Umpolung reaction novel

For the first time, diverse key intermediates (i.e., Breslow intermediates and derivatives) involved in NHC-catalysed processes have been generated and fully characterized by means of NMR spectroscopy and X-ray diffraction.This work is particularly significant since it opens novel perspectives for NHC-catalysed umpolung reactions. 

Not surprisingly, the hydrolysis of nitriles is the most atom economical reaction. The isohypsic transformations are the acid chloride-Schotten-Baumann sequence, carbodiimide and carbonyl-diimidazole methods, and the hydrolysis of nitriles. The oxidation of nitriles with hydrogen peroxide is hyperhypsic for the reasons mentioned in Section 9.1.1 for the nitrile to carboxylic acid transformation. The umpolung method, which electronically reverses the roles of the reacting partners, is hypohypsic since a net oxidation has occurred at the reactive carbon atom. This is a novel... 

Our initial studies uncovered a novel class of unconventional homoenolate nucleophiles accessible from the addition of an NHC to an unsaturated aldehyde. The umpolung addition of an alcohol across an enal to generate saturated esters has opened new possibilities for the development of useful acylatirai and oxidation reactions [99-106]. 

Radical cations, generated by single electron oxidation of enamines that are generated in situ by the reaction of aldehydes with a chiral imidazolidinone, serve as a new class of electrophiles for silyl enol ethers (Scheme 3-1and allylsilanes. This novel Umpolung strategy allows the introduction of a chiral center at the a-position of aldehydes with these silicon reagents. The chiral imidazolidinone works as an... 

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