Esterifications redox

A broad spectrum of chemical reactions can be catalyzed by enzymes Hydrolysis, esterification, isomerization, addition and elimination, alkylation and dealkylation, halogenation and dehalogenation, and oxidation and reduction. The last reactions are catalyzed by redox enzymes, which are classified as oxidoreductases and divided into four categories according to the oxidant they utilize and the reactions they catalyze 1) dehydrogenases (reductases), 2) oxidases, 3) oxygenases (mono- and dioxygenases), and 4) peroxidases. The latter enzymes have received extensive attention in the last years as bio catalysts for synthetic applications. Peroxidases catalyze the oxidation of aromatic compounds, oxidation of heteroatom compounds, epoxidation, and the enantio-selective reduction of racemic hydroperoxides. In this article, a short overview... 

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. 

Although the hypothesis of Egumi may be an oversimplification, it is certainly true that Fe /Feu is widely used in redox systems. Zn " in hydrolysis, esterification, and similar reactions, and molybdenum in nitrogenase. xanthine oxkkise. nitnite reductase, etc Putting abundance aside, discuss the specific chemical properties of these metals this make them well suited for their tasks. 

Exercise 15-28 In the conversion of 2-propanol to 2-propanone with chromic acid, which is the redox step, esterification or elimination What is the change in oxidation level of carbon in this reaction ... 

Recently Uniqema has introduced commercially a Surfmer under the trade name of Maxemul 5011. Maxemul is produced by esterification of an unsaturated fatty anhydride with a methoxy PEG such that the reactive group is close to the hydrophilic moiety [ 34 ]. Stable latexes with a solid content of 52% were produced in the seeded emulsion polymerization of film-forming methyl methacrylate/butyl acrylate/acrylic acid (3% Surfmer on monomers, constant monomer feeding rate over 4 h, potassium persulfate/sodium metabisulfate redox initiator). The latexes were stable to electrolytes but not to freeze-thaw. 

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. 

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