Enolizable a-haloketones

Transformation of enolizable a-haloketones to esters, carboxylic acids, or amides via alkoxide-, hydroxide-, or amine-catalyzed rearrangements, respectively. 

To summarize, when H s are present on the a-carbon opposite the leaving group, the electrocyclic mechanism usually operates when they are not, the semi-benzilic mechanism operates. Why does the electrocyclic mechanism proceed more quickly than the more reasonable semibenzilic mechanism for enolizable a-haloketones Deprotonation and electrocyclic ring closing are both very rapid reactions— the latter even when a strained ring is formed—and they must simply be faster than HO addition and migration, despite what our chemical intuition tells us. 

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). 

This reaction was first reported by Hantzsch and Weber in 1887. It is the formation of thiazole derivatives by means of condensation of a-haloketones (or aldehydes) and thioamides. Therefore, it is generally known as the Hantzsch thiazole synthesis. In addition, other names, including the Hantzsch synthesis, Hantzsch reaction, and Hantzsch thiazole reaction are also used from time to time. Besides thioamides, other thio-ketone derivatives such as thiourea, dithiocarbamates, and ketone thiosemicarbazone can also condense with a-halo ketones (or aldehydes) to form thiazoles. This reaction occurs because of the strong nucleophilicity of the sulfur atom in thioamides or thioureas, and normally gives excellent yields for simple thiazoles but low yields for some substituted thiazoles, as of dehalogenation. This reaction has been proven to be a multistep reaction, and the intermediates have been isolated at low temperatures, in which the dehydration of cyclic intermediates seems to be the slow step. It is found that a variety of reaction conditions might result in the racemized thiazoles that contain an enolizable proton at their chiral center, and it is the intermediate not the final product that is involved in the racemization. Therefore, some modifications have been made to reduce or even eliminate the epimeriza-tion upon thiazole formation. In addition, this reaction has been modified using a-tosyloxy ketones to replace a-haloketones. ... 

Silyl enol esters result from silyl-substituted ally alcohols (using RLi derivatives), from a-haloketones, enolizable aldehydes or ketones (using Nal/Mes-SiCl), from dialdehydes and (MesSi)2NH, and from anions of thioesters e.g.y Bu C=C(OSiMe3)SPh]. /3-Seleno- and /S-thiosilyl enol ethers are formed from the enone on reaction with RSH-MeaSiCl-CgHsN. On storing for long periods, silyl enol ethers condense to symmetric ketones, so should be redistilled before use. ... 

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