2,2 -Bithienyl, conformations studied

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... 

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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