Green Reduction in Water

Abstract

This chapter summarizes the major advances in aqueous‐phase reduction made in the past several years. The coverage includes water‐soluble ligands and catalysts, hydrogenation, and transfer hydrogenation, drawing examples from both achiral and chiral reactions, and a brief discussion of the role of water. Various functional groups are featured, including CC, CO, CN, CO 2 , and arenes. Although homogeneous metal catalysts are the focal point, examples of heterogeneous reduction are also highlighted, as are biomimetic catalysts.

A Versatile Ru Catalyst for the Asymmetric Transfer Hydrogenation of Both Aromatic and Aliphatic Sulfinylimines

“…In the case of the imine 1 i, bearing a carbamate substituent, the tert-butoxycarbonyl (Boc) group was also removed during the desulfinylation step, affording the diamine 2 i (Scheme 1), which was easily isolated as its dihydrochlo-A C H T U N G T R E N N U N G ride ( Table 2, entry 9). Very good results were obtained with chloro-substituted imines 1 j-l, irrespective of the position of the chlorine atom on the aromatic ring ( Table 2, entries [10][11][12]. These results also demonstrate the more general applicability of this catalyst in comparison with our previous system; for the imine derived from ortho-chloroacetophenone 1 j, less than 20 % conversion was observed after stir- Table 1.…”

“…[11] In the last decade, the asymmetric transfer hydrogenation (ATH) protocol has received a great deal more attention from chemists for the reduction of imines. This methodology is very convenient, for the following reasons: 1) it is operationally simple; 2) low loadings of the metal catalysts are normally used; 3) it avoids the handling of hazardous chemicals, such as molecular hydrogen or metallic hydrides; 4) the isolation of the reduction products is facilitated by the fact that volatile reaction byproducts are formed, such as acetone or carbon dioxide; 5) the reactions can be performed in environmentally benign solvents, like water; [12] and 6) the methodology has been demonstrated to be applicable to industrial processes. [13] Although the ATH process has been widely applied to the synthesis of chiral secondary alcohols by the reduction of ketones, [6,14] the number of examples of the stereoselective synthesis of amines by this method is more limited.…”

Extending the Chemistry of Hexamethylenetetramine in Ruthenium-Catalyzed Amine Oxidation

“…Nitrile and imine groups are considered as very important functional groups because they act as key building blocks for the synthesis of several organic compounds such as carboxylic acids, amines, amides, and heterocyclic compounds and several industrially important products such as pharmaceuticals, dyes, pesticides, polymers, etc. , The development of efficient, selective, cost-effective, and environmentally friendly catalyst systems for the synthesis of nitrile and imine compounds has always been one of the biggest pursuits of the scientific community. , Conventional methods available for synthesizing nitrile compounds include the ammoxidation method, the Sandmeyer type reaction, , the Rosenmund–von Braun reaction, dehydration of aldoximes and amides, and metal-catalyzed cyanation . Nitriles are also being synthesized from alcohols, aldehydes, and azides .…”

“…Many metal-based heterogeneous and homogeneous catalyst systems are also known to catalyze the oxidation of secondary amines . Though a plethora of methods are available for the syntheses of both imine and nitrile compounds from a variety of starting materials, most of the methods suffer from at least one of the following drawbacks: (a) use of toxic reagents, (b) poor atom economy, (c) harsh reaction conditions, and (d) poor selectivity. − …”

Synthesis and Catalytic Activity of Water-Soluble Ruthenium(II) Complexes Bearing a Naphthyridine–Carboxylate Ligand

“…The utilization of aqueous medium for a transition-metal-catalyzed reaction is of great interest because water is considered to be the “greenest” solvent . For homogeneous transition-metal-catalyzed reactions in aqueous medium, one of the intrinsic problems is the solubility of the catalysts in water .…”