Chemistry
Epoxide
Epoxide is a cyclic ether with a three-membered ring containing an oxygen atom and two carbon atoms. It is a highly reactive compound and is commonly used in organic synthesis as a building block for more complex molecules. Epoxides are also used as cross-linking agents in polymer chemistry.
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6 Key excerpts on "Epoxide"
eBook - PDF- Michael Dornbusch, Ulrich Christ, Rob Rasing(Authors)
- 2016(Publication Date)
- Vincentz Network(Publisher)
21 Properties and reactions of epoxy groups 2 Basic chemistry of the epoxy group Michael Dornbusch 2.1 Properties and reactions of epoxy groups Epoxides (oxiranes) are cyclic ethers that are characterised by high ring strain, which amounts to 114 kJ/mol in oxirane [1, 2] and 106 kJ/mol in oxetane [1] . The val-ues for cyclic hydrocarbons are in the same range: 115 kJ/mol for cyclopropane [1] and 111 kJ/mol for cyclobutane [1] . Equation 2.1: Important cyclic ethers and their systematic and trivial names The ring strain results from the bond angle of 60°, which is considerably less than the normal tetrahedral carbon angle of 109.5° and the C-O-C bivalent bond an-gle of 110° in ethers [7] . Small rings are stabilised by attached alkyl groups; thus the ring strain in 2-methyl-oxirane is 4 kJ/mol lower. The ring strain makes Epoxides much more reactive than other cyclic ethers. The ring strain in oxetane also enables it to react in mild conditions; its reactiv-ity ranks between that of oxirane and open-chain ethers [1] . In contrast, the higher homologues of the cyclic ethers are good solvents and are largely inert. The key reactions of Epoxides can be divided into two groups: • Reactions with nucleophiles in neutral solution and • Base-catalysed and acid-catalysed reactions M. Dornbusch, U. Christ, R. Rasing: Epoxy Resins © Copyright 2016 by Vincentz Network, Hanover, Germany Basic chemistry of the epoxy group 22 2.1.1 Reactions with nucleophiles As a general rule, ethers are inert to bases, which is why they serve as solvents in numerous organic reactions. By contrast, Epoxides undergo ring opening in mild conditions when attacked by nucleophiles, such as alkyl amines, in what is for-mally an addition reaction without elimination (see Equation 2.2).
eBook - ePub- Edwin Vedejs, Scott E. Denmark, Edwin Vedejs, Scott E. Denmark(Authors)
- 2016(Publication Date)
- Wiley-VCH(Publisher)
23Reactions of Epoxides (n → σ* )Tyler W. Wilson1 and Scott E. Denmark21 Gilead Sciences, Process Chemistry, 333 Lakeside Drive, Foster City, CA 94404, USA 2 University of Illinois, Department of Chemistry, 245 Roger Adams Laboratory, 600 South Mathews Avenue, Urbana, IL 61801, USA23.1 Introduction
The availability and versatility of Epoxides elevates them as a highly valued functional group in synthetic organic chemistry [1]. Epoxides are readily prepared by the direct oxidation of olefins and tremendous success has been achieved for both diastereoselective [2] and enantioselective preparations of oxiranes [3]. In parallel with advances for their synthesis, strategies for the site- and stereoselective opening of Epoxides have been developed that harness their strain energy for myriad transformations [4]. For example, the ring opening of Epoxides with nucleophiles, which can proceed with clean inversion of configuration by an SN 2 pathway, allows the construction of vicinal stereogenic centers with high stereochemical fidelity. Taken together, these processes of epoxidation and nucleophilic ring opening provide an extremely powerful method for transforming simple olefins into diverse arrays of chiral 1,2-difunctionalized building blocks.23.1.1 Lewis Acid-Catalyzed, Enantioselective Epoxide Opening
The ability of chiral Lewis acids to catalyze the desymmetrization of meso-Epoxides is now well established. Unlike most enantioselective transformations that involve enantiotopic face selection with compounds that contain no stereogenic centers, these reactions involve enantiotopic group selection with Cs -symmetric compounds containing two or more stereogenic centers. The power of this type of transformations is that a number of stereogenic centers can be unveiled in a single step (Eq. (23.1) ) [5]. The scope of the nucleophile for this process has evolved beginning with early studies by Nugent, who identified silyl azides as particularly effective nitrogen-based nucleophiles for Epoxide opening using a zirconium alkoxide complex as the catalyst [6]. Later, Jacobsen greatly extended the field of meso-Epoxide opening by employing chiral Cr- and Co-salen complexes for the reactions of Epoxides with silyl azides, alcohols, and carboxylic acids [5a]. More recently, the scope of this process has been expanded to include phenols and sulfur-based nucleophiles by Shibasaki and coworkers, who described the use of a gallium–lithium BINOL complex as the catalyst [7]. These along with many other contributions have now made the Lewis acid-catalyzed opening of meso
