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Substrate and Positional Selectivity in Electrophilic Substitution Reactions of Pyrrole, Furan, Thiophene, and Selenophene Derivatives
Authors:L I Belen'kii  I A Suslov  N D Chuvylkin
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
Abstract:The relative reactivity (substrate selectivity) of five-membered heterocycles on electrophilic substitution (pyrrole >> furan > selenophene > thiophene) and their positional selectivity (furan > selenophene ge thiophene > pyrrole) are not consistent. The indicated differences in positional selectivity (agr : beta 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+ le S+ < N+) and reflects the predominant role of the heteroatoms in stabilizing the sgr-complexes formed on beta-substitution, in which the positive charge is distributed between the heteroatom and one agr-carbon atom (in agr -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.
Keywords:benzo[b]selenophene  benzo[b]thiophene  benzo[b]furan  indole  pyrrole  selenophene  thiophene  furan  cationic sgr-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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