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

Abstract

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.

Chiral N-Heterocyclic Carbene-Catalyzed Annulations of Enals and Ynals with Stable Enols: A Highly Enantioselective Coates–Claisen Rearrangement

“…78 Kojic acid derivative 20 proved again to be a good substrate for the coupling with both aliphatic and aromatic enals (regardless of the substitution) in this oxidative method. However, we noted the decrease in percent conversion in the absence of base.…”

Cooperative N‐Heterocyclic Carbene (NHC) and Ruthenium Redox Catalysis: Oxidative Esterification of Aldehydes with Air as the Terminal Oxidant

“…As mentioned above, the major side product identified in these oxidative esterifications was the acid, [19][20][21] Notably, with aliphatic aldehydes this route was the major reaction pathway. It seems that whereas Breslow intermediates derived from aromatic aldehydes are efficiently oxidized by the Ru catalyst, the aliphatic congeners are not readily oxidized with the metal catalyst and reactions mainly occur via O 2 oxidation to eventually provide the corresponding acids.…”

“…[17] To the best of our knowledge, only one paper describing an NHC-catalyzed esterification between aldehydes and aliphatic alcohols with air as the terminal oxidant showing very limited substrate scope was published. [19,20,21] However, these oxidations do not necessarily need to proceed via acylazolium ions. Importantly, NHC-catalyzed oxidation of aldehydes to the corresponding acids with air as the terminal oxidant is documented.…”

NHC-catalysed highly selective aerobic oxidation of nonactivated aldehydes

“…An interesting conversion of aromatic aldehydes to acids with 15 mol % of an NHC-salt, DBU and CO 2 has been reported by Nair et al [24]. However Bode et al recently concluded that oxygen is the oxidant in these reactions [25]. An oxidative carboxylation of activated arylaldehydes with 10% water in DMF by 5 mol % of a sulfoxylalkyl-substituted NHC has been described by Yoshida et al [26].…”