Catalytic Action of Azolium Salts. VIII. Oxidative Aroylation with Arenecarbaldehydes Catalyzed by 1,3-Dimethylbenzimidazolium Iodide.

Refluxing of a mixture of benzaldehyde (1a), 1, 3-dimethylbenzimidazolium iodide (7), p-nitroaniline (9b) as an oxidizing agent, and 1, 8-diazabicyclo[5, 4, 0]-7-undecene (DBU) in MeOH afforded methyl benzoate (2a) in good yield. Other methyl arenecarboxylates 2 were similarly obtained from arenecarbaldehydes 1. We showed that this aroylation proceeds via the 2-aroyl-1, 3-dimethylbenzimidazolium salt (8). The 1, 2, 4-triazolium salt (18) and the naphtho[1, 2-d]imidazolium salt (19) were also effective catalysts for this oxidative aroylation. However, the aroylation did not proceed with the imidazolium salt (20). In the presence of flavins (25a-c), arenecarbaldehydes 1 reacted in MeOH under aerobic conditions catalyzed by the benzimidazolium salt 7 to give the corresponding methyl arenecarboxylates 2. 1-Methyl-3-[3-(10-phenylisoalloxazin-3-yl)propyl]benzimidazolium bromide (27) is an effective complex catalyst for this oxidative aroylation.