Electrophilic Addition

7 Key excerpts on "Electrophilic Addition"

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  eBook - ePub

  Essentials of Organic Chemistry

  For Students of Pharmacy, Medicinal Chemistry and Biological Chemistry

  • Paul M. Dewick(Author)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  8

  Electrophilic reactions

  In the preceding chapters we have seen how new bonds may be formed between nucleophilic reagents and various substrates that have electrophilic centres, the latter typically arising as a result of uneven electron distribution in the molecule. The nucleophile was considered to be the reactive species. In this chapter we shall consider reactions in which electrophilic reagents become bonded to substrates that are electron rich, especially those that contain multiple bonds, i.e. alkenes, alkynes, and aromatics. The π electrons in these systems provide regions of high electron density, and electrophilic reactions feature as the principal reactivity in these classes of compounds. We term the reactions electrophilic rather than nucleophilic, since it is the electrophile that provides the reactive species.

  8.1 Electrophilic Addition to unsaturated carbon

  The characteristic reaction of alkenes is Electrophilic Addition, in which the carbon–carbon π bond is replaced by two σ bonds.

  The π bond of an alkene results from overlapping of p orbitals and provides regions of increased electron density above and below the plane of the molecule. These electrons are less tightly bound than those in the σ bonds, so are more polarizable and can interact with a positively charged electrophilic reagent. This forms the first part of an Electrophilic Addition, in which the electrons are used to form a σ bond with the electrophile and leave the other carbon of the double bond electron deficient, i.e. it becomes a carbocation. This carbocation is then rapidly captured by a nucleophile, which donates its electrons to form the second new σ bond. This latter step is very much faster than the first step, and thus carbocation formation becomes the rate-determining step in this bimolecular

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  Organic Reaction Mechanisms 2007

  An annual survey covering the literature dated January to December 2007

  • A. C. Knipe(Author)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  CHAPTER 11

  Addition Reactions: Polar Addition

  P. KOČOVSKÝ Department of Chemistry, University of Glasgow

  Reviews

  Electrophilic Additions

  Halogenation and Related Reactions Additions of ArSX, ArSeX, and Related Reagents with Electrophilic Sulfur Additions of Hydrogen Halides and Other Brønsted Acids Additions of Electrophilic Carbon Additions of Electrophilic Nitrogen Additions Initiated by Metals and Metal Ions as Electrophiles Miscellaneous Electrophilic Additions

  Nucleophilic Additions

  Additions to Multiple Bonds Conjugated with C=O Additions to Multiple Bonds Activated by Other Electron-withdrawing Groups Additions of Organometallics to Activated Double Bonds Miscellaneous Nucleophilic Additions

  References

  Reviews

  During the coverage period of this chapter, reviews have appeared on the following topics: synthesis by addition reactions to alkenes;1 control of regio- and stereoselectivity in Electrophilic Addition reactions of allenes;2 addition reactions of E–E and E–H bonds (E = S, Se) to the triple bond of alkynes catalysed by Pd, Pt, and Ni complexes;3 palladium-catalysed carboetherification and carboamination reactions of γ -hydroxy- and γ -amino-alkenes for the synthesis of tetrahydrofurans and pyrrolidines;4 the mechanistic perspective of Heck–Mizoroki cross-coupling reaction;5 aldehydes from Pd-catalysed oxidation of terminal alkenes;6 electrophilic activation of alkenes by platinum(II) and comparison with the palladium(II) version;7 ligands for practical rhodium-catalysed asymmetric hydroformylation;8 transition metal-catalysed reactions using N -heterocyclic carbene ligands (besides Pd- and Ru-catalysed reactions);9 nucleophilic additions and substitutions in water;10 substitution at the α -carbons of α ,β -unsaturated carbonyl compounds (anti-Michael addition);11 trends in catalytic enantioselective conjugate addition reactions;12 the Morita–Baylis–Hillman reaction;13 the enantioselective Morita–Baylis–Hillman reaction and its aza counter-part;14 the aza-Baylis–Hillman reactions and their synthetic applications;15 recent advances in metal-catalysed asymmetric conjugate additions;16 catalytic enantioselective conjugate addition with Grignard reagents;17 and stereoselective conjugate addition reactions using in situ metallated terminal alkynes and the development of novel chiral P,N -ligands (PINAP).18

