Oxidative benzoin reactions

N-Heterocyclic Carbene/Carboxylic Acid Co-Catalysis Enables Oxidative Esterification of Demanding Aldehydes/Enals, at Low Catalyst Loading

We report the discovery that simple carboxylic acids, such as benzoic acid, boost the activity of N-heterocyclic carbene (NHC) catalysts in the oxidative esterification of aldehydes. A simple and efficient protocol for the transformation of a wide range of sterically hindered α- and β-substituted aliphatic aldehydes/enals, catalyzed by a novel and readily accessible N-Mes-/N-2,4,6-trichlorophenyl 1,2,4-triazolium salt, and benzoic acid as co-catalyst, was developed. A whole series of α/β-substituted aliphatic aldehydes/enals hitherto not amenable to NHC-catalyzed esterification could be reacted at typical catalyst loadings of 0.02-1.0 mol %. For benzaldehyde, even 0.005 mol % of NHC catalyst proved sufficient: the lowest value ever achieved in NHC catalysis. Preliminary studies point to carboxylic acid-induced acceleration of acyl transfer from azolium enolate intermediates as the mechanistic basis of the observed effect.

Vitamin B1-catalyzed aerobic oxidative esterification of aromatic aldehydes with alcohols

A straightforward aerobic oxidative esterification of aryl aldehydes with alcohols has been developed for the synthesis of substituted esters by employing vitamin B1 as a cost-effective, metal-free, and eco-friendly NHC catalyst. Air is used as a green terminal oxidant. The reaction is a useful addition to the existing NHC-catalytic oxidative esterification.

An aerobic oxidative esterification of aryl aldehydes with alcohols has been developed by employing vitamin B1 as a cost-effective, metal-free, and eco-friendly NHC catalyst.

Carbene-catalyzed oxidative acylation promoted by an unprecedented oxidant CCl3CN

An unprecedented example of a NHC-catalyzed acylation reaction is promoted by CCl₃CN as an oxidant.

N-Heterocyclic carbene-catalyzed oxidative [3 + 2] annulation of dioxindoles and enals: cross coupling of homoenolate and enolate

The N-heterocyclic carbene-catalyzed oxidative [3 + 2] annulation of dioxindole and enals was developed, giving the corresponding spirocyclic oxindole-γ-lactones in good yields with high to excellent diastereo- and enantioselectivities. The challenging aliphatic enals worked effectively using this strategy. The oxidative cross coupling of homoenolate and enolate via single electron transfer was proposed as the key step for the reaction.

N-heterocyclic carbene catalysed oxidative esterification of aliphatic aldehydes

Aliphatic aldehydes are readily transformed to the corresponding esters by oxidative carbene catalysis.

NHC-catalysed highly selective aerobic oxidation of nonactivated aldehydes

This publication describes a highly selective oxidation of aldehydes to the corresponding acids or esters. The reaction proceeds under metal-free conditions by using N-heterocyclic carbenes as organocatalysts in combination with environmentally friendly oxygen as the terminal oxidation agent.

N-Heterocyclic Carbenes as Organic Catalysts

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 rubrik of N-heterocyclic carbene (NHC) 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.

Oxidative amidation and azidation of aldehydes by NHC catalysis

N-heterocyclic carbene catalyzed oxidative amidations of various aldehydes to the corresponding hexafluoroisopropylesters by using the readily available organic oxidant A are described. The hexafluoroisopropylesters prepared in situ are shown to be highly useful active esters for amide bond formation. In addition, oxidative azidation of aldehydes is presented. These mild organocatalytic processes do not use any transition metal.

Oxidative formation of thiolesters in a model system of the pyruvate dehydrogenase complex

In the presence of a catalytic amount of 3-butyl-4-methylthiazolium bromide, the reaction of benzaldehydes with azobenzene in dichloromethane containing octanethiol and Et(3)N gave the corresponding S-octyl thiobenzoates in good yields. The thiolesters were produced by trapping of the 2-benzoylthiazolium salts with the thiol, which were generated through the azobenzene oxidation of the active aldehydes. This is the first example for the thiolester formation mimicking the function of the pyruvate dehydrogenase complex. An electron-withdrawing substituent at the 4-position of benzaldehyde enhanced the reaction rate. The effect of benzaldehyde substituents on the reaction rate was examined quantitatively on the basis of kinetic measurements, leading to a nonlinear correlation of log(k(obs)) with Hammett's substituent constants (sigma). The origin of the nonlinear Hammett plot was interpreted in terms of a shift in the rate-determining step of the multistep reaction with change of the electronic nature of substituent. Further support for this assumption was given by the observation that the reaction constant (rho) of the Hammett plot for the azobenzene substituent effect on the oxidation rate of 4-bromobenzaldehyde was much smaller than that of 4-cyanobenzaldehyde.

Nucleophilic Acyl Substitutions of Esters with Protic Nucleophiles Mediated by Amphoteric, Oxotitanium, and Vanadyl Species

[reaction: see text] A diverse array of oxometallic species were examined as catalysts in nucleophilic acyl substitution (NAS) reactions of methyl (or ethyl) esters with protic nucleophiles. Among them, oxotitanium acetylacetonate (TiO(acac)(2)) and vanadyl chloride (VOCl(2)-(THF)(x)()) served as the most efficient and water-tolerant catalysts. Transesterifications of methyl and/or ethyl esters with functionalized (including acid- or base-sensitive) 1 degrees and 2 degrees alcohols can be carried out chemoselectively in refluxed toluene or xylene in a 1:1 substrate stoichiometry using 1 mol % catalyst loading. The resultant products were furnished in 85-100% yields by simple aqueous workup to remove water-soluble catalysts. The new NAS protocol is also amenable to amines and thiols in 74-91% yields, albeit with higher loading (2.5 equiv) of protic nucleophiles. Representative examples of commercial interests such as Padimate O and antioxidant additives for plastics were also examined to demonstrate their practical applications. A 1:1 adduct between TiO(acac)(2) and a given 1-octadecanol was identified as (C(18)H(37)O)(2)Ti(acac)(2) and was responsible for its subsequent NAS of methyl esters.