Preparation of Sustainable Polar Aprotic Solvents from Biomass: One-Pot Two-Step Catalytic Reaction of Cellulose with N,N‑Dimethylurea over Ru/C

Inspired by innovative advancements in biomass valorization and the need for more environmentally benign polar aprotic solvents (PASs), a green and catalytic process is described for the synthesis of N,N′-dimethylimidazolidinone (DMI) and 1,3,4-trimethylimidazolidin-2-one (TMI) from cellulose, the most abundant and non-edible component of biomass. The notable properties of DMI and TMI include high boiling points, remarkable chemical stability, and being more eco-friendly than frequently used PASs such as dimethylformamide. These properties make DMI and TMI appealing solvents for the pharmaceutical industry. Cellulose depolymerization and reaction of intermediate products with N,N-dimethylurea (DMU) to produce PASs have been investigated in a one-pot, two-step process at elevated temperatures. Ruthenium supported on activated carbon (Ru/C) is an effective multifunctional catalyst for both C–C bond cleavage in cellulose and subsequent hydrogenation of the unsaturated products. The catalyst also promotes the condensation of hydroxy ketone intermediates with DMU to create cyclic PASs. The tandem reactions are challenging because of the varying conditions required in each step. An overall 85% selectivity for PASs was achieved starting from cellulose or sugar using DMI (the product) as a solvent. The optimized conditions for coupling of 1,2-propylene glycol (1,2-PG) with DMU were employed in a mechanistic study to produce PASs with both homogeneous and heterogeneous Ru catalysts. Catalytic oxidation of 1,2-PG to hydroxyacetone is the key step to produce TMI, and this step is promoted by electron-donating phosphine ligands on Ru.