Mixed macrocycles based on thiophenes and other unsaturated units

2 Mixed macrocycles based on thiophenes and other unsaturated units Mixed macrocycles based on -substitution in thiophene units 

Haley et al. synthesized thiophene analogues 4.38-4.40 of didehydrotribenzo [18]annulene by Eglinton cyclooligomerization in order to study their optical properties in comparison with the basic [18]annulene system (Chart 1.54). The Cs-symmetric 4.38 involving 2,3-thienylenes connected by butadiyne units can be regarded as the next higher expanded macrocycle in the series benzo[l,2-fc 3,4-b 5, 6-fc ]trithiophene 4.4 and trithienocyclotriyne (TTC) 4.34 [413]. 

Nitschke and Tilley developed a very effective synthesis of zirconacyclopentadiene-coupled macrocycles [414]. One of the examples was reacted with disulfur dichloride to give mixed macrocycle 4.41 in which three 3,4-thienylene units are connected by p-phenylene units and are responsible for cross-conjugation (Chart 1.54). 

Reagents and conditions (i) TiCh/Zn, Pyridine, THF, reflux (ii) H2SO4 (98%) 

The same authors extended the structural variety of thiophene-based macrocycles by synthesizing [24]annulene 4.53 in which two 2,2 -bithiophene units are connected via butatriene groups (Chart 1.56). The macrocycle showed clear paratropicity (antiaromaticity) whereas the dianion, a 26K-electron system which is obtained by alkali reduction, was strongly diatropic (aromatic) [421]. 

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