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  Organic Reaction Mechanisms 1966

  An annual survey covering the literature dated December 1965 through November 1966

  • B. Capon, M. J. Perkins, C. W. Rees, B. Capon, M. J. Perkins, C. W. Rees(Authors)
  • 2008(Publication Date)
  • Wiley-Interscience

    (Publisher)

  CHAPTER 5 Addition Reactions Electrophilic Additions A monograph on Electrophilic Additions to unsaturated systems has appeared.l Addition of halogens and related reactions. Addition reactions of halogens which accompany electrophilic aromatic substitution,2 and Electrophilic Addition to fluor0-01efins,~ have been reviewed. The mechanisms of the addition of halogens to olefins in non-polar media is complex and no single consistent mechanism has emerged. In this connection addition to dibenzobicyclo[2.2.2]octatriene (1) is of interest since it is known, largely from the work of Cristol, that radical addition gives unrearranged products, cis- and trans-($), whilst ionic addition gives the rearranged products, exo- and endo-@). Addition of iodine to (1) in non-polar solvents proceeds by & I I Radical h & __f Ionic & / / xy I I / xy or cis (2) y orexo (3) But (7) 1 P. B. D. de la Mare and R. Bolton, “Electrophilic Additions to Unsaturated Systems”, 2 P. B. D. de la Mare, J. S. Lomaa, and V. S. Del Olmo, Bull. Sot. Chim. Frame, 1966, 1157. 3 B. L. Dyatkin, E. P. Mochalina, and I. L. Knunyants, Usp. Khim., 35,979 (1966). Elsevier. London, 1966. Organic Reaction Mechanisms 1966 Edited by B. Capon, M. J. Perkins, C. W. Rees Copyright © 1967 by John Wiley & Sons, Ltd. Addition Reactions 125 an ionic mechanism to give endo-4-syn-8-diiodobenzobicyclo[3.2. lloctadiene [(3); X = Y = I ] solely, though this product slowly isomerizes at room temperature to an equilibrium mixture of starting materials, the exo-anti- isomer of [(3); X = Y = I] and the more stable isomer, trans-[(t); X = Y = I].

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  Organic Chemistry

  A Mechanistic Approach

  • Penny Chaloner(Author)
  • 2014(Publication Date)
  • CRC Press

    (Publisher)

  421 11.1 INTRODUCTION In this chapter, we will be studying addition reactions to carbon–carbon multiple bonds; this is the converse process of the eliminations that we studied in the previous chapter. Addition to carbon–heteroatom multiple bonds is coming up in Chapter 14. Nucleophiles, electrophiles, and radicals can all add across double bonds; first, we will concentrate on electrophiles and radicals, as nucleophiles only add readily when the double bond bears a group (such as a carbonyl, nitro, or nitrile; Chapter 17) capable of accepting electron density. Reactions with electrophiles or radicals add two moieties, atoms or groups, by a stepwise process; the two atoms or groups are not added simultaneously. However, there is another class of reactions where the two new bonds are made simultaneously—these are called concerted reactions. We should first recall the electronic structure of alkenes, with carbon atoms sp 2 hybridized and a sigma framework of bonds at approximately 120 ° to each other. Above and below the plane of the molecule is a π-orbital, derived from the two remaining p z orbitals ( 11.1). 11.1 The addition of two atoms or groups to an alkene is the most important reaction for this type of com- pound. The two electrons of the π-bond provide two of those needed to make the two new σ-bonds. 11.2 ELECTROPHILIC REACTIONS 11.2.1 REACTION MECHANISM This reaction is typified by the addition of a hydrogen halide, HX, or water, to an alkene. The process involves two steps, with a carbocation as the intermediate. In the first step (Figure 11.1), an electrophile, for example, a proton, is added to the carbon–carbon bond to form a carboca- tion. Notice how the curly arrow is drawn—the proton is being added to the “upper” carbon of the double bond and the electrons are “taken away” from the “lower” carbon, leaving it positively charged. In the second step, the counterion, for example, bromide, attacks the carbocation to give a saturated product.

