This report describes a density functional theory investigation into the reactivities of a series of aza‐1,3‐dipoles with ethylene at the BP86/TZ2P level. A benchmark study was carried out using QMflows, a newly developed program for automated workflows of quantum chemical calculations. In total, 24 1,3‐dipolar cycloaddition (1,3‐DCA) reactions were benchmarked using the highly accurate G3B3 method as a reference. We screened a number of exchange and correlation functionals, including PBE, OLYP, BP86, BLYP, both with and without explicit dispersion corrections, to assess their accuracies and to determine which of these computationally efficient functionals performed the best for calculating the energetics for cycloaddition reactions. The BP86/TZ2P method produced the smallest errors for the activation and reaction enthalpies. Then, to understand the factors controlling the reactivity in these reactions, seven archetypal aza‐1,3‐dipolar cycloadditions were investigated using the activation strain model and energy decomposition analysis. Our investigations highlight the fact that differences in activation barrier for these 1,3‐DCA reactions do not arise from differences in strain energy of the dipole, as previously proposed. Instead, relative reactivities originate from differences in interaction energy. Analysis of the 1,3‐dipole–dipolarophile interactions reveals the reactivity trends primarily result from differences in the extent of the primary orbital interactions.
A judiciously oriented external electric field (OEEF) can catalyze a wide range of reactions and can even induce endo/exo stereoselectivity of cycloaddition reactions. The Diels–Alder reaction between cyclopentadiene and maleic anhydride is studied by using quantitative activation strain and Kohn–Sham molecular orbital theory to pinpoint the origin of these catalytic and stereoselective effects. Our quantitative model reveals that an OEEF along the reaction axis induces an enhanced electrostatic and orbital interaction between the reactants, which in turn lowers the reaction barrier. The stronger electrostatic interaction originates from an increased electron density difference between the reactants at the reactive center, and the enhanced orbital interaction arises from the promoted normal electron demand donor–acceptor interaction driven by the OEEF. An OEEF perpendicular to the plane of the reaction axis solely stabilizes the exo pathway of this reaction, whereas the endo pathway remains unaltered and efficiently steers the endo/exo stereoselectivity. The influence of the OEEF on the inverse electron demand Diels–Alder reaction is also investigated; unexpectedly, it inhibits the reaction, as the electric field now suppresses the critical inverse electron demand donor–acceptor interaction.
We have quantum chemically explored the Diels–Alder reactivities of a systematic series of hetero‐1,3‐butadienes with ethylene by using density functional theory at the BP86/TZ2P level. Activation strain analyses provided physical insight into the factors controlling the relative cycloaddition reactivity of aza‐ and oxa‐1,3‐butadienes. We find that dienes with a terminal heteroatom, such as 2‐propen‐1‐imine (NCCC) or acrolein (OCCC), are less reactive than the archetypal 1,3‐butadiene (CCCC), primarily owing to weaker orbital interactions between the more electronegative heteroatoms with ethylene. Thus, the addition of a second heteroatom at the other terminal position (NCCN and OCCO) further reduces the reactivity. However, the introduction of a nitrogen atom in the backbone (CNCC) leads to enhanced reactivity, owing to less Pauli repulsion resulting from polarization of the diene HOMO in CNCC towards the nitrogen atom and away from the terminal carbon atom. The Diels–Alder reactions of ethenyl‐diazene (NNCC) and 1,3‐diaza‐butadiene (NCNC), which contain heteroatoms at both the terminal and backbone positions, are much more reactive due to less activation strain compared to CCCC.
An effective one-pot process of synthesizing 2,5-diformylfuran (DFF) directly from fructose was accomplished over carbon sphere (CS)-supported molybdenum oxides (MoO x /CS) catalysts. The MoO x /CS catalyst was first prepared by a glucose hydrothermal carbonization method and subsequently annealed under different atmospheres. The annealing treatment under air at 275 °C afforded abundant mesopores over the CS and exposed more active sites. Kinetics studies suggested that 5-hydroxymethylfurfural (HMF) was the key intermediate; acid and oxide sites were active toward the dehydration of fructose and aerobic oxidation of HMF to DFF, respectively. A relatively faster dehydration rate compared to oxidation rate was critical for the dehydration of fructose to HMF instead of decomposing fructose under oxidative conditions. Under the optimized reaction conditions, nearly 78% yield of DFF at 100% conversion of fructose was obtained in dimethyl sulfoxide under atmospheric pressure of oxygen at 160 °C within 2 h.
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