Aldehydes derived from 1,2,4-triazolium salts

However, even though this reaction proceeds in a very efficient way when an aromatic aldehyde is employed as the Michael donor, the use of aliphatic aldehydes is much more problematic due to the intrinsic instability of enoliz-able aldehydes in the basic media required in the reactions catalyzed by A-heterocyclic carbenes. In fact, pre-catalyst 119a performed poorly in this case, but this limitation was overcome with the use of bicyclic triazolium salt 120a derived from phenylalanine as catalyst precursor. This new A-heterocyclic... 

N-Heterocyclic carbenes (NHCs) have been well known for their unique reactivity to cleave the acyl C—H bond of aldehydes. In 2012, DiRocco and Rovis reported elegant asymmetric a-acylation reactions of tertiary amines 17 with various aldehydes 16 by employing a combination of an NHC catalyst and photoredox catalyst. Since the Breslow intermediate or NHC itself is sensitive to oxidative conditions, a judicious choice of oxidant and photoredox catalyst is crucial. After careful evaluation of the reaction conditions, Ru(bpy)3Cl2 was proved to be the optimal photoredox catalyst and m-dinitrobenzene (m-DNB) as the co-oxidant under irradiation with blue light. The aminoinda-nol-derived NHC C2 (generated in situ from the corresponding triazolium salt using NaH) exhibited the best asymmetric induction. An array of the desired products 18 was obtained in up to 94% yield and 92% ee (Scheme 2.7). 

Based on the elegant work from the group of Studer on redox activation of a,p-unsaturated aldehydes/ the You group realized enantioselective NHC-catalyzed Michael addition reactions to a,p-unsaturated aldehydes by redox oxidation. With 10 mol% of camphor-derived triazolium salt, developed by the authors group, the reactions of various dicarbonyl compounds... 

In 2013, the Chi group realized an NHC-catalyzed asymmetric p-functional-ization reaction of aldehydes via the transformation of saturated aldehydes to formal Michael acceptors via double oxidation. By using the catalyst derived from the chiral amino indanol triazolium salt in combination with quinone as the oxidant, the p-aryl substituted saturated aldehydes were converted to the o,p-unsaturated acyl azolium intermediates which further reacted with 1,3-dicarbonyl compounds or p-keto esters to generate the corresponding 5-lactones. It was found the use of LiCl and 4 A MS as additives was beneficial to improve the ee s of the products. Notably, the p-alkyl substituted saturated aldehydes were not viable substrates, probably due to the reduced acidity of the p-C—H bonds (Scheme 7.118). 

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