Epoxide Synthesis

4 Key excerpts on "Epoxide Synthesis"

• 

  eBook - ePub

  Green Oxidation in Organic Synthesis

  • Ning Jiao, Shannon S. Stahl, Ning Jiao, Shannon S. Stahl(Authors)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  4 Green Oxidative Synthesis of Epoxides

  Miquel Costas

  Facultat de Ciències, Departament de Química I Institut de Química Computacional i Catàlisi, Universitat de Girona, Girona, Spain

  4.1 Introduction

  Epoxides are strained three‐membered heterocycles that have a high interest in organic synthesis because they exhibit a very versatile reactivity (Scheme 4.1 ). Most remarkably, they can be ring‐opened by a number of nucleophiles with high control over regio‐ and stereochemistry [1 ]. This aspect makes epoxides convenient starting points for the preparation of a number of 1,2‐difunctionalized products.

  Scheme 4.1

  Representative examples of the reactivity of epoxides.

  Epoxides are most commonly prepared by three different methods (Scheme 4.2 ).

  Scheme 4.2

  General methods for preparation of epoxides.

  The most common method is the direct epoxidation of olefins, by reaction with an oxidant, most commonly a peroxide or peracid (Scheme 4.2 a), but also via a metal‐catalyzed oxidation reaction with a number of oxidants [2 –4 ]. Epoxides can be also prepared by ring closing of halohydrins on reaction with a base (Scheme 4.2 b). Halohydrins are initially formed by reaction of an olefin and hypochlorous acid. Alternatively, the hypochlorous acid can be prepared in situ by reaction of Cl2 and water. This method is used in the large‐scale production of propene oxide and in the production of epichlorohydrin [5 –7 ]. Finally, epoxides can be also synthetized by reaction of aldehydes or ketones with ylides (Scheme 4.2 c) [8 ].

  Epoxidation of olefins is a well‐established and particularly useful reaction because olefins are largely available starting materials. However, the vast majority of current methods are far from satisfactory from a number of perspectives. In the first place, some epoxidizing agents exhibit poor atom economy in their reactions, and delivery of the oxygen atom occurs at the expense of the production of a large number of byproducts. In addition, because of the rich reactivity of epoxides, chemoselectivity remains a critical aspect in epoxidations with aggressive oxidants and acidic conditions. Paradigmatically, the epoxidation performed with peracids, known as the Prilezhaev reaction, is widely employed in organic synthesis [9 ]. For example, epoxidations with electron poor peracids, such as

  m‐chloro perbenzoic acid

  (MCPBA ), are reliable reactions, with a wide substrate scope. However, they produce the corresponding carboxylic acid as a by‐product. Peracetic acid is also employed in large‐scale epoxidations [2

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Ullmann's Fine Chemicals

  • (Author)
  • 2014(Publication Date)
  • Wiley-VCH

    (Publisher)

  Epoxides Guenter Sienel, Peroxid-Chemie GmbH, Höllriegelskreuth, Federal Republic of Germany Robert Rieth, Peroxid-Chemie GmbH, Höllriegelskreuth, Federal Republic of Germany Kenneth T. Rowbottom, Laporte Industries Ltd., Widnes, Cheshire WA8 OJU, United Kingdom

  1. Introduction

  Epoxides, also known as oxiranes, are characterized by the following group:

  The epoxy group is a highly reactive moiety (see Chap. 2), which makes epoxides an important group of industrial organic intermediates. The most significant members of this group, ethylene oxide and propylene oxide, are treated in separate articles. Epichlorohydrin (Section 4.1) dominates among the raw materials for epoxy resins (→ Epoxy Resins).

  2. Reactions of Epoxides

  Polarity and ring strain make the oxirane ring highly reactive. Thus, epoxides participate in numerous reactions, which makes these compounds useful building blocks in organic synthesis (see Table 1. ) [1–6]. Often epoxides formed in an initial step react further to provide industrially important products, such as surfactants or detergents (tensides), antistatic- or corrosion-protection agents, additives to laundry deter-gents, lubricating oils, textiles, and cosmetics [1].

  Table 1. Reactions of epoxides.

