Thiophene, preferred orientation

The trifluoroacetylation, formylation and bromination of 1-tritylpyrrole occurs regioselec-tively at the 3-position in high yield (83JCS(P1)93>. 2-Alkyl substituents orient substitution in thiophene and selenophene into the 5-position. Since the a-directing power of the ring heteroatom is more pronounced in furan than thiophene, preference for 5-substitution is still further enhanced. In pyrrole, although the ratio of a to /3 reactivity is much smaller than in the other five-membered rings, 5-substituted 2-alkylpyrroles still appear to be the major products. 

Structural changes affect seriously an electrophilic substitution orientation in pyrroles owing to their low positional selectivity in reactions with electrophiles. Thus, in contrast to thiophene, selenophene, and, especially, furan analogues, even a relatively weak type 11 substituent in position 2 of the pyrrole ring is capable of overcoming the a-oiienting effect of the heteroatom and directs an electrophile preferably to the position 4 (68JCS(B)392). N-(p-Nitrophenyl)pyrrole-2-carbaldehyde... 

As an example, we discuss a structural study, by the Rietveld [10] method, of neutral poly thiophene [11]. This polymer is, sofar, the most ordered polyheterocycle, even though no oriented films have been obtained up to now, and so the only available samples are thermally annealed powders. Interestingly, aside from the expected (and found) co-planarity between adjacent rings, the detected 3-D order can be interpreted in terms of a discrete number of preferred arrangements, each of them shifted with respect to another along the chain axis, and co-present in the crystallites. This hypothesis accounts for the relative doping ease of polythiophene. 

To investigate the conformational energetic consequences of each of the possible regioisomers that can occur in PATs, the four oligomeric triads have been modeled in the gas phase by molecular mechanics and ab initio methods [57,58]. For the HT-HT example, both methods indicate that the thiophene rings prefer a trans coplanar orientation. Structures with the rings twisted... 

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