Hydrogen-Bond-Mediated Formation of C-N or C=N Bond during Photocatalytic Reductive Coupling Reaction over CdS Nanosheets.

Reductive amination of carbonyl compounds and nitro compounds represents a straightforward way to attain imines or secondary amines, but it is difficult to control the product selectivity. Herein, we report the selective formation of C-N or C=N bond readily manipulated through a solvent-induced hydrogen bond bridge, facilitating the swift photocatalytic reductive coupling process. The reductive-coupling of nitro compounds with carbonyl compounds using formic acid and sodium formate as the hydrogen donors over CdS nanosheets selectively generates imines with C=N bonds in acetonitrile solvent; while taking methanol as solvent, the C=N bonds are readily hydrogenated to the C-N bonds via hydrogen-bonding activation. Experimental and theoretical study reveals that the building of the hydrogen-bond bridge between the hydroxyl groups in methanol and the N atoms of the C=N motifs in imines facilitates the transfer of hydrogen atoms from CdS surface to the N atoms in imines upon illumination, resulting in the rapid hydrogenation of the C=N bonds to give rise to the secondary amines with C-N bonds. Our method provides a simple way to control product selectivity by altering the solvents in photocatalytic organic transformations.

Biomass-Derived Single Zn Atom Catalysts: The Multiple Roles of Single Zn Atoms in the Oxidative Cleavage of C-N Bonds.

The C-N bond cleavage represents one kind of important organic and biochemical transformation, which has attracted great interest in recent years. The oxidative cleavage of C-N bonds in N,N-dialkylamines into N-alkylamines has been well documented, but it is challenging in the further oxidative cleavage of C-N bonds in N-alkylamines into primary amines due to the thermally unfavorable release of α-position H from N-Cα-H and the paralleling side reactions. Herein, a biomass-derived single Zn atom catalyst (ZnN4-SAC) was discovered to be a robust heterogeneous non-noble catalyst for the oxidative cleavage of C-N bonds in N-alkylamines with O2 molecules. Experimental results and DFT calculation revealed that ZnN4-SAC not only activates O2 to generate superoxide radicals (·O2 -) for the oxidation of N-alkylamines to generate imine intermediates (C=N), but the single Zn atoms also served as the Lewis acid sites to promote the cleavage of C=N bonds in imine intermediates, including the first addition of H2O to generate α-hydroxylamine intermediates and the following C-N bond cleavage via a H atom transfer process.

High Nitrile Yields of Aerobic Ammoxidation of Alcohols Achieved by Generating •O2- and Br• Radicals over Iron-Modified TiO2 Photocatalysts.

Catalytic ammoxidation of alcohols into nitriles is an essential reaction in organic synthesis. While highly desirable, conducting the synthesis at room temperature is challenging, using NH3 as the nitrogen source, O2 as the oxidant, and a catalyst without noble metals. Herein, we report robust photocatalysts consisting of Fe(III)-modified titanium dioxide (Fe/TiO2) for ammoxidation reactions at room temperature utilizing oxygen at atmospheric pressure, NH3 as the nitrogen source, and NH4Br as an additive. To the best of our knowledge, this is the first example of catalytic ammoxidation of alcohols over a photocatalyst using such cheap and benign materials. Various (hetero) aromatic nitriles were synthesized at high yields, and aliphatic alcohols could also be transformed into corresponding nitriles at considerable yields. The modification of TiO2 with Fe(III) facilitates the formation of active •O2- radicals and increases the adsorption of NH3 and amino intermediates on the catalyst, accelerating the ammoxidation to yield nitriles. The additive NH4Br impressively improves the catalytic efficiency via the formation of bromine radicals (Br•) from Br-, which works synergistically with •O2- to capture H• from Cα-H, which is present in benzyl alcohol and the intermediate aldimine (RCH═NH), to generate the active carbon-centered radicals. Further, the generation of Br• from the Br- additive consumes the photogenerated holes and OH• radicals to prevent over-oxidation, significantly improving the selectivity toward nitriles. This amalgamation of function and synergy of the Fe(III)-doped TiO2 and NH4Br reveals new opportunities for developing semiconductor-based photocatalytic systems for fine chemical synthesis.

An enzyme-mimic single Fe-N3 atom catalyst for the oxidative synthesis of nitriles via C─C bond cleavage strategy.

The cleavage and functionalization of recalcitrant carbon─carbon bonds is highly challenging but represents a very powerful tool for value-added transformation of feedstock chemicals. Here, an enzyme-mimic iron single-atom catalyst (SAC) bearing iron (III) nitride (FeN3) motifs was prepared and found to be robust for cleavage and cyanation of carbon-carbon bonds in secondary alcohols and ketones. High nitrile yields are obtained with a wide variety of functional groups. The prepared FeN3-SAC exhibits high enzyme-like activity and is capable of generating a dioxygen-to-superoxide radical at room temperature, while the commonly reported FeN4-SAC bearing FeN4 motifs was inactive. Density functional theory (DFT) calculation reveals that the activation energy of dioxygen activation and the activation energy of the rate-determining step of nitrile formation are lower over FeN3-SAC than FeN4-SAC. In addition, DFT calculation also explains the catalyst's high selectivity for nitriles.