eBook - ePubOrganic Chemistry Study Guide
Key Concepts, Problems, and Solutions
- Robert J. Ouellette, J. David Rawn(Authors)
- 2014(Publication Date)
- Elsevier(Publisher)
16Ethers and Epoxides
Keys to the Chapter
The chemistry of ethers has substantially less variety than the chemistry of alcohols because several of the reactions of alcohols involve the O—H bond, namely dehydration and oxidation reactions are not possible for ethers. However, if comparable reactions are considered, such as substitution, then alcohols and ethers have similar reactivities. The reactions of Epoxides are the result of ring strain, which leads to the formation of ring-opened products.16.1 Structure of Ethers
Ethers contain an oxygen atom bonded to two alkyl groups, two aryl groups, or one of each. The geometry of ethers resembles that of alkanes, with the substitution of a methylene carbon atom by an sp2 -hybridized oxygen atom. Conformations of ethers resemble those of alkanes. The two nonbonding electron pairs of the ether oxygen are directed to the corners of a tetrahedron.16.2 Nomenclature of Ethers
The common names of simple ethers are based on the names of the alkyl or aryl groups bonded to the oxygen atom. The name results from listing the alkyl (or aryl) groups in alphabetical order and appending the name ether.The IUPAC name is based on the longest carbon chain bonded to the oxygen atom. The smaller group bonded to the oxygen atom is named as an alkoxy group and is regarded as a substituent on the longer chain.The three-, five-, and six-membered cyclic ethers have common names. Three-membered ring compounds are called Epoxides of the corresponding alkene from which they may be synthesized. The common names of five- and six-membered ring compounds are called tetrahydrofurans and tetrahydropyrans, respectively. In the IUPAC system, each ring size has a specific name. The names for cyclic ethers having three-, four-, five-, and six-membered rings are oxirane, oxetane, oxolane, and oxane, respectively. The oxygen atom in each ring is assigned the number 1, and the ring is numbered in the direction that gives the lowest numbers to substituents.
- David R. Klein(Author)
- 2017(Publication Date)
- Wiley(Publisher)
(c) There are two methods for naming Epoxides, although one of these methods will be less helpful because the two substituents (connected to the oxirane ring) are actually closed in a ring. This makes it difficult to name the compound as an oxirane. According to the first method for naming ethers, the parent is CHAPTER 13 451 cyclohexane, and the oxygen atom is considered to be an epoxy substituent connected to the parent at C1 and C2. 13.12. (a) There are two methods for naming Epoxides. In one method, the parent will be propane, and the oxygen atom is considered to be an epoxy substituent connected to the parent at C1 and C2. In addition, there is a phenyl substituent at C2, and the configuration of the chiral center is indicated. According to the second method for naming Epoxides, the parent is considered to be the oxirane ring. The methyl group and the phenyl group are both considered to be substituents, and their locations are identified with locants. Finally, the configuration of the chiral center is indicated (at the beginning of the name). In this case, the chiral center has the R configuration, as a result of the following prioritization scheme. (b) There are two methods for naming Epoxides. In one method, the parent will be heptane, and the oxygen atom is considered to be an epoxy substituent connected to the parent at C3 and C4. The configuration of each chiral center is indicated. According to the second method for naming Epoxides, the parent is considered to be the oxirane ring, which is connected to two substituents (a propyl group and an ethyl group). Their locations are identified with locants, and the configuration of each chiral center is indicated (at the beginning of the name). (c) There are two methods for naming Epoxides. In one method, the parent will be pentane, and the oxygen atom is considered to be an epoxy substituent connected to the parent at C2 and C3.
