- C-H bond activation by metal oxo species: Chromyl chloride oxidations of cyclooctane, isobutane, and toluene
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Chromyl chloride, CrO2Cl2, oxidizes cyclooctane, isobutane, and toluene under mild conditions (25-60 °C). The reactions give chlorinated products (chlorocyclooctane, tert-butyl chloride, and benzyl chloride) and a dark chromium-containing precipitate. Hydrolysis of the precipitate yields oxygenated products, such as ketones, aldehydes, chloro ketones, epoxides, and alcohols. Kinetic data show that all of the reactions are first order in CrO2Cl2 and first order in substrate, with no sign of an induction period. Primary isotope effects have been observed for t-d1-isobutane and d8-toluene. The kinetic and mechanistic data indicate that the reactions proceed by initial hydrogen atom transfer from the substrate to CrO2Cl2 The rates of hydrogen atom abstraction by CrO2Cl2 vary in the order cyclohexane ? and ΔS?, indicating a common mechanism for the four substrates. The pathways leading from the initially formed alkyl radicals to the observed products are described. The ability of CrO2Cl2 to abstract a hydrogen atom from alkanes is remarkable, as it is a closed-shell, diamagnetic species, not a radical. It is proposed that the hydrogen atom abstracting ability derives from the strong O-H bond formed on hydrogen atom transfer, in [Cl2(O)Cr(OH)]. The rates of the CrO2Cl2 reactions correlate with rates of hydrogen atom abstraction by oxygen radicals, assuming a CrO-H bond strength of 83 kcal/mol (similar to that in HMnO4-). The implications of this perspective for transition metal mediated hydrogen atom transfer reactions are discussed.
- Cook, Gerald K.,Mayer, James M.
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p. 7139 - 7156
(2007/10/02)
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- Methods for the introduction of a Phenylselenium Dichloride Group into he α-position of Carbonyl Compounds. Syntheses of Enones.
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Phenylselenium trichloride, PhSeCl3, direcly introduced, in fair yield, a PhSeCl2 group into the α-position of ketones with loss of HCl.To some extent, depending on the substrate, this reagent was also shown to act as a chlorinating agent toward ketones, yielding α-chloro ketones and α-phenylselenenyl ketones.The latter compounds were readily converted to selenium(IV) dichlorides by SO2Cl2 chlorination to significantly improve the overall yields of the selenetion process.The consecutive treatment of ketones with PhSeCl and SO2Cl2 could also be used for the introduction of a PhSeCl2 group, but this procedure was usually less efficient than the PhSeCl3-based one.Unsymmetrical ketones were selenated with poor regiocontrol.Aldehydes were primarily chlorinated by treatment with PhSeCl3, but consecutive treatment with PhSeCl3 and SO2Cl2 introduced a PhSeCl2 group into the α-position.Carboxylic acids and esters were ureactive toward PhSeCl3 and PhSeCl.PhSeCl3 underwent addition reactions with enones to introduce a PhSeCl2 group α or β to the carbonyl group, depending on the substrate.The carbonyl compounds substituted in the α-position with a PhSeCl2 group were easily converted to the corresponding α,β-unsaturated carbonyl compounds after hydrolysis/selenoxide elimination.Since the selenium(IV) intermediates involved were highly crustalline and easy to purify, the preparation of enones from symmetrical ketones via PhSeCl2 introduction/hydrolytic elimination was especially convenient to perform from the operational point of view.
- Engman Lars
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p. 4031 - 4037
(2007/10/02)
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- Metal-Catalyzed Organic Photoreactions. One-Step Synthesis of Chlorinated Ketones from Olefins by Photooxidation in the Presence of Iron(III) Chloride
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Under photooxidation in pyridine in the presence of FeCl3, mono- and disubstituted olefins gave α-chloro ketones, while tri- and tetrasubstituted olefins gave dichloro ketones with a C-C bond cleavage.The reaction was interpreted in terms of an electron-transfer mechanism occurring within the coordination sphere of the iron ion
- Kohda, Akira,Ueda, Keiko,,Sato, Tadashi
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p. 509 - 515
(2007/10/02)
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