Abstract:N-Heterocyclic carbenes catalyze the oxidation of allylic and benzylic alcohols as well as saturated aldehydes to esters with manganese(IV) oxide in excellent yields. A variety of esters can be synthesized, including protected carboxylates. The oxidation proceeds under mild conditions, with low loadings of a simple triazolium salt pre-catalyst in the presence of base. Substrates containing potentially epimerizable centers are oxidized while preserving stereochemical integrity. The acyl triazolium intermediate … Show more
“…Recently, K. A. Scheidt and co-workers have reported that, in the carbene-catalyzed redox reaction of aldehydes and related processes, a transient benzylic alcohol intemediate should be accessible through addition of N-heterocyclic carbene to any aldehyde. [21] …”
The benzoin condensation of aromatic and heteroaromatic aldehydes, catalyzed by electrochemically generated N-heterocyclic carbenes, has been set up in the absence of organic solvents and bases. a-Hydroxy ketones have been isolated in good to elevated yields, in short reaction times. Aldol products and carbene-aldehyde adducts have been obtained in elevated yields from linear and short branched aldehydes, respectively. A comparison with the use of classical organic solvents has been reporte
“…Recently, K. A. Scheidt and co-workers have reported that, in the carbene-catalyzed redox reaction of aldehydes and related processes, a transient benzylic alcohol intemediate should be accessible through addition of N-heterocyclic carbene to any aldehyde. [21] …”
The benzoin condensation of aromatic and heteroaromatic aldehydes, catalyzed by electrochemically generated N-heterocyclic carbenes, has been set up in the absence of organic solvents and bases. a-Hydroxy ketones have been isolated in good to elevated yields, in short reaction times. Aldol products and carbene-aldehyde adducts have been obtained in elevated yields from linear and short branched aldehydes, respectively. A comparison with the use of classical organic solvents has been reporte
“…This mechanism is consistent with numerous reports of NHC-catalyzed oxidative esterification of aldehydes with oxidants including azobenzene, 15 diphenoquinone, 16 and MnO 2 . 17 To test if our observations followed this proposal, we conducted the reaction under air in the presence of MeOH; to our surprise only the acid, rather than the expected ester was detected (entry 11). In contrast, the use of trans -azobenzene as the oxidant afforded cinnamic acid in the presence of water (entry 12) and methyl cinnamate in the presence of MeOH (entry 13).…”
mentioning
confidence: 90%
“…These observations are consistent with a number of reports of NHC-catalyzed oxidations of aldehydes with air or added stoichiometric oxidants. 7,8,15–17 …”
NHC-catalyzed oxidations using carbon dioxide as the stoichiometric oxidant have been carefully investigated. These studies support a secondary role of CO2 in suppressing side reactions and exogenous oxygen as the actual oxidant.
A highly efficient synthetic strategy toward Michael addition of indoles to α,β-unsaturated esters has been developed using Lewis acid InBr 3 as catalyst. The reactions generated 3-substituted indoles in high yields with excellent regio-selectivity in the presence of catalytic amount of InBr 3 under mild reaction conditions. The method is simple, efficient and practical.
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