Reagent

About: Reagent is a research topic. Over the lifetime, 60091 publications have been published within this topic receiving 1234928 citations. The topic is also known as: reagens.

Organic Synthesis “On Water”

Abstract: Water is the lingua franca of life on our planet and is the solvent of choice for Nature to carry out her syntheses.1 In contrast, our methods of making complex organic molecules have taken us far away from the watery milieu of biosynthesis. Indeed, it is fair to say that most organic reactions commonly used both in academic laboratories and in industry fail in the presence of water or oxygen. As a direct consequence of our attempts to mimick Nature's way of making new chemical bonds, we learned to rely on highly reactive nucleophilic and electrophilic reagents to gain control of the chemical reactivity and to channel chemical reactions down a desired pathway. The requirement for the protection of all protic functional groups, such as alcohols and amines, is another corollary of our reliance on these energetic species. Nevertheless, chemical transformations in aqueous solvents are not new to organic chemists. On the contrary, they have attracted attention of scientists for many years: the first use of water for an organic reaction could be dated back to Wohler's synthesis of urea from ammonium cyanate.2 From a true organic synthesis perspective, the earliest example could be the synthesis of indigo by Baeyer and Drewsen in 1882 (Scheme 1).3 In their synthesis, a suspension of o-nitrobenzaldehyde 1 in aqueous acetone was treated with a solution of sodium hydroxide. The immediate formation of the characteristic blue color of indigo 2 ensued, and the product subsequently precipitated. Scheme 1 Water possesses many unique physical and chemical properties: large temperature window in which it remains in the liquid state, extensive hydrogen bonding, high heat capacity, large dielectric constant, and optimum oxygen solubility to maintain aquatic life forms. These distinctive properties are the consequence of the unique structure of water.4,5 The structure and properties of water have been studied by scientists representing almost all fields of knowledge, and new theoretical models continue to emerge.6,7 Water is also known to enhance the rates and to affect the selectivity of a wide variety of organic reactions.8,9 In spite of these potential advantages, water is still not commonly used as a sole solvent for organic synthesis, at least in part because most organic compounds do not dissolve in water to a significant extent, and solubility is generally considered a prerequisite for reactivity: “corpora non agunt nisi soluta” (substances do not react unless dissolved). Consequently, in the many examples of “aqueous reactions” organic co-solvents are employed in order to increase the solubility of organic reactants in water.9,10 Alternatively, hydrophilicity of the reactants is increased by the introduction of polar functional groups, again to make the resulting compound at least partially water soluble.11 However, these manipulations tend to diminish and even negate the advantages of low cost, simplicity of reaction conditions, ease of workup, and product isolation that water has over traditional solvents. Therefore, the currently burgeoning field of organic synthesis in aqueous media encompasses a large family of reactions. The solubility of reacting species and products can range from complete to partial to practically none, so that reaction mixtures can be both homogeneous and heterogeneous. The amount of water can also range widely, from substoichiometric quantities to a large volume in which the reactants are suspended or dissolved. Several terms have been used in the literature to describe reactions in aqueous millieu. In water, in the presence of water, and on water are commonly found in the recent publications and are often used interchangeably to describe reactions that proceed under very different conditions.12,13 There is also a growing number of examples micellar catalysis in the presence of non-ionic surfactants, such as Triton X-100 and PTS (a tocopherol-based amphiphile).14-18 In this review, we attempt to survey organic transformations that benefit from being performed on water under the conditions defined by Sharpless and co-workers: when insoluble reactant(s) are stirred in aqueous emulsions or suspensions without the addition of any organic co-solvents. In many cases, it is impossible to ascertain whether the reaction is occuring in or on water, but as long as the reaction mixture remains heterogeneous and the overall process appears to benefit from it (either in terms of increased reaction rate or enhanced selectivity), it qualifies. The ‘on water’ moniker reflects the defining attribute of these reactions: the lack of solubility of the reactant(s) in water. A considerable rate acceleration is often observed in reactions carried out under these conditions over those in organic solvents.19 Furthermore, in many cases a significant rate increase of on water reactions over reactions carried out neat indicates that rate acceleration is not merely a consequence of increased concentration of the reacting species. Naturally, the degree of on water acceleration varies between different reaction classes, and even when it is modest, there are other advantages to carrying out reactions in this manner. Firstly, water is an excellent heat sink due to its large heat capacity, making exothermic processes safer and more selective, especially when they are carried out on large scale. Secondly, reactions of water-insoluble substrates usually lead to the formation of water-insoluble products. In such cases, product isolation simply involves filtration of solid products (or phase separation in case of liquids). Finally, the growing list of examples wherein reactions performed on water are not only faster but also more selective (whether chemo-, regio-, or enantio-) underscores the significant potential for process intensification for reactions performed on water. Although claims of the ecological advantages and “greenness” of water are almost invariably found in the opening paragraphs of reports describing aqueous reactions, they should be taken with a grain of salt. The low cost, relative abundance, and inherent safety of water notwithstanding, the environmental impact of a process is determined by many factors, such as the efficiency of the reaction in terms of atom economy,20 the nature of solvents used in the reaction workup, the residual concentration of regulated organic compounds and metal catalysts remaining in the aqueous waste, and the costs of its clean up or disposal.21,22 The mere finding that a process performs as well in water as it does in an organic solvent tells us little about its potential environmental impact. The field of aqueous organic synthesis has been regularly and comprehensively reviewed.9,10,23-27 In addition, recent reviews focusing on microwave assisted organic synthesis in water,28 reactions in near-critical water,29 and biocatalysis in water30 have been published. Accordingly, these topics are not covered in the present review.

