Beta-irradiation of biomass in the presence of 1-alkyl-3-methyl-imidazolium ionic liquids: model compound reactions to clarify the reaction at the reducing end of aldoses and cellulose

Abstract Biomass pretreatments often involve swelling in imidazolium-based ionic liquids (ILs) or beta-irradiation. The combination of both pretreatments can be used intentionally or can also occur inadvertently—for instance, upon irradiation of insufficiently washed biomass that still contains IL residues from an earlier pretreatment step. Previous work has shown a high reactivity of the reducing ends of cellulose upon irradiation in the presence of 1-ethyl-3-methyl-imidazolium (EMIm) ionic liquids which was now studied in detail by means of carbohydrate model compounds (monosaccharides, cellobiose, and cellodextrins). The model reactions show a selective loss of C-1 at the unprotected reducing end. This way, D-glucose is converted into D-arabinose, while D-galactose, the C-4-epimer of glucose, gives D-lyxose, the C-3 epimer of arabinose. The yield of the pentoses from the corresponding hexoses is proportional to the irradiation dose and reaches approx. 15% at an irradiation dose of 800 kGy. Also in cellobiose and cellodextrins, the terminal, reducing glucopyranose unit was converted into the lower arabino-homologue. In all of these processes, the removed C 1 -unit reacts as an umpoled formaldehyde synthon, confirmed by trapping in Stetter-type reactions with nitroethylene or cyclohexen-3-one. The mechanism of the fragmentation reaction is explained by the irradiative formation of the IL-derived N -heterocyclic carbene (NHC) which, as a strong nucleophile, reacts with the reducing ends of carbohydrates. The initially formed, well-known Breslow intermediate undergoes a retro-benzoin cleavage to give the lower homologue and the umpoled HCHO equivalent. By transfer of this IL-bound C 1 -synthon to suitable acceptors (trapping agents), the NHC can be regenerated so that a reaction cycle is completed and its action becomes catalytic. The structure elucidation of the products of the model compound reactions is presented, along with a discussion of the formation mechanisms. Graphical abstract