Selectivity In Organic Reactions Research Articles

Role of transition metal nanocomposites in organic reactions: A state of art as an alternative to conventional catalysts

Nanocomposites play vital role as catalyst during organic reactions because of high aspect ratio and greater surface area of nanostructured materials. The strong interfacial interaction of reactant with the surface of nanocomposites during reaction gives an enhancement in rate and yield of the product. The present review reveals a critical analysis regarding the function of transition metal based nanocomposites during catalytic action in various organic reactions. Catalytic action of transition metal based nanocomposites in combination with other carbon based nanomaterials, polymeric materials as well as with other metals or metal oxides are extensively elaborated in this review. The change in yields, duration of reaction, atom economy, and effect of solvent along with selectivity of organic reactions are monitored by various transition metal based nanocomposites. Reduction, oxidation, epoxidation, hydrogenation, halogenation and coupling reactions are appreciably reviewed considering the catalytic effect of these nanocomposites. The role of different nanocatalysts in various known name reactions in organic chemistry is also studied. Hence, present review explores new methods of using transition metal based nanostructured catalysts following the principle of green chemistry by replacing the traditional catalysts and reagents in organic synthesis.

Selective photocatalytic oxidation of aromatic alcohols using B-g-C3N4/Bi2WO6 composites

In this work, B-g-C3N4/Bi2WO6 composites were synthesized by simple hydrothermal reaction and calcination, and their morphology, composition and physicochemical properties were characterized. The results show that the composite can selectively oxidize aromatic alcohols to aromatic aldehydes under visible light. The conversion rate of benzyl alcohol can reach 98.9% and the selectivity can reach 99% in 4 h, which are 1.59 times and 5.03 times that of Bi2WO6 alone, respectively. Further photoelectric performance investigation revealed that the recombination of B-g-C3N4 with Bi2WO6 effectively enhanced the separation of photogenerated carriers of B-g-C3N4 and suppressed the generation of holes on Bi2WO6, thereby improving the photocatalytic activity and the selectivity of the catalytic reaction of p-benzyl alcohol. In addition, the electronic effect and steric hindrance effect of substituents on the benzene ring on the selective catalytic oxidation of aromatic alcohols were also explored. Our work also reveals that the bandgap structure of the composites and the efficient separation of photogenerated electron transport are key factors to improve the photocatalytic reactivity and selectivity of organic reactions.

ChemInform Abstract: Deciphering Selectivity in Organic Reactions: A Multifaceted Problem

AbstractReview: 46 refs.

Deciphering Selectivity in Organic Reactions: A Multifaceted Problem.

Computational chemistry has made a sustained contribution to the understanding of chemical reactions. In earlier times, half a century ago, the goal was to distinguish allowed from forbidden reactions (e.g., Woodward-Hoffmann rules), that is, reactions with low or high to very high activation barriers. A great achievement of computational chemistry was also to contribute to the determination of structures with the bonus of proposing a rationalization (e.g., anomeric effect, isolobal analogy, Gillespie valence shell pair electron repulsion rules and counter examples, Wade-Mingos rules for molecular clusters). With the development of new methods and the constant increase in computing power, computational chemists move to more challenging problems, close to the daily concerns of the experimental chemists, in determining the factors that make a reaction both efficient and selective: a key issue in organic synthesis. For this purpose, experimental chemists use advanced synthetic and analytical techniques to which computational chemists added other ways of determining reaction pathways. The transition states and intermediates contributing to the transformation of reactants into the desired and undesired products can now be determined, including their geometries, energies, charges, spin densities, spectroscopy properties, etc. Such studies remain challenging due to the large number of chemical species commonly present in the reactive media whose role may have to be determined. Calculating chemical systems as they are in the experiment is not always possible, bringing its own share of complexity through the large number of atoms and the associated large number of conformers to consider. Modeling the chemical species with smaller systems is an alternative that historically led to artifacts. Another important topic is the choice of the computational method. While DFT is widely used, the vast diversity of functionals available is both an opportunity and a challenge. Though chemical knowledge helps, the relevant computational method is best chosen in conjunction with the nature of the experimental systems and many studies have been concerned with this topic. We will not address this aspect but give references in the text. Usually, a computational study starts with the validation of the method by means of benchmark calculations vs accurate experimental data or state-of-the-art calculations. Finally, computational chemists can bring more than the sole determination of the reaction pathways through the analysis of the electronic structure. In our case, we have privileged the NBO analysis, which has the advantage of describing interactions on the basis of terms and concepts that are shared within the chemical community. In this Account, we have chosen to select representative reactions from our own work to highlight the diversity of situations than can be addressed nowadays. These include selective activation of C(sp(3))-H bonds, selective reactions with low energy barriers, involving closed shell or radical species, the role of noncovalent interactions, and the importance of considering side reactions.

Kinetic Analysis for the Multistep Profiles of Organic Reactions: Significance of the Conformational Entropy on the Rate Constants of the Claisen Rearrangement.

The significance of kinetic analysis as a tool for understanding the reactivity and selectivity of organic reactions has recently been recognized. However, conventional simulation approaches that solve rate equations numerically are not amenable to multistep reaction profiles consisting of fast and slow elementary steps. Herein, we present an efficient and robust approach for evaluating the overall rate constants of multistep reactions via the recursive contraction of the rate equations to give the overall rate constants for the products and byproducts. This new method was applied to the Claisen rearrangement of allyl vinyl ether, as well as a substituted allyl vinyl ether. Notably, the profiles of these reactions contained 23 and 84 local minima, and 66 and 278 transition states, respectively. The overall rate constant for the Claisen rearrangement of allyl vinyl ether was consistent with the experimental value. The selectivity of the Claisen rearrangement reaction has also been assessed using a substituted allyl vinyl ether. The results of this study showed that the conformational entropy in these flexible chain molecules had a substantial impact on the overall rate constants. This new method could therefore be used to estimate the overall rate constants of various other organic reactions involving flexible molecules.

