Carbanions addition to carbonyl groups

The effect of substituents in the benzene ring provides an interesting example of the dual nature of carbanion addition to carbonyl groups. When an electron-releasing group is conjugated with the aldehyde function, a resonance form such as (XIV) apparently lowers the usual positive charge on the carbonyl carbon atom to a point where the addition... 

Nucleophilic additions to carbonyl groups lead to alcohols which on dehydration, furnish alkenes70,71. This two-step protocol has been extremely useful for diene and polyene synthesis with wide variation in the carbonyl substrate and the nucleophilic addendum. Diene synthesis using aldol-type condensation as well as phenyl sulphonyl carbanion (the Julia reaction) are also discussed in this section. 

The reactions to be discussed in this section involve carbanion addition to carbonyl centers with a potential leaving group. The tetrahedral intermediate formed in the addition step then reacts by expulsion of the leaving group. The overall transformation results in the... 

Enders, D., Breuer, K. Addition of acyl carbanion equivalents to carbonyl groups and enones. Comprehensive Asymmetric Catalysis l-lll 1999,3, 1093-1102. 

Addition of Acyl Carbanion Equivalents to Carbonyl Groups and Enones... 

The most general methods for the syntheses of 1,2-difunctional molecules are based on the oxidation of carbon-carbon multiple bonds (p. 117) and the opening of oxiranes by hetero atoms (p. 123fl.). There exist, however, also a few useful reactions in which an a - and a d -synthon or two r -synthons are combined. The classical polar reaction is the addition of cyanide anion to carbonyl groups, which leads to a-hydroxynitriles (cyanohydrins). It is used, for example, in Strecker s synthesis of amino acids and in the homologization of monosaccharides. The ff-hydroxy group of a nitrile can be easily substituted by various nucleophiles, the nitrile can be solvolyzed or reduced. Therefore a large variety of terminal difunctional molecules with one additional carbon atom can be made. Equally versatile are a-methylsulfinyl ketones (H.G. Hauthal, 1971 T. Durst, 1979 O. DeLucchi, 1991), which are available from acid chlorides or esters and the dimsyl anion. Carbanions of these compounds can also be used for the synthesis of 1,4-dicarbonyl compounds (p. 65f.). 

The addition reaction of enolates and enols with carbonyl compounds is of broad scope and of great synthetic importance. Essentially all of the stabilized carbanions mentioned in Section 1.1 are capable of adding to carbonyl groups, in what is known as the generalized aldol reaction. Enolates of aldehydes, ketones, esters, and amides, the carbanions of nitriles and nitro compounds, as well as phosphoms- and sulfur-stabilized carbanions and ylides undergo this reaction. In the next section we emphasize the fundamental regiochemical and stereochemical aspects of the reactions of ketones and aldehydes. 

A number of reactions of the general type shown in Equation 5.26 lead to carbanions by loss of a carbon group. These processes occur when R " is a stabilized carbanion they can also be considered as the reverse of nucleophilic additions to carbonyl (Chapter 8). 

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