Vitamin C Is an Ideal Substitute for Hydrazine in the Reduction of Graphene Oxide Suspensions

Abstract: The preparation of solution-processable graphene from graphite oxide typically involves a hydrazine reduction step, but the use of such a reagent in the large-scale implementation of this approach is not desirable due to its high toxicity. Here, we compare the deoxygenation efficiency of graphene oxide suspensions by different reductants (sodium borohydride, pyrogallol, and vitamin C, in addition to hydrazine), as well as by heating the suspensions under alkaline conditions. In almost all cases, the degree of reduction attainable and the subsequent restoration of relevant properties (e.g., electrical conductivity) lag significantly behind those achieved with hydrazine. Only vitamin C is found to yield highly reduced suspensions in a way comparable to those provided by hydrazine. Stable suspensions of vitamin C-reduced graphene oxide can be prepared not only in water but also in common organic solvents, such as N,N-dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP). These results open the perspective ...

Enhanced electrocatalytic CO2 reduction via field-induced reagent concentration

Abstract: Electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO) is the first step in the synthesis of more complex carbon-based fuels and feedstocks using renewable electricity. Unfortunately, the reaction suffers from slow kinetics owing to the low local concentration of CO2 surrounding typical CO2 reduction reaction catalysts. Alkali metal cations are known to overcome this limitation through non-covalent interactions with adsorbed reagent species, but the effect is restricted by the solubility of relevant salts. Large applied electrode potentials can also enhance CO2 adsorption, but this comes at the cost of increased hydrogen (H2) evolution. Here we report that nanostructured electrodes produce, at low applied overpotentials, local high electric fields that concentrate electrolyte cations, which in turn leads to a high local concentration of CO2 close to the active CO2 reduction reaction surface. Simulations reveal tenfold higher electric fields associated with metallic nanometre-sized tips compared to quasi-planar electrode regions, and measurements using gold nanoneedles confirm a field-induced reagent concentration that enables the CO2 reduction reaction to proceed with a geometric current density for CO of 22 milliamperes per square centimetre at -0.35 volts (overpotential of 0.24 volts). This performance surpasses by an order of magnitude the performance of the best gold nanorods, nanoparticles and oxide-derived noble metal catalysts. Similarly designed palladium nanoneedle electrocatalysts produce formate with a Faradaic efficiency of more than 90 per cent and an unprecedented geometric current density for formate of 10 milliamperes per square centimetre at -0.2 volts, demonstrating the wider applicability of the field-induced reagent concentration concept.

Facile Catalyst Separation Without Water: Fluorous Biphase Hydroformylation of Olefins

Abstract: A novel concept for performing stoichiometric and catalytic chemical transformations has been developed that is based on the limited miscibility of partially or fully fluorinated compounds with nonfluorinated compounds. A fluorous biphase system (FBS) consists of a fluorous phase containing a dissolved reagent or catalyst and another phase, which could be any common organic or nonorganic solvent with limited or no solubility in the fluorous phase. The fluorous phase is defined as the fluorocarbon (mostly perfluorinated alkanes, ethers, and tertiary amines)-rich phase of a biphase system. An FBS compatible reagent or catalyst contains enough fluorous moieties that it will be soluble only or preferentially in the fluorous phase. The most effective fluorous moieties are linear or branched perfluoroalkyl chains with high carbon number; they may also contain heteroatoms. The chemical transformation may occur either in the fluorous phase or at the interface of the two phases. The application of FBS has been demonstrated for the extraction of rhodium from toluene and for the hydroformylation of olefins. The ability to separate a catalyst or a reagent from the products completely at mild conditions could lead to industrial application of homogeneous catalysts or reagents and to the development of more environmentally benign processes.