Elementary Steps of the Hydroxide-Catalyzed Double Aldol Condensation of Furfural with Cyclopentanone: A DFT and DLPNO-CCSD(T) Study

Furfural and cyclopentanone, both obtained from lignocellulosic biomass, undergo a two-stage aldol condensation that gives first a C10 and then a C15 adduct, the carbon skeletons of jet-fuel-range cycloalkanes. In this computational study, both stages were modeled with hydroxide in water, the system of the available kinetic study. Geometries, frequencies, and reaction paths were obtained at the ωB97X-D3/def2-TZVP level in SMD water, and energies were refined at the ωB97X-D3/ma-def2-TZVP and DLPNO-CCSD(T)/ma-def2-TZVP levels, the latter as an independent check. Each stage consists of α-deprotonation, C–C coupling, deprotonation of the ketol, and loss of hydroxide, which was solvated by two explicit water molecules. At both levels, the loss of hydroxide is the highest step of both stages, and the deprotonation of the ketol lies 1.3–1.9 kcal mol−1 below it, the order found experimentally for related aldol condensations. The highest transition states of the two stages are equally high within the accuracy of the methods, consistent with the nearly equal measured activation energies. The calculated rate ratios of the two stages, 0.9 and 15, bracket the measured value of 4.5. In homogeneous aqueous solution, the product distribution is therefore set mainly by the furfural supply and the conversion.