A haloketone in
organic chemistryis a functional groupconsisting of a ketonegroup or more general a carbonylgroup 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 preferred conformationof a haloketone is that of a cisoidwith the halogen and carbonyl sharing the same plane as the steric hindrancewith 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] ] .
*Haloketones and halo carbonyl compounds in general are synthesized by reaction of
carbonylcompounds with halogenation agents:
halogens, bromine and chlorine give monosubstitution, fluorinegives polysubstitution
Hell-Volhard-Zelinsky halogenationa carboxylic acidreacts with bromine in presence of phosphorus tribromide.
* In the
Nierenstein reactionan acyl chloride reacts with diazomethane
Efforts are reported in
asymmetric synthesisof halocarbonyls through organocatalysis. In one study an acid chlorideis converted into an α-halo-ester with a strong base ( sodium hydride), a bromine donor and an organocatalyst based on prolineand quinine["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 Lectka J. Org. Chem.; 2006; 71(23) pp 8946 - 8949; (Note) DOI|10.1021/jo061522l] :
In the proposed
reaction mechanismthe base first converts the acid chloride to the ketene, the organocatalyst then introduces chirality through its quininoid tertiary amine, forming a ketene adduct.
Haloketones take part in several reaction types. In reaction with a
nucleophile2 electrophilic sites are available and in reactions with a base several acidic protons exist due to the presence of two electron withdrawing groups. The carbon halogen bond experiences increases polarityfrom the inductive effectof the carbonyl group making the carbon atom more electropositive.
nucleophilic aliphatic substitutionreactions with potassium iodidein acetone, 1-chloro-2-propanone reacts faster than n-propylchloride by a factor of 36000.
* In the
Favorskii rearrangementa base abstacts first an acidic α-proton and the resulting carbanionthen displaces the halogen.
* The same sequence is observed in the
Bingel reactionwith fullerenes
* In crossed
Aldol reactions between haloketones and aldehydes the initial reaction product is an halohydrinwhich can subsequently form a oxiranein the presence of base.
* Haloketones are important in
heterocyclic chemistry. An example is the use of haloketones in the Hantzsch Pyrrole Synthesisand the Hantzsch thiazole synthesis.
* Haloketones react with phosphites in the
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