One-Step Synthesis of N-Heterocyclic Compounds from Carbohydrates over Tungsten-Based Catalysts

A one-step reaction system has been developed to convert glucose and other renewable carbohydrates into important N-heterocyclic compounds in ammonia solution. Under optimal conditions, 2-methyl pyrazine (MP) and 4(5)-methyl imidazole (MI) were produced within 15 min with combined yields of around 40%. While the formation of 4(5)-MI was identified as a noncatalytic process, the yield of 2-MP was highly influenced by the presence of catalysts. In particular, nearly 3-fold yield enhancement of 2-MP was achieved over several tungsten-based catalysts. Control experiments, isotope-labeling tests, ESI-MS, and NMR analysis revealed that the formation of 2-MP follows a fragmentation mechanism, while small tungsten clusters such as [HW2O7](-) and [W4O13](2-) were the catalytically active species facilitating both glucose fragmentation and the subsequent cyclization reaction to generate pyrazine rings. The work exemplifies the possibility of sustainable production of important N-containing, heterocyclic chemicals from woody biomass, where the identification and understanding of novel catalysts are the key.