- Haloketone
A haloketone in
organic chemistry is afunctional group consisting of aketone group or more general acarbonyl group with a α-halogen substituent . The general structure is RR'C(X)C(=O)R where R is an alkyl or aryl residue and X any one of the halogens. The preferredconformation of a haloketone is that of acisoid with the halogen and carbonyl sharing the same plane as thesteric hindrance with the carbonyl alkyl group is generally larger ["The Chemistry of α-Haloketones and Their Utility in Heterocyclic Synthesis" Ayman W. Erian, Sherif M. Sherif and Hatem M. Gaber Molecules 2003, 8, 793-865 [http://www.mdpi.org/molecules/papers/81100793.pdf Online Article] ] .Haloketone synthesis
*Haloketones and halo carbonyl compounds in general are synthesized by reaction of
carbonyl compounds with halogenation agents:
**halogen s, bromine and chlorine give monosubstitution,fluorine gives polysubstitution
**Tetrabutylammonium tribromide
**N-Bromosuccinimide
*In theHell-Volhard-Zelinsky halogenation acarboxylic acid reacts with bromine in presence ofphosphorus tribromide .
* In theNierenstein reaction an acyl chloride reacts with diazomethaneAsymmetric synthesis
Efforts are reported in
asymmetric synthesis of halocarbonyls throughorganocatalysis . In one study anacid chloride is converted into an α-halo-ester with a strong base (sodium hydride ), a bromine donor and an organocatalyst based onproline andquinine ["Scalable Methodology for the Catalytic, Asymmetric -Bromination of Acid Chlorides" Cajetan Dogo-Isonagie, Tefsit Bekele, Stefan France, Jamison Wolfer, Anthony Weatherwax, Andrew E. Taggi, and Thomas LectkaJ. Org. Chem. ; 2006; 71(23) pp 8946 - 8949; (Note) DOI|10.1021/jo061522l] :In the proposed
reaction mechanism the base first converts the acid chloride to theketene , the organocatalyst then introduces chirality through its quininoidtertiary amine , forming a ketene adduct.Haloketone reactions
Haloketones take part in several reaction types. In reaction with a
nucleophile 2 electrophilic sites are available and in reactions with a base several acidic protons exist due to the presence of twoelectron withdrawing group s. The carbon halogen bond experiences increasespolarity from theinductive effect of the carbonyl group making the carbon atom more electropositive.*In
nucleophilic aliphatic substitution reactions withpotassium iodide inacetone , 1-chloro-2-propanone reacts faster than n-propylchloride by a factor of 36000.
* In theFavorskii rearrangement a base abstacts first an acidic α-proton and the resultingcarbanion then displaces the halogen.
* The same sequence is observed in theBingel reaction with fullerenes
* In crossedAldol reaction s between haloketones andaldehyde s the initial reaction product is anhalohydrin which can subsequently form aoxirane in the presence of base.
* Haloketones are important inheterocyclic chemistry . An example is the use of haloketones in theHantzsch Pyrrole Synthesis and theHantzsch thiazole synthesis .
* Haloketones react with phosphites in thePerkow reaction .See also
*
Organic chemistry References
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