Harnessing Carbenoid Reactivity From Imidazoles and Oxiranes

The combination of azole compounds, such as 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 (NHC). In addition to its primary function, surplus oxirane serves as a scavenger, removing acidic byproducts that are formed from the aldehyde through oxidative NHC 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 both the formyl and the oxirane groups, yielded thermosets with glass transition temperatures above 100°C.