Halogenation Of Ketones Research Articles

Copper Cocatalyst Modulated Radical Generation for Selective Heterogeneous Photosynthesis of α-Haloketones.

The α-haloketones are important precursors for synthetic chemistry and pharmaceutical applications; however, their production relies heavily on traditional synthetic methods via halogenation of ketones that are toxic and environmentally risky. Here, we report a heterogeneous photosynthetic strategy of α-haloketone production from aromatic olefins using copper-modified graphitic carbon nitride (Cu-C3N4) under mild reaction conditions. By employing NiX2 (X = Cl, Br) as the halogen source, a series of α-haloketones can be synthesized using atmospheric air as the oxidant under visible-light irradiation. In comparison with pristine carbon nitride, the addition of Cu as a cocatalyst provides a moderate generation rate of halogen radicals and selective reduction of molecular oxygen into •OOH radicals, thus leading to a high selectivity to α-haloketones. The Cu-C3N4 also exhibits high stability and versatility, rendering it a promising candidate for solar-driven synthetic applications.

ChemInform Abstract: Halogenation of Ketones with N‐Halosuccinimides under Solvent‐Free Reaction Conditions.

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Halogenation of ketones with N-halosuccinimides under solvent-free reaction conditions

Several aryl substituted ketones, cyclic ketones, 1,3-diketones and a β-ketoamide were halogenated with N-halosuccinimides under solvent-free reaction conditions (SFRC) at various temperatures (20–80 °C), whereas less enolized ketones required the presence of an acid catalyst ( p-toluenesulfonic acid, PTSA). Bromination of substituted acetophenones obeys first order kinetics v= k Br[ketone] and the following correlation with the keto–enol equilibrium constant: log k Br=0.3p K E+ C 1, less enolized substrates being more reactive; the moderate positive charge developed in the rate determining step was confirmed by the Hammett correlation ( ρ=−0.5). On the other hand, in cyclic ketones an opposite relation was observed: log k Br=−0.6p K E+ C 2, indicating higher reactivity of substrates with higher enolization constant ( K E). The important role of the nature of the solvent (MeCN, MeOH) in preorganization of the ketone–NBS–PTSA mixture prior to SFRC bromination was found.

Nitrosation of ketones: clear evidence for the involvement of the enol tautomers

Nitrosation of acetone and ethyl methyl ketone in the presence of a fairly high concentration of Cl–, Br–, or SCN– is first-order in [ketone] and [H+] and zero-order in [HNO2] and [Cl–], [Br–], and [SCN–]. This demonstrates that reaction occurs by nitrosation of the enol form of the ketone, and that under these conditions, enolisation is rate-limiting. The measured rate constant for enolisation in the nitrosation experiments is in excellent agreement with that obtained earlier for halogenation and for hydrogen-exchange reactions. The same behaviour occurs with 1,3-dichloroacetone, except that here the enolisation is not acid-catalysed–again this agrees with the bromination work. With lower concentrations of added nucleophiles, for all four ketones, the rate equation includes terms first-order in [HNO2] and also in [Cl–] or [Br–], showing that the reaction of the enol with the nitrosating species is now rate-limiting. There is a very close analogy with the kinetic behaviour found in the halogenation of ketones. Analysis of the kinetic data reveals the reactivity sequence NOCl > NOBr > NOSCN (well known in N-nitrosation), and the reactivity of the enols is as expected for electrophilic addition. With acetylacetone, the rate-limiting step is always the reaction of the enol, due in part to the lower reactivity of the enol. In the absence of added nucleophiles, both acetone and ethyl methyl ketone react with N2O3, whereas the less-reactive 1,3-dichloro-acetone and acetylacetone react preferentially with H2NO2+ or NO+—a similar trend occurs in N-nitrosation of amines.

Intramolecular general acid catalysis in the halogenation of ketones

Measurements have been made of the rate of iodination in aqueous solution of 2-oxobicyclo[2.2.2]octane-1-carboxlic acid and its methyl ester. These reactions are of zero order with respect to iodine, the measured velocities representing the rate of ionization or enolization. In self-buffered or acid solutions the observed rate can be represented by v=kH[RCO2H][H+]+k1[RCO2–][H+]+k3[RCO2H][RCO2–]. k1 Is ca. 200 times greater than the corresponding rate constant for the methyl ester, which we attribute either to intramolecular general acid catalysis, or to electrostatic stabilization of the transition state for the normal acid-catalysed mechanism of enolization. k3 Is probably attributable to bifunctional catalysis, since the rate of the iodination of the ionized form of the keto-acid, catalysed by acetic acid, is at least 16 times greater than for the corresponding methyl ester. Kinetic measurements on the halogenation of several other keto-carboxylic acids are also reported.

Examination of the evidence for ‘Hal B I’ and ‘Hal B II’ mechanisms for halogenation of ketones

The evidence for base catalysed ‘Hal B I’ and ‘Hal B II’ mechanisms for halogenation of ketones has been reevaluated. Our results, together with those of Cullis and Hashmi, support the traditional mechanism of halogenation. Alternative interpretations are suggested for the ‘apparent’ evidence for ‘Hal B I’ and ‘Hal B II’ mechanisms. The kinetics of the reactions of 1- and 3-bromobutan-2-one with aqueous hydroxide ion have been determined.

Halogenation of Ketones. VIII. Studies on the Mechanisms of the two Base-Catalyzed Halogenations of 2-Butanone. Indications of Non-Enolic Mechanisms.

Halogenation of Ketones. VIII. Studies on the Mechanisms of the two Base-Catalyzed Halogenations of 2-Butanone. Indications of Non-Enolic Mechanisms.

Halogenation of Ketones. VI. On the Deuteration of Butanone-2 in Aqueous Acetate, and Acetate Buffers.

Halogenation of Ketones. VI. On the Deuteration of Butanone-2 in Aqueous Acetate, and Acetate Buffers.