  2.1. Reactions with Compounds Containing Ionizable Hydrogen

  Reactions of epoxides with oxygen, sulfur, or carbon anions, usually in the presence of either acid or alkaline catalysts, affords β-hydroxy compounds (see Table 1. ). Unsymmetrically substituted epoxides may yield two isomers the ratio of which is controlled by pH [7].

  The base-catalyzed reaction follows an SN 2 substitution mechanism; attack of the nucleophile X− occurs predominantly at the sterically less hindered and more electron-deficient carbon atom. This substitution leads to Walden inversion at this carbon atom. The major product obtained from 1 is isomer 2

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Applications of Domino Transformations in Organic Synthesis, Volume 1

  • Prof. Scott A. Snyder(Author)
  • 2015(Publication Date)
  • Thieme

    (Publisher)

  1.2 Cation– ð Cyclizations of Epoxides and Polyepoxides K. W. Armbrust, T. Halkina, E. H. Kelley, S. Sittihan, and T. F. Jamison General Introduction Domino reactions of polyepoxides are of general interest for the synthesis of a variety of polycyclic ethers, including polyether ionophores, squalene-derived polyethers, and ma-rine ladder polyethers. [1] A number of biosynthetic hypotheses invoke polyepoxide cas-cades to rapidly generate complex polyethers; the two most cited are the Cane–Celmer– Westley and Nakanishi proposals. Respectively, these hypotheses account for the ob-served formation of all-exo and all-endo cascade products, borrowing terminology from Baldwin)s classification of ring-closing processes. [2–4] In general, smaller-ring products (formed by what is generally termed an exo pathway) are kinetically favored over larger-ring products ( endo ). That is, five-membered tetrahydrofuran rings form preferentially to their six-membered tetrahydropyran counterparts. Similarly, six-membered tetrahydro-pyrans form preferentially to seven-membered oxepane rings. A number of total synthe-ses have been inspired by these proposed cascades, and many creative methods have been developed to generate either exo or endo products in a selective fashion. The polyepoxide starting materials are often prepared via the powerful Sharpless and Shi asymmetric ep-oxidations of alkenes. [5,6] Complete transfer of stereochemical information is generally observed, as most epoxide openings proceed with stereospecific inversion of configura-tion. For the large majority of cascades discussed in this review, mechanistic details re-main unresolved, but most can nonetheless be categorized on the basis of how they are designed.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Science of Synthesis: Houben-Weyl Methods of Molecular Transformations Vol. 37

  Ethers

  • Craig J. Forsyth(Author)
  • 2014(Publication Date)
  • Thieme Chemistry

    (Publisher)

  The literature on this topic is extensive; thousands of papers report using this method for the installation of an epoxide group somewhere in a synthetic sequence. Although there is a lack of general reviews, Berti’s review on the stereochemistry of Epoxide Synthesis pro- vides a comprehensive discussion of the main mechanistic aspects, and an excellent selec- tion of the previous literature. [1] Stereoelectronic factors are crucial in determining the success of epoxide ring formation via intramolecular nucleophilic substitution. To achieve optimal overlap between the attacking alkoxide nucleophile and the s * orbital of the C¾X bond (X = leaving group), the geometry of the transition state must be as close as possible to the requisite anti-periplanar arrangement (Scheme 2). The importance of this stereoelectronic requirement for epoxide ring formation can be illustrated using the substituted trans-Decalin series. [2] Whereas the ring closure of 3A to 4A must proceed through a high-energy boat-like transition state in the first case, the reaction from 3B to epoxide 4B proceeds via the ground-state chair conformation. The lack of bond reorganization required in the second case ensures smooth transformation of the alkoxide in just over 1 minute at room temperature (Scheme 3). Scheme 1 Synthesis of Epoxides via Ring Closure of X-Hydrins R 1 HO R 2 X 1 R 1 O R 2 2 base X = Cl, Br, , OSO 2 R 3 , OCOR 3 , OSi(OR 3 ) 3 , SR 3 , NR 3 2 Scheme 2 Transition-State Assembly for Epoxide Ring Formation by an Intramolecular S N 2 Process O − X σ ∗ (C X) 407 for references see p 429 Not surprisingly, the factors that populate the reactive conformation typically result in a significant decrease in the activation energy for the ring closure. For example, the gem- dimethyl (or Thorpe-Ingold) effect leads to considerable enhancement in the rate of epoxide ring closure.

  Sign up to read

  Learn more about book