The CH/π hydrogen bond in chemistry. Conformation, supramolecules, optical resolution and interactions involving carbohydrates

The CH/π hydrogen bond is an attractive molecular force occurring between a soft acid and a soft base. Contribution from the dispersion energy is important in typical cases where aliphatic or aromatic CH groups are involved. Coulombic energy is of minor importance as compared to the other weak hydrogen bonds. The hydrogen bond nature of this force, however, has been confirmed by AIM analyses. The dual characteristic of the CH/π hydrogen bond is the basis for ubiquitous existence of this force in various fields of chemistry. A salient feature is that the CH/π hydrogen bond works cooperatively. Another significant point is that it works in nonpolar as well as polar, protic solvents such as water. The interaction energy depends on the nature of the molecular fragments, CH as well as π-groups: the stronger the proton donating ability of the CH group, the larger the stabilizing effect. This Perspective focuses on the consequence of this molecular force in the conformation of organic compounds and supramolecular chemistry. Implication of the CH/π hydrogen bond extends to the specificity of molecular recognition or selectivity in organic reactions, polymer science, surface phenomena and interactions involving proteins. Many problems, unsettled to date, will become clearer in the light of the CH/π paradigm.

CH/π hydrogen bonds in organic and organometallic chemistry

This treatise is an update to a preceding highlight (CH/π hydrogen bonds in crystals) published in this journal 5 years ago (M. Nishio, CrystEngComm, 2004, 6, 130–156). After the introductory part (sections 1 and 2), we survey recent results (mostly since 2004) relevant to the CH/π hydrogen bond: crystal conformation, packing and host/guest chemistry (section 3). Section 4 summarizes the results obtained by crystallographic database (CSD and PDB) analyses. In section 5, several topics in related fields (selectivity in organic reactions, surface chemistry, structural biology, drug design and high-level ab initio calculations of protein/substrate complexes and natural organic compounds) are introduced, and in the final part we comment on the prospects of this emerging field of chemistry.

Synthesis, characterization and evaluation of sulfonic resins as catalysts

Ion-exchange resins have been often used as catalysts especially those based on styrene–divinylbenzene copolymers with sulfonic acid groups in the aromatic rings of polymer chains. That is due to the advantages of heterogenous catalysis over the homogeneous acid catalysis. Moreover, resin catalysts can often lead to high selectivity in organic reactions due to the matrix effects. Therefore, the study of copolymers synthesis conditions to determine the type of polymer structure produced as well as the characterization of sulfonic resins obtained thereof are of great interest. The current paper describes the synthesis, characterization and evaluation as catalysts of sulfonic resins derived from polymer supports synthesized by aqueous suspension polymerization of styrene and divinylbenzene. The reaction conditions were varied and polymer supports with different physical properties and morphological characteristics were obtained. The polymer supports were chemically modified by sulfonation. The resultant sulfonic resins had their catalyst activity evaluated in the esterification of acetic acid with n-butanol.

Competition and selectivity of organic reactions on semiconductor surfaces: reaction of unsaturated ketones on Si(100)-2 x 1 and Ge(100)-2 x 1.

A combined experimental and theoretical study of a model system of multifunctional unsaturated ketones, including ethyl vinyl ketone (EVK), 2-cyclohexen-1-one, and 5-hexen-2-one, on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces was performed in order to probe the factors controlling the competition and selectivity of organic reactions on clean semiconductor surfaces. Multiple internal reflection infrared spectroscopy data and density functional theory calculations indicate that EVK and 2-cyclohexen-1-one undergo selective [4 + 2] hetero-Diels-Alder and [4 + 2] trans cycloaddition reactions on the Ge(100)-2 x 1 surface at room temperature. In contrast, on the Si(100)-2 x 1 surface, evidence is seen for significant ene and possibly [2 + 2] C=O cycloaddition side products. The greater selectivity of these compounds on Ge(100) versus Si(100) is explained by differences between the two surfaces in both thermodynamic factors and kinetic factors. With 5-hexen-2-one, for which [4 + 2] cycloaddition is not possible, a small [2 + 2] C=C cycloaddition product is observed on Ge(100) and possibly Si(100), even though the [2 + 2] C=C transition state is calculated to be the highest barrier reaction by several kilocalories per mole. The results suggest that, due to the high reactivity of clean semiconductor surfaces, thermodynamic selectivity and control will play important roles in their selective functionalization, favoring the use of Ge for selective attachment of multifunctional organics.

ChemInform Abstract: High Pressure and Selectivity in Organic Reactions

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Models for selectivity in organic reactions

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Organic Reactions in the Solid Phase Prepared by Co-Deposition of Reagent Vapours Onto Cold Surface

Abstract New approach is proposed to influence reactivity and selectivity of organic reactions. It is based on structural ordering conditions for reagents using solids prepared from vapour phase.

ChemInform Abstract: Phase Transfer Catalysis and Selectivity in Organic Synthesis

Besides practical advantages and high versatility, phase transfer catalysis (PTC) gives important improvements in selectivity of organic reactions. This paper presents a discussion of selectivity changes induced under phase transfer conditions. Three main aspects are considered: chemoselectivity, regioselectivity and stereoselectivity.

Phase Transfer Catalysis and Selectivity in Organic Synthesis

AbstractBesides practical advantages and high versatility, phase transfer catalysis (PTC) gives important improvements in selectivity of organic reactions. This paper presents a discussion of selectivity changes induced under phase transfer conditions. Three main aspects are considered: chemoselectivity, regioselectivity and stereoselectivity.