N-Heterocyclic carbene-catalyzed oxidations

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 generated under catalytic conditions can be employed as a chiral acylating agent in the desymmetrization of meso-diols.

Study on the Reactivity of Aldehydes in Electrolyzed Ionic Liquids: Benzoin Condensation – Volatile Organic Compounds (VOCs) vs. Room Temperature Ionic Liquids (RTILs)

“…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] …”

On the Role of CO2 in NHC-Catalyzed Oxidation of Aldehydes

“…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).…”

“…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 …”

Highly Efficient InBr3 Catalyzed Michael Addition of Indoles to α,β‐Unsaturated Esters

“…16 173.7, 136.9, 127.4, 126.2, 121.5, 118.6, 113.3, 109.1, 51.4, 34.9, 32.4, 20.4. Methyl 3-(1H-indol-3-yl)propanoate 17 1, 122.0, 121.4, 119.3, 118.6, 111.1, 51.1, 34.7, 20.6.…”