Halogenation of carbonyl compounds

A similar catalytic procedure for enantioselective formation of C-Br and C-Cl bonds has been reported recently by the Lectka group [83]. The concept of this a-halogenation of carbonyl compounds is tandem asymmetric halogenation and esterification (Scheme 3.28). Inexpensive acyl halides, 74, are used as starting... 

Recent advances in catalytic asymmetric electrophilic a-halogenation of carbonyl compounds are described in two reviews.293,294. 

Organocatalysis has led to the development of new methods for the asymmetric a-halogenation of carbonyl compounds leading to the formation of stereogenic C-X centers. Hence, details of direct enantioselective fluorination, chlorination, and bromination reactions will be presented in the following sections. 

Oestreich M. Strategies for catalytic asymmetric electrophihc a halogenation of carbonyl compounds. Angew. Chem. Int. Ed. 2005 44 2324-2327. 

Halogenation of carbonyl compounds should be carried out in acid solution. Attempts in basic solution lead to multiple substitutions and C-C bond cleavage. 

Diastereoselective halogenations using removable chiral auxiliaries have been the purpose of recent significant developments. a-Halo aldehydes, a-halo ketones and a-halo carboxylic acid derivatives are very useful precursors involved for total syntheses of pharmaceutical drugs and phytochemicals. In most cases, the biological activity is associated with one of the two enantiomers. So, diastereoselective halogenation of carbonyl compounds and carboxylic acid derivatives has attracted considerable attention in recent years, as a tool for the production of enantiomerically pure substances. The numerous examples of diastereoselective halogenation of compounds with non removable chiral adjuvants are outside the scope of this review. 

Synthetic Transformation 25.2 Acid-Catalyzed a-Halogenation of Carbonyl Compounds... 

Ueda M, Kano T, Maruoka K. Organocatalyzed direct asymmetric a-halogenation of carbonyl compounds. Org. Biomol. Chem. 2009 7(10) 2005-2012. 

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