Facile Rearrangement of 3‑Oxoalkyl Radicals
is Evident in Low-Temperature Gas-Phase Oxidation of Ketones
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Abstract
The
pulsed photolytic chlorine-initiated oxidation of methyl-<i>tert</i>-butyl ketone (MTbuK), di-<i>tert</i>-butyl
ketone (DTbuK), and a series of partially deuterated diethyl ketones
(DEK) is studied in the gas phase at 8 Torr and 550–650 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 γ-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–Beckwith ring expansion.
Without this rearrangement, peroxy radicals derived from MTbuK and
DTbuK cannot undergo HO<sub>2</sub> 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 mol<sup>–1</sup>. The MTbuK oxidation
experiments also show several minor channels, including β-scission
of the initial radicals and cyclic ether formation