2025/08/05 by Matthias Steiner, Johanna M. Uher, Jürgen Lindner +2 · 1 voice
Chemistry · #N-Heterocyclic Carbenes in Organic and Inorganic Chemistry #Synthetic Organic Chemistry Methods #Catalytic Cross-Coupling Reactions
paper · pdf · doi:10.26434/chemrxiv-2025-q70q9
openalex publication_date 2025/08/05 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/15
The combination of azole compounds like 1-methylimidazole and oxiranes (e.g. phenyl glycidyl ether) gives carbenoid reactivity at elevated temperatures. Benzoin condensation was performed with 5 mol% azole and 10 mol% oxirane under air at temperatures of 70 °C and above, achieving conversions of up to 85% of benzaldehyde and yields of up to 64% of benzoin. The lower benzoin yield is due to the formation of oxidative benzoin follow-up products under these reaction conditions. A variety of combinations of azole compounds and oxiranes have been shown to catalyze benzoin condensation. Thus, a modular, potentially inexpensive method of generating carbenoid reactivity has been revealed. The proposed mechanism for catalyst formation involves oxirane opening by, for example, 1-methylimidazole, which forms a zwitterionic methylimidazolium adduct with the oxirane. Then, the acidic proton in the 2-position of the imidazolium core is deprotonated by the zwitterion's alkoxide releasing the corresponding N-heterocyclic carbene. In addition to its primary function, surplus oxirane serves as a scavenger, removing acidic byproducts that are formed from the aldehyde through oxidative N-heterocyclic carbene catalysis. This property enables benzoin condensation without the exclusion of oxygen. The practical utility of this catalytic system was demonstrated by polymerizing simple bifunctional aldehyde/oxirane monomers - namely 4-(2-oxiranylmethoxy)-benzaldehyde, 3-(2-oxiranylmethoxy)-benzaldehyde and vanillin-based 2-methoxy-4-(2-oxiranylmethoxy)-benzaldehyde - using 5 mol % 1-methylimidazole in a solventless manner and without excluding air. The monomers polymerized via the formyl and the oxirane groups, yielded thermosets with glass transition temperatures above 100 °C.