Paper
Nitrosation of 2-aryl-1-bromo-1-fluorocyclopropanes with nitrosonium tetrafluoroborate as an approach to the synthesis of 5-aryl-4-bromo-4-fluoroisoxazolines
Published Jul 1, 2016 · O. B. Bondarenko, A. I. Komarov, N. Zyk
Russian Chemical Bulletin
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Abstract
Nitrosation of cyclopropanes is a direct pathway to the construction of five membered [NO] containing hetero cycles such as isoxazoles and isoxazolines. Successful pro ceeding of this electrophilic process is largely determined by a possibility of the formation of stable carbocationic intermediates. Thus, the nitrosation of arylated cyclopro panes proceeds, as a rule, through the formation of benzyl carbocation independent of the nature of nitrosating agent and leads to 5 arylisoxazolines.1—3 In the case of 2 aryl 1,1 dichloro and 1,1 dibromocyclopropanes, dihalome thyl cation turns out to be the most stabilized. The subse quent heterocyclization and spontaneous aromatization under the reaction conditions result in the formation of 3 arylisoxazoles with the halogen atom at position 5 of the heterocycle.4—7 Nitrosation of fluorine containing 1,1 dihalocyclopropanes is of special interest: the presence of only one fluorine atom in compound can crucially change both chemical and biological properties of the compound. In the present work, we study the trans formation of 2 aryl 1 bromo 1 fluorocyclopropanes upon the action of nitrosonium tetrafluoroborate. The reaction of 2 aryl 1 bromo 1 fluorocyclopro panes 1a—c with nitrosonium tetrafluoroborate was car ried out at room temperature in the presence of a 10% excess of nitrosating agent, using nitromethane as a sol vent. The nitrosation of the small ring and subsequent heterocyclization resulted in the formation of isoxazolines 2a—c (Scheme 1).** As it is seen from the structure of obtained isoxazo lines, in our case, in contrast to the known examples of nitrosation of arylated gem dichloro and dibromocyclo propanes, the nitrosation heterocyclization reaction pro ceeded through the formation of a benzyl carbocation, that is apparently explained by high electronegativity of the fluorine atom.
Nitrosation of 2-aryl-1-bromo-1-fluorocyclopropanes with nitrosonium tetrafluoroborate leads to the formation of isoxazolines, with the fluorine atom playing a crucial role in the reaction.
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