N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools
CHAPTER 14: N-Heterocyclic Carbenes as Organic Catalysts
Published:04 Nov 2016
Special Collection: 2016 ebook collectionSeries: Catalysis Series
P. Chiang, S. Díez-González, and J. W. Bode, in N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools, ed. S. Diez-Gonzalez, The Royal Society of Chemistry, 2nd edn, 2016, pp. 534-566.
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The synthetic utility of azolium salts as catalysts was thought to be limited to the generation of acyl anion equivalents for use in benzoin and Stetter reactions. The discovery, in 2004, of new catalysts, substrates, and reaction manifolds has ignited a new generation of reactions that fall under the general rubric of N-heterocyclic carbene-catalyzed reactions. These powerful new processes include the catalytic generation of activated carboxylates for α-functionalized aldehydes, enantioselective annulations via catalytically generated ester enolate equivalents, and the NHC-catalyzed generation of formal homoenolate equivalents. The history of these new reactions and an overview of the reactions, their substrate scope, and mechanistic pathways are summarized in this chapter.