eBook - PDF- David R. Klein(Author)
- 2020(Publication Date)
- Wiley(Publisher)
We saw several ways to form Epoxides and many reagents that open Epoxides. Note that opening an Epoxide provides two functional groups on adjacent carbon atoms. O OH Nuc Whenever you see two adjacent functional groups, you should think of Epoxides. Let’s see an example. SKILLBUILDER LEARN the skill 14.6 INSTALLING TWO ADJACENT FUNCTIONAL GROUPS Propose a synthesis for the following transformation: OMe OH Enantiomer + SOLUTION Always approach a synthesis problem by initially asking two questions. 1. Is there a change in the carbon skeleton? No, the carbon skeleton is not changing. 2. Is there a change in the functional groups? Yes, the starting material has no functional groups, and the product has two adjacent functional groups. The answers to these questions dictate what must be done. Specifically, we must install two adjacent functional groups. This suggests that we consider using a ring-opening reaction of an Epoxide. Using a retrosynthetic analysis, we draw the Epoxide that would be necessary. OMe OH O The regiochemistry of this step requires that the methoxy group must be placed at the more substituted position. This dictates that the Epoxide must be opened under acidic conditions to ensure that the nucleophile (MeOH) attacks at the more substituted position. Our next step is to determine how to make the Epoxide. We have seen a couple of ways to make Epoxides, both of which start with an alkene. O At this point, we can start working forward, focusing on converting the starting material into the desired alkene. The starting material has no functional groups, and we have seen only one method for introducing a functional group into an alkane. Specifically, we must employ a radical bromination. Br Br 2 hν 634 CHAPTER 14 Ethers and Epoxides; Thiols and Sulfides PRACTICE the skill APPLY the skill need more PRACTICE? At this point, we just need to bridge the gap.
eBook - PDF- David R. Klein(Author)
- 2021(Publication Date)
- Wiley(Publisher)
624 CHAPTER 13 Ethers and Epoxides; Thiols and Sulfides Thiols and Sulfides Thiols R 2 R 1 Br R 2 R 1 SH NaSH RSH RSH S S R R A disulfide + NaOH/H 2 O Br 2 HCl, Zn Sulfides R SH R S R 1) NaOH 2) RX MeX X R S R R S R Me + + − R S R O O Sulfone Na O 4 Sulfoxide R S R O Sulfide R S R H 2 O 2 H 2 O 2 Enantioselective Epoxidation R OH O R OH O R OH (CH 3 ) 3 COOH Ti[OCH(CH 3 ) 2 ] 4 (+)-DET (CH 3 ) 3 COOH Ti[OCH(CH 3 ) 2 ] 4 (–)-DET Ring-Opening Reactions of Epoxides 1) NaCN 2) H 3 O + 1) NaSR 2) H 3 O + 1) RMgBr 2) H 3 O + 1) LiAlH 4 2) H 3 O + 1) NaOR 2) H 3 O + [H + ] H 2 O [H + ] ROH HX HO OR HO X HO OH OH RS OH R OH H OH RO OH NC Acid-catalyzed Strong nucleophile O Preparation of Epoxides R R H H cis O R R H H cis MCPBA 1) Br 2 , H 2 O 2) NaOH Review of Concepts and Vocabulary 625 REVIEW OF CONCEPTS AND VOCABULARY SECTION 13.1 • Ethers are compounds that have an oxygen atom bonded to two groups, which can be alkyl, aryl, or vinyl groups. • The ether group is a common structural feature of many nat- ural compounds and pharmaceuticals. SECTION 13.2 • Unsymmetrical ethers have two different alkyl groups, while symmetrical ethers have two identical groups. • The common name of an ether is constructed by assigning a name to each R group, arranging them in alphabetical order, and then adding the word “ether.” • The systematic name of an ether is constructed by choosing the larger group to be the parent alkane and naming the smaller group as an alkoxy substituent. SECTION 13.3 • Ethers of low molecular weight have low boiling points, while ethers with larger alkyl groups have higher boiling points due to London dispersion forces between the alkyl groups. • Ethers are often used as solvents for organic reactions. SECTION 13.4 • The interaction between ethers and metal ions is very strong for polyethers, compounds with multiple ether groups. • Cyclic polyethers, or crown ethers, are capable of solvating metal ions in organic (nonpolar) solvents.
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