Substrate and Positional Selectivity in Electrophilic Substitution Reactions of Pyrrole, Furan, Thiophene, and Selenophene Derivatives |
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Authors: | L I Belen'kii I A Suslov N D Chuvylkin |
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Institution: | (1) Russian Academy of Sciences, N. D. Zelinsky Institute of Organic Chemistry, Moscow, 119991, Russia;(2) Russian Academy of Sciences, Higher Chemical College, Moscow, 125820, Russia |
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Abstract: | The relative reactivity (substrate selectivity) of five-membered heterocycles on electrophilic substitution (pyrrole >> furan > selenophene > thiophene) and their positional selectivity (furan > selenophene thiophene > pyrrole) are not consistent. The indicated differences in positional selectivity ( : ratios) of the parent heterocycles show up essentially in orientation on electrophilic substitution reactions of their derivatives and the corresponding benzannelated systems. It was shown that the positional selectivity was reduced in a sequence corresponding to the change in relative stability of the onium states of the elements (O+ < Se+ S+ < N+) and reflects the predominant role of the heteroatoms in stabilizing the -complexes formed on -substitution, in which the positive charge is distributed between the heteroatom and one -carbon atom (in -isom ers four atoms of the ring participate in delocalization of the charge). This interpretation was confirmed by quantum-chemical calculations carried out by the RHF/6-31G(d), MP2/6-31G(d)//RHF(6)-31G(d), and B3LYP/6-31G(d) ab initio methods. |
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Keywords: | benzo[b]selenophene benzo[b]thiophene benzo[b]furan indole pyrrole selenophene thiophene furan cationic -complexes" target="_blank">gif" alt="sgr" align="BASELINE" BORDER="0">-complexes quantum-chemical calculations MP2/6-31G(d) and B3LYP/6-31G(d) methods relative stability of the onium states of chalcogens positional selectivity substrate selectivity electrophilic substitution |
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