2,2 -Bithienyls conformations

Interest in the chemistry of polythiophenes has stemmed from their potential as organic conducting polymers . The simplest polythiophenes are bithienyls, which are 2 -16 ( = 0), 2,3 -32, or 3,3 -33. Furthermore, the molecules can rotate round the intermolecular C-C bond and thus each isomer can possess various conformations. 

For 2,3 -bithienyl 32, the conformation could not be obtained due to the disorder in the crystals. Similarly, from the X-ray of 3,3 -bithienyl 33, it was not possible to determine if the /7-conformation holds for all the molecules. The C-S bond lengths in bithienyl do not differ much from that in thiophene and, consequently, the thienyl substituent does not alter the geometry of thiophene to any extent. 

Bithienyl 116a exists in two conformations in the gas phase as shown by electron diffraction studies [254]. 

Lunazzi et al. have determined the conformation of 2,2 -bithienyl by means of a liquid crystal (nematic phase) NMR study (73JCS(P2)751) they found 78% of s-trans and 22% of s-cis rotamers (the estimated interconversion barrier is about 20 kj mol , see Section 2.9). The same compound using the same approach was studied by Khetrapal and Kunwar (74MP (28)441), the 0 angles determined by X-ray (Section 2.1) and electron diffraction (Section 2.2) are 0° and 34°, respectively they concluded that 2,2 -bithienyl is a mixture of cis- and frans-planar structures but they were unable to calculate the proportions (for theoretical calculations, see Section 2.14.3). 

Compounds 124 and 127 were studied at the lowest ab initio level (STO-3C) to conclude that the structures correspond to fully planar s-cis (both heteroatoms on the same side, sZ) and s-trans forms (sE), the former being the most stable (81 JCS(P2)127). STO-3C and 4-31C calculations were carried out on bifurans 115,116, and 117 by Orti and Tomas (84THE(108) 199), the authors conclude that the sE/sZ preferences depend on the position of the oxygen atoms in the case of 115 the preferred conformation is the sE. A similar conclusion (STO-3C calculations) was reached for bithienyls 119 and 120 where for 118 the most stable conformation is the sE (86JCS(P2)907) in this case the rotational barrier was calculated (25.1 kj mol ) and it agrees with that determined by DNMR (21 8 kJ mol, see Section 2.9). Compounds 115, 118, the Se and Te analogues as well as mixed derivatives were studied at the B3LYP/6-311 ++C(d,p) and... 

The conformations of unsubstituted 2,2 -, 3,3 -, and 2,3 -bithienyls were derived from SCF MO calculations (70JA1453, 72T4419). The calculated... 

A recent example illustrates the drastic difference in the conformation of bithienyl in the presence or the absence of a substituent in the vicinity of the pivot bond. In tetrameric-thienyl 152, the two central rings are coplanar whereas the peripheral thiophens are twisted in 153 the two central rings are almost perpendicular and the peripheral thiophenes are conjugated. It results in very different UV properties (10TL2956). 

P. Bucci, M. Longeri, C. A. Veracini, L. Lunazzi, Nematic phase nuclear magnetic resonance investigations of rotational isomerism. HI. Conformational preferences and interconversion barrier of 2,2 -bithienyl, J. Am. Chem. Soc., 96, 1305-1309 (1974). 

E.S.R. Spectra.— The e.s.r. spectra of some thienylphenyl and trithienyl-methyl radicals have been analysed. The complex e.s.r. spectrum that arises on treatment of 2,2 -bithienyl with potassium at — 80 °C is interpreted as due to two rotamers of the radical anion of 2,2 -bithienyl. It is believed that the two aromatic rings are held in a planar or nearly planar conformation, where the sulphur atoms are in a cw- or /I a/w-position, for a sufficient time for each rotamer to give an individual spectrum. 2-Phenyl-thiophen is claimed to behave similarly. The ketyls from thiophen-2-aldehyde and 2-acetylthiophen also give overlapping spectra, assigned to the 0-cis- and O-frawj -isomers present in unequal amounts. Well-resolved e.s.r. spectra of the 2- and 3-thenyl and 5-methyl-2-thenyl radicals, produced by the reaction of t-butoxyl radicals with the methylthiophens, were also obtained, and the spin distributions in the radicals studied were compared... 

Optically active 3,3 -bithienyls have served as excellent models for the study of conformational and configurational effects on the c.d. curves of atropisomeric compounds. 

Bi- and Poly-heterocycles.—Extensive work on the synthesis, optical resolution, and conformation of 3,3 -bithienyls, as well as detailed studies of their c.d. curves, has appeared. " Most 3,3 -bithienyls were prepared by the coupling of 3-thienyl-lithium derivatives followed by the introduction of the appropriate substituents by electrophilic substitution or by the modification of substituents by reduction, - ° oxidation, - - or side-chain bromination. In particular, halogen-metal exchange in bromo-substituted 3,3 -bithienyls was very useful for the modification of substituents. - - Ullman coupling of 3-bromo-2,4-dimethoxycarbonylthiophen was used for the synth is of 2,2, 4,4 -tetra-... 

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