Enolizable ketones, halogenation

Unfortunately, alkylation of thallium(i) salts of cyclic 1,3-diketones results in predominant O-alkylation. In a reaction of possibly general applicability to highly enolized ketones, triphenyldibromophosphorane effects the conversion of acetylacetone into 4-bromopent-3-en-2-one under mild conditions. The reaction of enolizable ketones with triphenylphosphine and carbon tetrachloride gives rise to variously halogenated and homologated derivatives. ... 

Ketones containing an enolizable hydrogen can be halogenated at the a position (the carbon adjacent to the carbonyl) with bromine, chlorine, NBS, or NCS. The reaction probably proceeds via addition of X2 to the enol form of the carbonyl (secs. 9.2.A, 9.8.A). Elimination of HX from the addition product generates an other enol, which tautomerizes to the a-haloketone. Reaction of cyclohexanone with bromine, for example, would give 2-bromocyclohexanone (174) and a similar reaction with NCS (173) would give 2-chloro-cyclohexanone (172). 

Haloform reactions are generally performed with halogens in the presence of hydroxide [251] or directly with hypohalites [252]. Alternative methods affording carboxylic acids from methyl ketones (or other enolizable substrates) include the aerobic oxidation in the presence of a catalytic amount of dinitrobenzene [253] with a base in a dipolar aprotic solvent such as DMF [254] or HMPT (hexamethylphospho-ric triamide) [255, 256] and the use of stoichiometric quantities of hypervalent iodide derivatives [95, 257] or nitrosylpentacyanoferrate [258]. Furthermore, metal catalysts can be used, and systems such as tert-butyl hydroperoxide in the presence of rhenium oxide [259], oxygen in combination with a copper complex [260], heteropolyacids [261] and Mn"/Co" systems [262] were found to be applicable. Finally, aryl ketones are selectively oxidized to aliphatic carboxylic acids by treatment with periodate [81] in the presence of ruthenium trichloride [263]. 

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