Chemistry
Regioselectivity
Regioselectivity refers to the preference of a chemical reaction to occur at a specific site or region of a molecule. This selectivity is determined by the electronic and steric properties of the reactants and can be influenced by reaction conditions and catalysts. Regioselective reactions are important in organic synthesis as they allow for the selective formation of specific products.
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3 Key excerpts on "Regioselectivity"
- Vasyl Andrushko, Natalia Andrushko, Vasyl Andrushko, Natalia Andrushko(Authors)
- 2013(Publication Date)
- Wiley(Publisher)
These methods allow nowadays the syntheses of complex chiral organic structures that previously seemed unattainable to be synthesized in the laboratory. The synthetic efforts toward many drugs and natural prod- ucts have largely been driven by their intriguing chemical structures and interesting biological properties. 168 There are four types of selectivity that can be distin- guished in a chemical process: 1. Chemoselectivity—a selective reactivity of one func- tional group in the presence of another. For example, H 3 C O hydrogenation H 3 C OH H 3 C OH H 3 C O chemoselectivity hydroformylation CHO CHO Regioselectivity hydrophosphonylation (R) O O O (R) O O (R) OH PO(OMe) 2 (R) O O (S) OH PO(OMe) 2 NH 2 Ph N H (S) Ph N H (R) Ph enantioselectivity intramolecular hydroamination diastereoselectivity FIGURE 1.45. Selectivity of chemical conversions. THE MAIN STRATEGIES TO ACCESS OPTICALLYACTIVE PRODUCTS 37 if two chemically different functionalities are present in a molecule such as C C and C O (Figure 1.45) and both can be hydrogenated, the chemoselectivity refers to the extent to which both C O or C C bonds will be hydrogenated. 2. Regioselectivity—a property of a chemical process to produce selectively one isomer over the other theoret- ically possible structural isomers. Regioselective reactions are usually addition or elimination reactions. One such example is the hydroformylation reaction. As the formyl group can be attached to either the primary (terminal C-atom or the secondary (internal C-atom), two isomers (linear and the branched respectively) can be formed (Figure 1.45). 3. Enantioselectivity—a degree to which one of the possible enantiomers of a chiral product can be pref- erentially formed during a chemical process. Although a substrate is achiral, the enantiopure or enantio-enriched catalyst may give rise to the forma- tion of one specific product-enantiomer.
eBook - ePubStereochemistry and Stereoselective Synthesis
An Introduction
- Mihály Nógrádi, László Poppe, József Nagy, Gábor Hornyánszky, Zoltán Boros(Authors)
- 2016(Publication Date)
- Wiley-VCH(Publisher)
There are, however, some processes, when it is problematic to separate the mixture of regioisomeric products. In such instances, Regioselectivity may be of help, as, for example, in the enzyme-catalyzed reaction shown in Figure 7.9. Separation of the sulfate esters of 1- and 2-naphthol can be accomplished by regioselective hydrolysis using a sulfatase enzyme. A similar procedure can be carried out with sulfate esters of o- and m-, as well as with o- and p- substituted phenols. Figure 7.9 Regioselective hydrolysis of sulfate esters with sulfatase enzyme. 7.2.2.2 Product Regioselectivity By Regioselectivity in a chemical reaction, usually product selectivity taken in a narrower sense is understood. Even within this category, more than one type can be distinguished, and Regioselectivity is often mixed up with the concept of chemoselectivity. According to one interpretation, Regioselectivity means competition between different regions of the same functional group. For instance, selectivities in the direct 1,2-addition, as well as in Michael - and 1,4-additions, are put by some authors into this category (as quoted in association with chemoselectivity shown in Figure 7.7). According to an alternative interpretation, Regioselectivity means selectivity manifested between groups of the same constitution but of different connectedness. The examples in the following two sections demonstrate this case. Regioselectivity in Enol and Enolate Formation A classic example for Regioselectivity associated with functional groups is the dependence of enol/enolate formation on reaction conditions (Figure 7.10). Figure 7.10 Regioselectivity in enolate formation. Variations of Regioselectivity in enolization can be traced back fundamentally to the difference of kinetically and thermodynamically controlled enolization mechanisms. Protons of the ketones in Figure 7.10 (where R is a simple electron donating alkyl group) are somewhat more acidic at the methyl group
eBook - PDF- (Author)
- 2006(Publication Date)
- Elsevier Science(Publisher)
Theoretical Aspects of Chemical Reactivity A. Toro-Labbé (Editor) © 2007 Published by Elsevier B.V. Chapter 7 Using the reactivity–selectivity descriptor f r in organic chemistry a b Christophe Morell, a André Grand, b Soledad Gutiérrez-Oliva, and b Alejandro Toro-Labbé a Département de Recherche Fondamentale sur la Matière Condensée, Service de Chimie Inorganique et Biologique, LAN (FRE2600), CEA-Grenoble, 17, rue des Martyrs, 38054 Grenoble Cedex 9, France and b Laboratorio de Química Teórica Computacional (QTC), Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Correo 22, Santiago, Chile 1. Introduction Besides the yield, there are many requirements for a chemical reaction to be interesting in organic synthesis, the regio-and stereo-selectivity 1 are crucial factors to understand reaction mechanisms. Indeed, one of the most exciting challenges for a chemist is to control the regio-and stereo-selectivity of the chemical species involved at the different steps of the overall synthesis process. A variety of well-known organic reactions such as additions to alkenes, 1 Diels Alders (DA) reactions, 1 − 3 electrophilic aromatic substi-tution, 4 cyclo-additions 4 and nucleophilic additions 1 on ketones or aldehydes present relatively high regio-and/or stereo-selectivity that have been rationalized using differ-ent theoretical tools and models. In this context, it can be observed that there is a lack of a theoretical model that allows the rationalization of reaction mechanisms in terms of a universal reactivity–selectivity descriptor. In this contribution, a theoretical model in which regio-and stereo-selectivity are characterized through a unique reactivity– selectivity descriptor defined in terms of the electronic properties of the species involved is proposed and then applied to analyse few classical organic reactions.
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