Chemoselectivity

4 Key excerpts on "Chemoselectivity"

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  eBook - ePub

  Stereochemistry and Stereoselective Synthesis

  An Introduction

  • Mihály Nógrádi, László Poppe, József Nagy, Gábor Hornyánszky, Zoltán Boros(Authors)
  • 2016(Publication Date)
  • Wiley-VCH

    (Publisher)

  Part III General Characteristics of Stereoselective Reactions

  Selectivity is an all-important key feature of chemical reactions. Selectivity enables multistep reactions to take place economically. Moreover, without selectivity it would be impractical to analyze and purify the products of reactions. After defining stereospecificity and stereoselectivity, this part provides the reader with a systematic analysis of the various types of selectivties with special emphasis on selectivities leading to single enantiomeric products.

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  Chapter 7 Types and Classification of Selectivities

  Selectivity in chemistry is interpreted in various ways. The terms Chemoselectivity, regioselectivity, and stereoselectivity (including dia- and enantioselectivity) are widely used. However, there is no general agreement on their precise meaning. Therefore, the following chapters are dedicated to this topic and attempt to provide a uniform interpretative framework.

  7.1 Main Types of Selectivity

  In chemistry, two main types of selectivities can be defined; one depends mainly on the properties of the substrate, while the other differs in the products of a reaction [1]. These two main types of selectivities are shown in the following figures.

  7.1.1 Substrate Selectivity

  A reagent or a catalyst is substrate selective (Figure 7.1 ) when it transforms different substrates (S1 , S2 , …) to products (P1 , P2 , …) under identical conditions at different rates (k1 ≠ k2 ).

  Figure 7.1

  Substrate-selective reactions.

  7.1.2 Product Selectivity

  A reagent or catalyst is product selective (Figure 7.2 ) when it permits the formation of more than one product at different rates (k1 ≠ k2 ) from a single substrate (S) whereby the products (P1 , P2 , …) are formed in a ratio differing from the one statistically expected.1

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  eBook - ePub

  Directed Selectivity in Organic Synthesis

  A Practical Guide

  • Tanja Gaich, Ekkehard Winterfeldt(Authors)
  • 2014(Publication Date)
  • Wiley-VCH

    (Publisher)

  29), render them into passive volume, which means that they influence just by their sheer size.

  The wide range of options to use silane groups of different reactivity for chemoselective transformations is nicely demonstrated by an example from the benzleukodienes (33) [16].

  Having seen these impressive examples, we shall not be surprised by the silyl groups in the following chapters on regioselectivity and stereoselectivity. Chemoselectivity poses particularly demanding problems if the same functional group is present at different positions of a molecule as in sugars or glycosides. In this case, there may be options to rely on the sterical situation, especially if one can reversibly retreat to cyclic or bicyclic structures. Very often, however, the assistance of protecting groups will have to be considered, at least as long as purely chemical transformations are employed. There are quite encouraging signals, however, from various types of enzymatic reactions.

  It is, unfortunately, absolutely impossible to discuss the progress and the future possibilities in this field in this chapter but we include at least one example to demonstrate the capacity of these tools [17].

  It is hard to see that any type of conventional hydrolysis could compete with these results.

  1.2 Regioselectivity

  Regioselectivity is of particular importance with fundamental starting materials carrying functional groups that offer two reactive positions, such as olefins, acetylenes, epoxides, anhydrides, and imides. There are additionally the two enolate structures of ketones, as well as unsaturated carbonyl groups (1,2- vs 1,4-addition). In addition, there are a number of aromatic and heteroaromatic compounds posing various problems with regard to regioselective substitution.

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  Theoretical Aspects of Chemical Reactivity

  • (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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  eBook - ePub

  Modern Organic Synthesis

  An Introduction

  • George S. Zweifel, Michael H. Nantz, Peter Somfai(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  Regiochemistry, the preferential addition of the reagent at only one of the two possible regions or directions, exemplified in the Robinson annulation by the preferential alkylation of 2-methylcyclohexanone by the derived enolate at C(2) and not at C(6).
• Chemoselectivity, selective reaction of one functional group in the presence of other functional groups, exemplified by the preferential reaction of an aldehyde in the presence of a keto group.
• Stereoselectivity, the exclusive or predominant formation of one of the several possible stereoisomeric products, exemplified by the preferential formation of cis-3-methylcyclohexanol on reduction of 3-methylcyclohexanone with lithium aluminum hydride in THF or Et2 O.
• Efficiency, fewest number of steps.
• High yields in each step, of paramount concern in any chemical reaction is the yield.
• Availability and costs of starting materials.
• Most environmentally friendly route. Ideally, the atoms of the substrate and any additional reagents used for the reaction should appear in the final product, called “atom economy”19 —no by-products are formed, isolation of desired product is facilitated, and waste disposal is minimized (e.g., Diels–Alder reaction, metal-catalyzed reactions such as the example below20 ):
• Simplicity of selected procedures. Over the years, a large number of reagents have been developed that require special techniques for handling. If possible, one should use procedures that are less demanding in their execution.
• Isolation and purification of reaction products.21 Despite recent advances in methodologies for the synthesis of very complex molecules, one important aspect of synthesis has not been affected much over the last decades: isolation and purification. An excellent review entitled “Strategy-Level Separations in Organic Synthesis: From Planning to Practice” discusses various techniques for the separation of reaction mixtures.22