Redox esterification

Various aldehydes 184 and alcohols have been shown to be competent in the redox esterification of unsaturated aldehydes in the presence of the achiral mesityl triazo-lium pre-catalyst 186. Both aromatic and aliphatic enals participate in yields up to 99% (Table 13). Tri-substituted enals work well (entry 3), as do enals with additional olefins present in the substrate (entries 4 and 7). The nucleophile scope includes primary and secondary alcohols as well as phenols and allylic alcohols. Intramolecular esterification may also occur with the formation of a bicyclic lactone (entry 8). 

Bode and co-workers further extended redox esterification to include carbon-carbon bond breaking of formyl-cyclopropanes [113]. Both esters and thioesters are formed in high yield and good enantioselectivities (Scheme 31). The M-mesityl substituted triazolium salt 191 proved to be the most efficient pre-catalyst providing complete suppression of the benzoin reaction. Electron-deficient substituents, such as phenyl ketone, readily provide ester formation. 

Scheme 31 Redox esterification of chiral enantioenriched formylcyclopropanes...

In 2006, Zeitler demonstrated the use of alkynyl aldehydes in redox esterification [114]. As in previous examples, the author proposes the formation of an activated carboxylate that acts as an acylating agent Eq. 19. A variety of a,P-unsaturated carboxylic esters 199 are formed in moderate yields with E-selectivity up to >95 5. 

Zeitler K (2006) Stereoselective synthesis of (E)-a, 3-unsaturated esters via carbene-catalyzed redox esterification. Org Lett 8 637-640... 

Schmidt A, Habeck T, Snovydovych B, Eisfeld W (2007) Addition reaction and redox esterifications of carbonyl compounds by A-heterocyclic carbenes of indazole. Qrg Lett 9 3515-3518... 

Sohn SS, Bode JW (2006) A-Heterocyclic carbene catalyzed C-C bond cleavage in redox esterifications of chiral formylcyclopropanes. Angew Chem Int Ed 45 6021-6024... 

Zeitler K (2006) Stereoselective synthesis of (E)-afi—unsaturated esters via carbene-catalyzed redox esterification. Qrg Lett 8 637-640 Zeitler K, Mager I (2007) An efficient and versatile approach for the immobilization of carbene precursors via copper-catalyzed [3+2]-cycloaddition and their catalytic apphcation. Adv Synth Cat 349 1851-1857 Zhao GL, Cordova A (2007) A one-pot combination of amine and heterocyclic carbene catalysis direct asymmetric synthesis of fi-hydroxy and fS-malonale esters from a,fS-unsaturated aldehydes. Tetrahedron Lett 48 5976-5980 Zhou ZZ, Ji FQ, Cao M, Yang GF (2006) An efficient intramolecular Stetter reaction in room temperature ionic liquids promoted by microwave irradiation. Adv Synth Cat 348 1826-1830... 

Soluble supported catalysts of this class are also accessible. Thus, various thia-zolium, imidazolium and triazolium structures have been recently immobilized on PEG via dick chemistry [365], Thiazolium resins (151) were the most efficient catalysts (10mol.%) for the intramolecular Stetter reaction. Compound 154 (Scheme 10.23) was obtained in 81% with the supported catalyst whilst an 84% yield was obtained when the non-supported analog was employed. Imidazolium systems like 152 were also able to promote the carbene-catalyzed diastereoselective redox esterification. In this case, 155 was obtained with higher yields (77%) and stereoselectivities (18 1 E Z ratio) than when using the non-supported system (63% and 11 1). This can be associated with the positive influence of steric hindrance on the results of this reaction. 

NHCs catalyse a one-pot synthesis of hydroxamic esters, via reaction of nitrosoben-zenes, aldehydes and enals in an aza-benzoin-type process, followed by an internal redox esterification. ... 

Oximes (56) and a, 0-unsaturated aldehydes (57) undergo a redox esterification to oxime esters (58) catalysed by a triazolium salt. A wide variety of oxime and enal types are tolerated. 

Unlike the NHC-catalyzed redox esterifications described above, which were first reported in 2004, chemists have long recognized the potential of azolium... 

Very recently [126] the above described catalytic activity of [C2CiIm][OAc] in the benzoin condensation has been reported to provide a novel route for biomass processing. 5-Hydroxymethylfurfural - that can be produced effectively from plant biomass resources such as glucose or cellulose [127] - can undergo a facile benzoin condensation (see Fig. 12) in [C2Cilm][OAc], catalyzed by the solvent itself under very mild conditions (60-80°C) with excellent yields (up to 98% within 1 h). The product of the reaction, 5,5 -di(hydroxymethyl)furoin has been suggested for use as a possible C12 kerosene/jet fuel [126]. Later, also a redox esterification reactirai has been reported in this ionic liquid [174]. 

Scheme 18.16 Redox esterification of a-halo aldehydes, according to Rovis and coworkers.

Building on the formal redox esterification of ynals discovered by Zeitler [104], the group of Bode showed that the unsaturated acyl azoHum intermediate could be trapped by enols to yield dihydropyranones 128 enantioselectively (Scheme 18.24) [11a]. Mechanistically, the enol 127 could perform a 1,4-addition on the acyl azoHum followed by a lactonization. Instead, the authors postulate a 1,2-addition/ Coates-Claisen rearrangement pathway that is supported by kinetic and computational studies [105]. Another notable finding was a strong counterion dependence on the reaction rate in the absence of added base. The data presented is consistent with the chloride ion in 74 effectively playing the role of base to deprotonate the triazoUum cation. 

Unlike the NHC-catalyzed redox esterifications described above, which were first reported in 2004, chemists have long recognized the potential of azolium salts such as thiamine to promote esterification reactions in the presence of an external oxidant. These reactions are inspired by the biological process whereby an activated aldehyde or Breslow intermediate is oxidized by... 

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