In our studies on the catalytic activity of Group IVB transition metal Lewis acids, Hf(OTf)4 was identified as a highly potent catalyst for ”one-pot, three-component” Biginelli reaction. More importantly, it was found that solvent-free conditions, in contrast to solvent-based conditions, could dramatically promote the Hf(OTf)4-catalyzed formation of 3,4-dihydro-pyrimidin-2-(1H)-ones. To provide a mechanistic explanation, we closely examined the catalytic effects of Hf(OTf)4 on all three potential reaction pathways in both “sequential bimolecular condensations” and “one-pot, three-component” manners. The experimental results showed that the synergistic effects of solvent-free conditions and Hf(OTf)4 catalysis not only drastically accelerate Biginelli reaction by enhancing the imine route and activating the enamine route but also avoid the formation of Knoevenagel adduct, which may lead to an undesired byproduct. In addition, 1H-MMR tracing of the H-D exchange reaction of methyl acetoacetate in MeOH-d4 indicated that Hf(IV) cation may significantly accelerate ketone-enol tautomerization and activate the β-ketone moiety, thereby contributing to the overall reaction rate.

Iron-catalyzed oxidative amination of benzylic C-(sp 3 )−H bonds with anilines bearing electron-withdrawing groups (EWGs) or electron-donating groups (EDGs) is realized based on simple variations of N-substituents on imidazolium cations in novel ionic Fe(III) complexes. The structural modification of the imidazolium cation resulted in regulation of the redox potential and the catalytic performance of the iron metal center. Using DTBP as oxidant, [HItBu][FeBr 4 ] showed the highest catalytic activity for anilines bearing EWGs, while [HIPym][FeBr 4 ] was more efficient for EDG-substituted anilines. This work provides alternative access to benzylamines with the advantages of both a wide substrate scope and iron catalysis.

Inspired by the recent discovery that aryldiazoacetates could be excited with visible light to generate carbenes in a mild and catalyst‐free manner, we tested the possibility to induce the cyclization of fluorenes from biaryldiazoacetate precursors simply by visible light irradiation at ambient temperature. The experimental results showed that 455 nm blue light is most effective on the synthesis of a diversity of fluorenes and indenoarenes in excellent yields. Analysis of UV absorbance and electronic transition properties of biaryldiazoacetates revealed that their absorbance peaks in the blue light region are due to the n–π* transition of diazo group. Their small ϵ420 nm values well explains the fact that only when exposed to intense blue light could biaryldiazoaetates be excited to generate carbenes. Theoretical calculations and kinetic isotopic effect (KIE) study illustrated that the fluorene cyclization proceeds via the intramolecular carbene C−H insertion pathway rather than the cyclopropanation pathway. In addition, a furan ring‐opening mechanism for the unexpected formation of an indene‐based unsaturated aldehyde was also proposed based on the calculated potential energy surfaces of three plausible reaction pathways.

In the past decade, selenocyclization has been extensively exploited for the preparation of a wide range of selenylated heterocycles with versatile activities. Previously, selenium electrophile-based and FeCl3-promoted methods were employed for the synthesis of selenylated benzoxazines. However, these methods are limited by starting material availability and low atomic economy, respectively. Inspired by the recent catalytic selenocyclization approaches based on distinctive pathways, we rationally constructed an efficient and greener double-redox catalytic system for the access to diverse selenylated benzoxazines. The coupling of I2/I− and Fe3+/Fe2+ catalytic redox cycles enables aerial O2 to act as the driving force to promote the selenocyclization. Control and test redox experiments confirmed the roles of each component in the catalytic system, and a PhSeI-based pathway is proposed for the selenocyclization process.

A P(V)-N activation method based on nucleoside phosphoropiperidate/DCI system has been developed for improved synthesis of diverse UDP-furanoses. The reaction conditions including temperature, amount of activator, and reaction time were optimized to alleviate the degradation of UDP-furanoses to cyclic phosphates. In addition, an efficient and facile phosphoramidite route was employed for the preparation of furanosyl-1-phosphates.