This Focus Review describes different methods that have been reported for the synthesis of imines. It is organized according to the methods used for imine synthesis starting with metal catalysis, including Ru, Au, V, Cu, Mn, Co and Pd catalysis. Other methods, such as photocatalysis, electrocatalysis, organocatalysis, and so on, are also emphasized. Ample information on the condensation of carbonyl compounds/alcohols with amines and direct oxidation of amines to give imines is discussed. Furthermore, among various metal‐catalyzed reactions, specific attention has been paid to copper‐catalyzed imine synthesis, as copper is less toxic than other heavy metals, comparatively inexpensive, and is easily accessible.

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Abstract:The copper (I)-catalyzed direct synthesis of imines from amines under mild aerobic conditions is described. The method is applicable for the synthesis of various imines from corresponding amines such as benzylic, aliphatic, cyclic secondary, heteroaromatic species and the oxidative condensation of benzylamines with anilines extends the scope of the CuCl catalytic system. Noteworthy, solvent-free procedure, air as a benign oxidant, and the cheap and easy availability of the catalyst are the vital advantages of the method.

Removal of color and reduction of chemical oxygen demand (COD) in an industrial azo dye effluent containing chiefly reactive dyes were investigated under single-pass conditions at a dimensionally stable anode (DSA) in a thin electrochemical flow reactor at different current densities, flow rates, and dilutions. With 50% diluted effluent, decolorization was achieved up to 85-99% at 10-40 mA/ cm2 at 5 mL/min flow rate and 50-88% at 30-40 mA/ cm2 at high (10-15 mL/min) flow rates. The COD reduction was maximum (81%) at 39.9 mA/cm2 or above when solution-electrode contact time (Ct) was as high as 21.7 s/cm2 and decreased as Ct declined at a given current density. Cyclic voltammetric studies suggesting an indirect oxidation of dye molecules over the anode surface were carried out at a glassy carbon electrode. The effect of pH on decolorization and COD reduction was determined. An electrochemical mechanism mediated by OCl- operating in the decolorization and COD reduction processes was suggested. The effluent was further treated with NaOCI. The oxidized products from the treated effluents were isolated and confirmed to be free from chlorine-substituted products by IR spectroscopy. From the apparent pseudo-first-order rate data, the second-order rate coefficients were evaluated to be 2.9 M(-1) s(-1) at 5 mL/ min, 76.2 M(-1) s(-1) at 10 mL/min, and 156.1 M(-1) s(-1) at 15 mL/ min for color removal, and 1.19 M(-1) s(-1) at 5 mL/min, 1.79 M(-1) s(-1) at 10 mL/min, and 3.57 M(-1) s(-1) at 15 mL/min for COD reduction. Field studies were also carried out with a pilot-scale cell at the source of effluent generation of different plants corresponding to the industry. Decolorization was achieved to about 94-99% with azo dye effluents at 0.7-1.0 L/min flow costing around Indian Rupees 0.02-0.04 per liter, and to about 54-75% in other related effluents at 0.3-1.0 L/min flow under single-pass conditions.

The efficient Cu(I)-catalyzed oxidative homocoupling of terminal alkynes in the presence of a base using an amine as a ligand and oxygen as an oxidant yields the symmetrical 1,3-diynes with yields of up to 99%. The outcome of the couplings critically depends on the proper choice of base and ligand as well as reaction conditions. Best results were observed with 2.0 mol % CuCl, 1.5 mol % TMEDA or DBEDA, and DBU or DABCO in acetonitrile.