Nitrobenzene aldehyde oxidations catalyzed by the conjugate bases of thiazolium ions

N-Heterocyclic carbene-catalyzed oxidations

“…Several reports exist of thiazolium or benzimidazolium reagents catalyzing the oxidation of aromatic aldehydes to esters. 29–33,35 A representative example is the recent disclosure by Connon and co-workers of an approach utilizing a thiazolium catalyst and azobenzene as an oxidant 55. While this method gives good yields for the oxidation of substituted benzaldehydes (46–97% yield, 8 substrates), application to unactivated substrates results in poor yields of the ester.…”

“…Using a non-enzymatic thiazolium-based N -heterocyclic carbene (NHC), very brief studies concerning the oxidative transformation converting aromatic aldehydes to esters were reported 29–32. More recently, Miyashita33 has demonstrated that other NHCs derived from triazolium and benzimidazolium salts, are capable of catalyzing the oxidation of aryl aldehydes.…”

Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes

“…In 1977, Castells reported the oxidation of aryl aldehydes to the corresponding methyl esters with thiazolium salts and nitrobenzene oxidants. 202 …”

Carbene Catalysts

“…The first redox process of this type using heterocyclic carbenes was reported by Castells and co-workers in 1977 in which aldehydes were oxidized to esters in one-pot in the presence of nitrobenzene [104]. Furfural 169 is converted into methyl 2-furoate 170 in 79% yield (Equation 15).…”