Current Green Chemistry - Volume 2, Issue 3, 2015

Three-component one-pot Kabachnik-Fields condensation of carbonyl compounds with amines and phosphites is perhaps the most exploited reaction for the preparation of structurally diverse α-aminophosphonates. Most of the described protocols might be considered to some extent as green ones with solvent-free and microwave-assisted ones being the most effective.

Novel tendencies in the induction of the Michaelis-Arbuzov reaction become apparent when studying the current literature on the preparation of organophosphorus compounds. These alternatives involve catalysis with either metal salts and complexes or the application of metallic and non-metallic Lewis acids, microwave irradiation and performing the reaction in ionic liquids. The outcome of these developments is broadening the structural diversification of potential substrates and products, as well as the economic aspects by saving time and costs. In this review, we illustrate these achievements with representative examples and mechanistic discussions.

The goal of this review is to describe recent advances in metal catalyzed reactions and organocatalysis leading to α-monofluoro-, α,α-gem-difluoro- or trifluoromethyl phosphonate derivatives with a particular emphasis on introducing chiral centers in these compounds.

In this minireview, the possible utilization of the microwave (MW) technique in solid–liquid (S–L) phase C-alkylations is summarized. In the S–L phase C-alkylation of a series of simple active methylene containing compounds, such as diethyl malonate, ethyl acetoacetate and ethyl cyanoacetate, as well as P=O-containing derivatives, such as tetraethyl methylenebisphosphonate, diethyl cyanomethylphosphonate and diethyl ethoxycarbonylmethylphosphonate, in most of the cases, there is no need to use a phase transfer catalyst, as MW promotes the reaction in itself. Most of the Calkylations was performed under solvent-free conditions. The elimination of the catalyst and solvent from the system means an environmentally friendly (“green”) accomplishment.

This mini review presents recent applications of polymer-supported catalyst in different green synthetic approaches, and is focused on the use of these types of catalysts in phase transfer catalysis processes. Phase transfer catalysis is now a well established method in organic synthesis in both industry and academia, being recognized as a versatile method because of its simple experimental operations, mild reaction conditions, inexpensive and environmentally benign reagents and solvents. The discussions involved only some limited aspects related to the use of polymer-supported catalysts in phase transfer catalysis especially concerning ammonium and phoshonium catalysts and cinchona grafted on organic supports. The illustration of efficiency and reusability but also highlights the scope and limitations associated with the supported catalytic systems from a green chemistry view are presented.

The last decade has seen a tremendous outburst in modifying chemical processes to make them ‘green’ or at least acceptable so as to attain sustainability of our environment. People are keen to know new inventions in the field of chemistry but at the same time, they are also eager to know how far the newly developed processes are benign to us! Hence, alternative benign organic methodologies have become an imperative part of organic syntheses and chemical reactions. Role of catalysts in organic synthesis is obvious and thus they find huge applications and uses. Efforts have been made to reduce toxicity level of catalysts by multidirectional modifications, but the most fruitful way is to design an organic reaction without catalyst(s), if feasible! If this important achievement can be coupled with energy consideration, one of the most alarming issues in the 21^st Century, then it would be the great as far green chemistry practice and sustainability are concerned. The present review summarizes the latest developments on catalyst-free organic reactions reported during the last decade with a focus on such reactions occurring in aqueous and non-aqueous media under room temperature condition. Various named reactions, one-pot multi-component reactions, condensations, oxidations, ring opening of epoxides and the synthesis of various pharmaceutically important heterocyclic compounds are discussed among others.

Novel nano-structured Ni(Pd)/SiO2 catalytic systems doped with carbon nanotubes (CNT) were prepared by the sol-gel method. The catalysts proved to be effective for transformation of nitroarenes into anilines by transfer hydrogenation method with the use of hydrazine hydrate as hydrogen donor. Elaborated catalysts provide remarkable advantages such as low metal loading, avoidance of toxic ligands, high yields and recyclability. No significant loss of activity was observed on fivefold use of the catalysts.

A chiral 1,2-amino alcohol derivative, (S)-(+)-2-(N-benzylamino)butan-1-ol (BAB), was synthesized from its Schiff base by catalytic hydrogenation over palladium on carbon, in various solvents (toluene, methanol, hexane, dichloromethane, tetrahydrofuran, ethyl acetate), under mild conditions (room temperature, atmospheric pressure). Preparation of the Schiff base was also optimized. This compound is a resolving agent for preparing optically active, practically important cyclopropanecarboxylic acids.

This short review summarises the most important results obtained during the past decade in the field of palladium catalysed aminocarbonylation. In this reaction, using amines as N-nucleophiles, the carbonylation of enol-triflates/aryl-triflates or their synthetic surrogates, alkenyl halides/aryl halides takes place resulting in the formation of unsaturated carboxamides or aryl carboxamides. In the latter case, 2-ketocarboxamides are also formed via double carbon monoxide insertion. The efficiency of aminocarbonylation is illustrated by several high-yielding syntheses including transformation of both simple model substrates and skeletons of biological importance. As green chemistry aspects of this reaction, the ambient conditions, high activity, low catalyst loading, the possibility of tuning selectivities and high functional group tolerance should be mentioned.