Benzoin (and neither the Breslow intermediate nor the NHC-aldehydetetrahedral adduct) has been unambiguously identified as the oxidised species in aerobic NHC-catalysed aldehyde esterifications.In the previous communication,1 the first examples of broad scope, efficient, N-heterocyclic carbene (NHC)-catalysed aerobic oxidative methyl esterifications of aromatic aldehydes in the absence of alkylating agents, solid stoichiometric oxidants or co-oxidation catalysts were reported. Previously these reactions (when O 2 /air had been used as the oxidant) had been postulated to proceed either through the oxidation of the Breslow intermediate or its immediate precursor. We wished to ascertain the nature of the oxidised intermediates that were involved in these aerobic oxidative esterifications, and began by examining the scope of the process with respect to the alcohol component. If the reaction proceeds through a highly electrophilic acyl triazolium 14 ( Fig. 1), one would not expect to observe significant differences between esterifications using different alcohols (Scheme 1). Experiments were carried out using two sets of conditions (Scheme 1): a 1 : 1 THF-alcohol solvent mixture (condition set A) and use of 3.0 equivalents of alcohol in THF solvent (condition set B). We commenced our study with a series of alcohols which form esters 3 (amenable to different deprotection methodologies). Both benzylic and allylic alcohols were suitable substrates; providing the corresponding products 4 and 5 in good (condition set B) to excellent yields (condition set A) yields. The formation of the trichloroethanol-derived 6 proceeded in ca. 60% yield irrespective of the conditions employed. Interestingly, the synthesis of the corresponding trifluoro-analogue 7 was limited by the volatility of the alcohol, and could only be formed under condition set B. Similar difficulties were encountered with the more hindered hexafluoroisopropanol -resulting in the formation of 8 in low yield. The more hindered and less acidic isopropanol proved resistant to esterification: ester 9 could not be generated.(1)Using precatalyst 2 and DBU, 1 could also be cleanly oxidised to the acid 10 in THF/H 2 O (10 : 1 v/v) in excellent isolated yield (eqn (1) H NMR spectroscopy using styrene (114 mL, 1 mmol, 1 equiv.) as an internal standard; yield of isolated product given in parentheses. Transformation to 7, 8 and 9 performed at rt.

Although ultimately not isolable for X‐ray structural characterization, the free perfluorinated trityl cation was shown to be observable in neat triflic acid, which represents milder conditions than previous reports of this cation in “magic acid” or oleum. A triflate‐bound species could be generated in organic solvents using stoichiometric amounts of triflic acid and was shown to be synthetically viable for hydride abstraction from Et3SiH. It was demonstrated that the para‐position on the ‐C6F5 rings is the primary point of attack for decomposition of the cation.

A highly efficient, broad scope, additive-free mild protocol for the oxidative carbene-catalysed esterification of aldehydes (including the related aqueous oxidation to acids) has been developed.Esters are a general class of molecules which are so commonplace that their undoubted synthetic importance defies quantification. 1 The ester functional group is most commonly accessed via the stoichiometric activation of a carboxylic acid as an acyl halide, anhydride or activated ester with subsequent acyl transfer to an alcohol nucleophile.Recently, interest has grown rapidly in the development of an alternative oxidative esterification of aldehydes with alcohols catalysed by N-heterocyclic carbenes (NHCs, 1, Fig. 1). 2,3 These transformations are purported to involve the addition of the carbene catalyst to an aldehyde 2 to form the Breslow intermediate 3, which is then converted by a stoichiometric oxidant e.g. nitrobenzene, 4 MnO 2 , 5,6 TEMPO, 7 diphenoquinone, 8,9 azobenzene, 10,11 phenazine or riboflavin; 12 (the use of anodic electrochemical oxidation to replace the stoichiometric oxidant has also been very recently demonstrated 13 ) to an electrophilic acyl azolium ion 4. This species can then transfer the acyl group to an alcohol 5 to afford ester 6 and the carbene, which re-enters the catalytic cycle. The potential of this novel strategy as a synthetic tool in natural product synthesis has recently begun to be appreciated. 5b,e In the presence of molecular oxygen in place of the stoichiometric oxidant it has been proposed that a separate oxidative fate for the Breslow intermediate involving addition of O 2 to 3 affords the internal peroxy zwitterion 7 14 which then reacts with either the substrate aldehyde 2 (via peroxyacid intermediates) or another molecule of 3 to generate a carboxylate ion 8; 15 this can be trapped by either a proton or an alkyl halide 10 14-17 to afford either the acid 9 or the ester 11 respectively. 18,19 von Wangelin et al. 2a (Fig. 1) have recently classified these distinct NHC-mediated esterifications as being either 'oxidative' (i.e. the oxidation of 3 to 4 by an added oxidant) or 'oxygenative' (i.e. the conversion of 3 to 7 by O 2 ). 20 While significant progress has been made -a mild, additive-free NHC-mediated protocol of broad scope for the aerobic oxidation of aldehydes has yet to be developed.During an investigation into the use of magnetite nanoparticles as a co-catalyst for the NHC-mediated oxidative esterification of aldehydes, we observed partial esterification of benzaldehyde (12) in methanol under aerobic conditions in the absence of the nanoparticle. After reproducing the result several times, we embarked on a study to determine the factors which influence the efficiency of this catalytic process (Table 1).Preliminary experiments revealed that 12 could be esterified to a conveniently detectable extent in methanolic THF (1 : 1 v/v) in the presence of triazolium precatalysts (15 mol%) and a

A N-heterocyclic carbene catalyst incorporating a bulky yet electron-deficient N-aryl substituent promotes chemoselective and enantioselective intermolecular crossed benzoin condensations.