Zirconium-Based Catalysts in Organic Synthesis

Zirconium is a silvery-white malleable and ductile metal at room temperature with a crustal abundance of 162 ppm. Its compounds, showing Lewis acidic behavior and high catalytic performance, have been recognized as a relatively cheap, low-toxicity, stable, green, and efficient catalysts for various important organic transformations. Commercially available inorganic zirconium chloride was widely applied as a catalyst to accelerate amination, Michael addition, and oxidation reactions. Well-designed zirconocene perfluorosulfonates can be applied in allylation, acylation, esterification, etc. N-Chelating oganozirconium complexes accelerate polymerization, hydroaminoalkylation, and CO₂ fixation efficiently. In this review, the applications of both commercially available and synthesized zirconium catalysts in organic reactions in the last 5 years are highlighted. Firstly, the properties and application of zirconium and its compounds are simply introduced. After presenting the superiority of zirconium compounds, their applications as catalysts to accelerate organic transformations are classified and presented in detail. On the basis of different kinds of zirconium catalysts, organic reactions accelerated by inorganic zirconium catalysts, zirconium catalysts bearing Cp, and organozirconium catalysts without Cp are summarized, and the plausible reaction mechanisms are presented if available.

Rongalite-Mediated Transition Metal- and Hydride-Free Chemoselective Reduction of α-Keto Esters and α-Keto Amides

A transition metal- and hydride-free protocol has been developed for the chemoselective reduction of α-keto esters and α-keto amides using rongalite as a reducing agent. Here, rongalite acts as a hydride-free reducing agent via a radical mechanism. This protocol offers the synthesis of a wide range of α-hydroxy esters and α-hydroxy amides with 85-98% yields. This chemoselective method is compatible with other reducible functionalities such as halides, alkenes, amides, and nitriles. The use of inexpensive rongalite (ca. $0.03/1 g), mild reaction conditions, and gram-scale synthesis are some of the key features of this methodology. Also, cyclandelate, a vasodilator drug, has been synthesized in gram scale with 79% yield.

Ru–g-C3N4 as a highly active heterogeneous catalyst for transfer hydrogenation of α-keto amide into β-aminol or α-hydroxyl amide

This work reports a sustainable route for the catalytic transfer hydrogenation (CTH) of α-keto amide into β-aminol via an efficient heterogeneous catalyst wherein ruthenium is incorporated on an active graphite sheet of a carbon nitride support (Ru–g-C₃N₄).

DMF/NaOH/H2O: a metal-free system for efficient and chemoselective reduction of α-ketoamides

A metal-free method for efficient and chemoselective reduction of α-ketoamides using DMF/NaOH/H₂O system has been developed.