Current Organic Chemistry - Volume 21, Issue 9, 2017

Over the past decade, polyoxometalates (POMs) have attracted extensive attention as homogeneous and heterogeneous catalysts or nanocatalysts for organic transformation. The most advantageous feature of POMs is the possibility of the catalyst design at atomic/molecular levels. They could be designed as multifunctional catalysts, based on different metal atoms or heteroatoms in their structure, counter cations, second structures, and etc. There are different ways for heterogenization of POMs for catalytic applications in literature. Two major groups have been reported for this purpose: exchanging the heteropoly acid (HPAs) protons with cations and immobilization of HPAs on a suitable solid support. The obtained heterogeneous catalysts have special properties and could be used in the wide range of organic reactions. Recent developments in this emerging area are discussed in this review, covering a little discussion about why the POMs are active catalysts. Then different POMcatalyzed organic reactions including oxidation, reduction, multicomponent, esterification and transesterification, C-C coupling, isomerization reactions, etc. were illustrated. According to the multifunctionality of the POM-base catalysts, this review compiles the recent advances (focusing on the period 2000-2016) in POM-catalyzed reactions which have attracted the interest of the scientific community addressing on development of environmentally benign synthetic processes.

Within the Green Chemistry context, one of the major challenges is the replacement of conventional homogeneous catalysts by heterogeneous catalysts, which are easy to handle and usually recyclable since they can be readily recovered. This paradigm change is becoming particularly significant with the development of zeolites, either in their native acidic form or modified by metal ions, as catalysts in organic chemistry. Due to the current burgeoning of this new field, the present review only covers some typical but representative aspects. Typical reactions of carbohydrates and other acid-sensitive organic compounds can be catalyzed by H-zeolites, advantageously replacing dangerous and toxic strong acids. Metal-doped zeolites can act as catalysts for various transformations. Some cycloadditions, including the well-known 'click' reaction, and coupling, specially Cu-catalyzed reactions. This new kind of heterogeneous catalysts can be recycled and reused, in sharp contrast to conventional homogeneous catalysts.

Amines which are organic molecules can be employed in several significant syntheses such as the synthesis of biologically active molecules, polymers, drugs, dyes and pharmaceutical products. The critical issue in the synthesis of amines is to choose the most efficient heteregenous catalyst in order to obtain the best catalytic activity, reusability and durability. For this purpose, up to know, many studies have been performed on the reduction of nitro compounds to synthesize amines in the presence of various heteregenous catalysts. Addressed herein, recent developments have been gathered on the production of amines by the help of heteregenous catalysts such as platinum-, gold-, palladium-, silver-, nickel-, iron-, copper-, ruthenium-, iridium-, rhodium-, ionic liquid- and carbon-based monometallic, bimetallic and multimetallic heterogenous catalysts. Those catalysts have showed superior characteristics in terms of the amount, recoverability, reusability and the structural integrity of catalyst during the reaction. Thus, the main goal of this study is to enlighten the scientists and to discuss (review) the critical investigations on the reduction of nitro groups by heterogenous catalysts in last years.

It is now extensively established that the ideal way to achieve the development in heterogeneous catalysis is to utilize semiconducting titanium dioxide (TiO2) based particles with high surface area as well as mesoporous structure. This can efficiently accommodate substrates on its surface to interact with each other. Most of the reduction-oxidation type organic transformations, degradation of hazardous organic moieties are feasible at moderate reaction conditions with the help of TiO2 nanoparticles due to their ability to form composites with enhanced performance, better metal support interactions, chemical stability, selectivity and presence of reactive acid-base sites on its surface. This review article provides a balanced and comprehensive analysis of some innovative and ambitious strategies for modification of TiO2 host material for heterogeneous catalysis: a) making its composites with other materials; and b) doping by metal or non-metal.

Use of graphene oxide (GO) nanosheets as ‘carbocatalyst’ has emerged with overwhelming prospects since the seminal report from C. W. Bielawski in 2010. Water dispersible GO nanosheets are prepared from graphite powder via conventional oxidation and subsequent exfoliation. Use of GO as the catalyst is however focussed primarily on (i) its easy reducing nature, which in turn could promotes various oxidation reactions and (ii) acidic nature due to the presence of carboxylic acid groups mostly on its edges that in turn could act as solid heterogeneous acid and mediate various acid-catalyzed reactions. Notable advantages of catalytic uses of GO are: obtaining easily from renewable sources, metal-free, avoiding use of toxic salts, reusability etc. The present review article aims at summarizing catalytic applications in promoting diverse organic reactions since 2010 along with its future prospects.