- Deciphering Reactivity and Selectivity Patterns in Aliphatic C-H Bond Oxygenation of Cyclopentane and Cyclohexane Derivatives
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A kinetic, product, and computational study on the reactions of the cumyloxyl radical with monosubstituted cyclopentanes and cyclohexanes has been carried out. HAT rates, site-selectivities for C-H bond oxidation, and DFT computations provide quantitative information and theoretical models to explain the observed patterns. Cyclopentanes functionalize predominantly at C-1, and tertiary C-H bond activation barriers decrease on going from methyl- and tert-butylcyclopentane to phenylcyclopentane, in line with the computed C-H BDEs. With cyclohexanes, the relative importance of HAT from C-1 decreases on going from methyl- and phenylcyclohexane to ethyl-, isopropyl-, and tert-butylcyclohexane. Deactivation is also observed at C-2 with site-selectivity that progressively shifts to C-3 and C-4 with increasing substituent steric bulk. The site-selectivities observed in the corresponding oxidations promoted by ethyl(trifluoromethyl)dioxirane support this mechanistic picture. Comparison of these results with those obtained previously for C-H bond azidation and functionalizations promoted by the PINO radical of phenyl and tert-butylcyclohexane, together with new calculations, provides a mechanistic framework for understanding C-H bond functionalization of cycloalkanes. The nature of the HAT reagent, C-H bond strengths, and torsional effects are important determinants of site-selectivity, with the latter effects that play a major role in the reactions of oxygen-centered HAT reagents with monosubstituted cyclohexanes.
- Martin, Teo,Galeotti, Marco,Salamone, Michela,Liu, Fengjiao,Yu, Yanmin,Duan, Meng,Houk,Bietti, Massimo
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- Some studies on the solvolysis of 1-chloro-1-alkyl cycloalkanes
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The effect of the bulk of the sidechain on the rate of solvolysis of 1-alkyl cyclopentyl, cyclohexyl, and cycloheptyl chlorides has been studied.With the exception of the t-butyl systems, the ratio of solvolysis rates for the three ring systems falls in a given series.The slower rate of solvolysis in the six-membered ring system may be due to an extra activation energy contribution caused by the conversion of a neutral chair form to the twist boat or half chair conformation, prior to the actual solvolysis.In the case of five- and seven-membered ring systems the formation of an intermediate carbonium ion is sterically favoured (I-strain or eclipsing interaction) consistent with earlier findings.The faster rate of solvolysis of 1-t-butylcycloalkyl chlorides is likely due to a rearrangement reaction where alkyl participation enhances the rate of solvolysis.
- Ranganayakulu, K.,Vasumathi Devi, M.,Balaji Rao, R.,Rajeswari, K.
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p. 1484 - 1489
(2007/10/02)
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