Umpolung

Umpolung or polarity inversion in organic chemistry is the chemical modification of a functional group with the aim of the reversal of polarity of that group.

Quotes

 * The carbonyl group is electrophilic at the carbon atom and hence is susceptible to attack by nucleophilic reagents. Thus, the carbonyl group reacts as a formyl cation or as an acyl cation. A reversal of the positive polarity of the carbonyl group so it acts as a formyl or acyl anion would be synthetically very attractive. To achieve this, the carbonyl group is converted to a derivative whose carbon atom has the negative polarity. After its reaction with an electrophilic reagent, the carbonyl is regenerated. Reversal of polarity of a carbonyl group has been explored and systematized by Seebach.
 * George S. Zweifel and Michael H. Nantz Modern Organic Synthesis (2006), Ch. 1. Synthetic Design


 * Umpolung in a synthesis usually requires extra steps. Thus, one should strive to take maximum advantage of the functionality already present in a molecule.
 * George S. Zweifel and Michael H. Nantz Modern Organic Synthesis (2006), Ch. 1. Synthetic Design


 * The most utilized Umpolung strategy is based on formyl and acyl anion equivalents derived from 2-lithio- 1,3-dithiane species.
 * George S. Zweifel and Michael H. Nantz Modern Organic Synthesis (2006), Ch. 1. Synthetic Design