A-Carbon of carbonyl compounds

The removal of functional groups from the a-carbon of carbonyl compounds is an important transformation in organic synthesis. Anionic tellurium reagents offer additional useful methods to attain this. 

Leaving groups on the a-carbon of carbonyl compounds (e.g., Br and OR) are reduced away by one-electron reducing agents. a-Bromocarbonyl compounds are reduced to the corresponding enolates by Zn in the Reformatsky reaction (Chapter 2). After the initial electron transfer, several pathways are possible, but all lead to the enolate. The enolate is usually allowed to react immediately with an electrophile such as another carbonyl compound. Before the advent of strong, non-nucleophilic bases, the Reformatsky reaction was the only way to prepare enolates of simple carbonyl compounds quantitatively. 

Two representative organocatalytic reaction systems can be considered for nucleophilic a-substitution of carbonyl compounds, the issue of this chapter. One involves the in situ formation of a chiral enamine through covalent bond between organo-catalyst (mainly a chiral secondary amine such as proline) and substrate (mainly an aldehyde), followed by asymmetric formation of new bond between the a-carbon of carbonyl compound and electrophile. Detachment of organocatalyst provides optically active a-substituted carbonyl compound, and the free organocatalyst then participates in another catalytic cycle (Figure 6.1a) [2]. 

Reactions at the a Carbon of Carbonyl Compounds Enois and Enoiates 821... 

CHAPTER 18 REACTIONS AT THE a CARBON OF CARBONYL COMPOUNDS Ends and Enolates... 

An umpolung reaction at the a-carbon of carbonyl compounds has been reported, in which nucleophilic o -arylation and a-alkylation of ketones is effected by polarity inversion of V-alkoxyenamines, without isolation of enamines or imines being necessary. The mild conditions for a-arylation do not require transition-metal species. ... 

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