c And 4b were obtained through palladi um(0)-catalyzed coupling of the mono- and distannanes of the tetrahydr opyranyl derivative of 1 with the appropriate monobromo compound (Stil le's reaction). Finally, the paper reports force-field calculations wh ich suggest that the low lambda(max) values measured for 1, 3c, and 4b are also the result of intramolecular hydrogen-bonding interactions w hich favor highly twisted conformations in solution.