Facile Rearrangement of 3-Oxoalkyl Radicals is Evident in Low-Temperature Gas-Phase Oxidation of Ketones

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• 作者：Adam M. Scheer ; Oliver Welz ; Darryl Y. Sasaki ; David L. Osborn ; Craig A. Taatjes
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：September 25, 2013
• 年：2013
• 卷：135
• 期：38
• 页码：14256-14265
• 全文大小：656K
• 年卷期：v.135,no.38(September 25, 2013)
• ISSN：1520-5126

文摘

The pulsed photolytic chlorine-initiated oxidation of methyl-tert-butyl ketone (MTbuK), di-tert-butyl ketone (DTbuK), and a series of partially deuterated diethyl ketones (DEK) is studied in the gas phase at 8 Torr and 550鈥?50 K. Products are monitored as a function of reaction time, mass, and photoionization energy using multiplexed photoionization mass spectrometry with tunable synchrotron ionizing radiation. The results establish that the primary 3-oxoalkyl radicals of those ketones, formed by abstraction of a hydrogen atom from the carbon atom in 纬-position relative to the carbonyl oxygen, undergo a rapid rearrangement resulting in an effective 1,2-acyl group migration, similar to that in a Dowd鈥揃eckwith ring expansion. Without this rearrangement, peroxy radicals derived from MTbuK and DTbuK cannot undergo HO₂ elimination to yield a closed-shell unsaturated hydrocarbon coproduct. However, not only are these coproducts observed, but they represent the dominant oxidation channels of these ketones under the conditions of this study. For MTbuK and DTbuK, the rearrangement yields a more stable tertiary radical, which provides the thermodynamic driving force for this reaction. Even in the absence of such a driving force in the oxidation of partially deuterated DEK, the 1,2-acyl group migration is observed. Quantum chemical (CBS-QB3) calculations show the barrier for gas-phase rearrangement to be on the order of 10 kcal molp>鈥?p>. The MTbuK oxidation experiments also show several minor channels, including 尾-scission of the initial radicals and cyclic ether formation.