Reactivities of Methylenetriangulanes and Spirocyclopropanated Bicyclopropylidenes toward Bromine. Relative Stabilities of Spirocyclopropanated versus Methyl-Substituted Bromonium Ions
| Autor: | de Meijere A, Fiebig T, Apeloig Y, Ruasse Mf, Sergei I. Kozhushkov, Galland B, Thomas Späth, Xavier P |
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| Rok vydání: | 2002 |
| Předmět: | |
| Zdroj: | The Journal of Organic Chemistry. 67:4100-4114 |
| ISSN: | 1520-6904 0022-3263 |
| DOI: | 10.1021/jo0162686 |
| Popis: | The bromine additions to methylenecyclopropane (1), bicyclopropylidene (2), and spirocyclopropanated methylenecyclopropanes and bicyclopropylidenes 3-6 in methanol at 25 degrees C proceed essentially with the same rate as those to the corresponding oligomethyl-substituted ethylenes. An increasing number of spiroannelated three-membered rings enhances the rate of bromination and stabilizes the intermediate cyclopropyl bromonium cations against ring opening in the course of bromine addition. Calculations at the B3LYP/6-311G(d,p) level show that unsymmetrical bromonium ions are the intermediates, and that they are stabilized by the spiroannelation with cyclopropane rings. The bromonium ion derived from 1 is less stable by 6.3 kcal mol-1 than that from isobutene. One or two spirocyclopropane rings as in 3 and 4 stabilize the corresponding bromonium ion by 9.6 and 16.4 kcal mol-1, respectively, while one or two alpha-cyclopropyl substituents as in ethenylcyclopropane (7) and 1,1-dicyclopropylethene (8) stabilize the corresponding bromonium ions by 13 and 29 kcal mol-1, respectively. The experimental bromination rates of all the studied alkenes correlate reasonably well (r2 = 0.93) with calculated relative energies of the corresponding bromonium ions. The correlation is even better within the series of methylenecyclopropanes 1, 3, and 4 (r2 = 0.974) and bicyclopropylidenes 2, 5, and 6 (r2 = 0.999). The experimental bromination rates also correlate fairly well with the first ionization energies of the corresponding alkenes 1-12 (with r2 = 0.963) and 13-19 (with r2 = 0.991). The calculated preferred nucleophilic attack of a water molecule at both the C1' and C1 atoms of representative bromonium ions conforms well to the experimentally observed product distribution. |
| Databáze: | OpenAIRE |
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