Reactivity and Chemical Transformations

Heteroatoms in five-membered 7i-excessive heteroaromatic compounds are responsible for the chemical behaviour of the molecules as a whole. The heteroatoms not only give electrons to form an aromatic n-electron system but also determine the direction of the attack of electrophilic or nucleophilic agents. In fused 7i-excessive heterocycles containing two or more heteroatoms, the reactivity of compounds and their physical properties are substantially affected by both the mutual arrangement of the heteroatoms and the electronic effects associated with their nature. 

This series was confirmed experimentally. For example, the following series of reactivities of different positions in these compounds was obtained in studies of acetylation, formylation and chlorination of thiophene (140), thieno[2,3-fe]thiophene (142) and thieno[3,2-Z ]thiophene (145) C(2) (145) C(2) (142) C(2) (140) C(3) (142) C(3) (145) C(3) (140) (72CS137). In formylation, position 4 in thieno[3,4-f ]thiophene (148) was found (70RTC77) to be more readily subjected to the attack of an electrophilic species compared to position 6. 

The comparative reactivities in electrophilic substitution of selenolo[3,2-Z ]thio-phene (143) and -selenophene (144) with respect to thieno[3,2-Z ]thiophene (142) were studied by the method of competitive reactions (80CS206). The reactivity was found to change in the following order 144 143 142. This series agrees well with a higher substitution rate in selenophene compared to thiophene in acetylation, formylation and chlorination reactions. 

Competitive metallation of two-component systems, viz., thieno[3,2-Z ]thiophene (142) and selenolo[3,2-Z ]thiophene (143), selenolothiophene 143 and thiophene (140), selenolothiophene 143 and selenophene (141), thiophene (140) and selenophene (141), was investigated (74IZV1575). The reactions were carried out with a deficient amount of Bu Li in anhydrous diethyl ether at 25 + 1 °C followed by hydrolysis with heavy water. Mass-spectrometric analysis of the products demonstrated that the relative reactivities of the compounds increase in the following order 143 142 141 140 (74IZV1575). 

The reactivities of thieno[2,3-Z ]thiophene (142) and selenolo[2,3-Z ]thiophene (143) were also compared using competitive acetylation of their mixture with thiophene (140) or selenophene (141) by acetyl chloride in the presence of tin chloride (CHCI3, 25 °C) (76ZOR1574). The selenophene rings were found to be more reactive than the thiophene ring. 

---

In the latter case we have a system of alternating charge densities what presses to take into account long-range electrostatic interactions caused by charges at dots of the crystalline lattice of support as well as by those of the atoms limiting its surface. Such a counting of electrostatic effects is not commonly used for description of the reactivity and chemical transformations of admolecules within subsurface layer that affects the correctness of the results obtained. 

---