Reversal of the Carbonyl Group Polarity Umpolung

Umpolung in a synthesis usually requires extra steps. Thus, one should strive to take maximum advantage of the functionality already present in a molecule. 

Since formyl and acyl anions are not accessible, one has to use synthetic equivalents of these anions. Several reagents are synthetically equivalent to formyl or acyl anions, permitting the Umpolung of carbonyl reactivity. 

Formyl and Acyl Anions Derived from 1,3-Dithianes  

The most utilized Umpolung strategy is based on formyl and acyl anion equivalents derived from 2-lithio-l,3-dithiane species. These are readily generated from 1,3-dithianes (thioacetals) because the hydrogens at C(2) are relatively acidic (p f 31). In this connection it should be noted that thiols (EtSH, pi 11) are stronger acids compared to alcohols (EtOH, 16). Also, the lower ionization potential and the greater polarizability of the valence electrons of sulfur compared to oxygen make the divalent sulfur compounds more nucleophilic in Sj,2 reactions. The polarizability factor may also be responsible for the stabilization of carbanions a to sulfur.  

The anions derived from dithianes react with alkyl halides to give the corresponding alkylated dithianes. Their treatment with HgCl2-HgO regenerates aldehydes or ketones, respectively, as depicted below. 

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