The selective oxo-functionalization of hydrocarbons under mild conditions with molecular oxygen as the terminal oxidant continues to be a hot topic in organic synthesis and industrial chemistry. Though many oxidation protocols in combination with transition metal salts, enzymes, organometallic catalysts, or organocatalysts have been summarized recently, a review that focuses solely on the metal-free allylic/benzylic oxidation strategies with molecular oxygen is still unavailable. This critical review will summarize recent significant advances achieved in this important field under the scope of green chemistry, which covers the promising applications and brief mechanistic profiles involving three kinds of efficient catalysts, namely N-hydroxyimides, homogeneous/heterogeneous light-sensitive molecules, and heteroatom-doped carbon materials, and concerns the sustainability of these methods, as well as predicts the potential utilization of available but unreported analogous catalysts or catalytic systems in this field. Special emphasis will also be placed on the burgeoning metal-free strategies with visible light irradiation from the long-term greenness and sustainability of these oxidation processes due to their established appealing performances under ambient conditions.

Quaternary ammonium-based polymeric ionic liquids (PILs) are novel CO₂ sorbents as they have high capacity, high stability and high binding energy. Moreover, the binding energy of ionic pairs to CO₂ is tunable by changing the hydration state so that the sorbent can be regenerated through humidity adjustment. In this study, theoretical calculations were conducted to reveal the mechanism of the humidity swing CO₂ adsorption, based on model compounds of quaternary ammonium cation and carbonate anions. The electrostatic potential map demonstrates the anion, rather than the cation, is chemically preferential for CO₂ adsorption. Further, the proton transfer process from water to carbonate at the sorbent interface is successfully depicted with an intermediate which has a higher energy state. By determining the CO₂ adsorption energy and activation energy at different hydration states, it is discovered that water could promote CO₂ adsorption by reducing the energy barrier of proton transfer. The adsorption/desorption equilibrium would shift to desorption by adding water, which constitutes the theoretical basis for humidity swing. By analyzing the hydrogen bonding and structure of the water molecules, it is interesting to find that the CO₂ adsorption weakens the hydrophilicity of the sorbent and results in release of water. The requirement of latent heat for the phase change of water could significantly reduce the heat of adsorption. The special “self-cooling” effect during gas adsorption can lower the temperature of the sorbent and benefit the adsorption isotherms.

In an effort to examine the interaction between dioxygen and iron-macrocyclic complexes, and to understand how this interaction was affected by those different macrocyclic ligands, dioxygen binding with iron-porphyrin, iron-phthalocyanine, iron-dibenzotetraaza[14]annulene, and iron-salen complexes is investigated by means of quantum chemical calculations utilizing Density Functional Theory (DFT). Based on the analysis of factors influencing the corresponding dioxygen binding process, it showed that different macrocyclic ligands possess different O–O bond distances, and different electronic configurations for the bound O₂ and non-aromatic macrocyclic ligands favor dioxygen activation. Furthermore, the smaller the energy gap between the HOMO of iron–macrocyclic complexes and the LUMO of dioxygen, the more active the bound O₂ becomes, with a longer O–O bond distance and a shorter Fe–O bond length.