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  Organic Reaction Mechanisms 2010

  An annual survey covering the literature dated January to December 2010

  • A. C. Knipe(Author)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  Chapter 11 Addition Reactions: Polar Addition

  P. Ko ovský

  Department of Chemistry, University of Glasgow, Glasgow, UK

  Reviews Electrophilic Additions Halogenation and Related Reactions Additions of ArSX, ArSeX, and Related Reagents with Electrophilic Sulfur Additions of Hydrogen Halides and Other Brønsted Acids Additions of Electrophilic Carbon Additions of Electrophilic Nitrogen Additions Initiated by Metals and Metal Ions as Electrophiles Miscellaneous Electrophilic Additions Nucleophilic Additions

  Additions to Multiple Bonds Conjugated with C—O

  Additions to Multiple Bonds Activated by Other Electron-withdrawing Groups Additions of Organometallics to Activated Double Bonds Miscellaneous Nucleophilic Additions References

  Reviews

  During the coverage period of this chapter, reviews have appeared on the following topics: computational analysis of the reaction mechanisms in terms of reaction phases, including hidden intermediates and hidden transition states;1 cationic polymerization of vinyl monomers in aqueous media, ranging from monofunctional oligomers to long-lived polymer chains;2 Mirozoki–Heck reaction;3 the Heck reaction applied to 1,3- and 1,2-unsaturated derivatives with emphasis on molecular complexity;4 oxidative Heck reaction;5 progress in the Heck and Suzuki reaction catalysed by Pd-N-heterocyclic carbene complexes;6 Brønsted acid versus metal catalysis in hydroamination reactions;7 scope and limitations of the hydroarylation reactions;8 transition-metal-catalysed alkene and alkyne hydroacylation;9 recent advances in carbocupration of α-heterosubstituted alkynes;10 regioselective reductive cross-coupling of unsymmetrical alkynes;11 copper- and rhodium-catalysed enantioselective conjugate addition;12, 13 cyclic carbometallation of alkenes, arenes, alkynes, and allenes;14 intramolecular 1,2- and 1,4-addition reactions;15 conjugate addition reactions of carbon nucleophiles to electron-deficient dienes;16 new development of oxa-Michael reaction;17 progress on the catalytic asymmetric Michael addition reaction;18 cinchona alkaloid-catalysed nucleophilic conjugate addition to electron-deficient C C bonds;19 development of chiral thiourea catalysts and their application to asymmetric catalytic reactions;20 privileged chiral catalysts in asymmetric Morita–Baylis–Hillman (MBH)/aza-MBH reaction;21 chiral, chelating hydroxyalkyl and hydroxyaryl N-heterocyclic carbenes and their application in copper-catalysed asymmetric conjugate addition;22 addition and cycloaddition reactions of phosphinyl- and phosphonyl-2H-azirines, nitrosoalkenes, and azoalkenes;23 inter- and intra-molecular reactions of epoxides and aziridines with π-nucleophiles;24 applications of dynamic nuclear polarization to the study of reactions and reagents in organic and biomolecular chemistry;25 and selectivity in metal-catalysed C C bond cleavage of alkylidenecyclopropanes.26

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  The Organometallic Chemistry of the Transition Metals

  • Robert H. Crabtree(Author)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  * back donation, and so the polyene is left with a net positive charge, favorable for nucleophilic attack.

  Encouraging nucleophilic abstraction by starting from a high‐oxidation‐state metal is shown in Eq. (8.29 ) (tripod = HC(C5 H5 N)3 ). The acetate abstracts the alkyl from the metal, the two electrons in the M-C bond reducing the metal from rare Ni(IV) to the much stabler Ni(II) state[16 ]:

  8.29

  8.5 Electrophilic Addition and Abstraction

  In common with 2e nucleophiles, zero‐electron electrophiles such as H+ or Me+ can attack a ligand. Unlike nucleophiles, however, they can also attack the M-L bond or the metal itself because, as 0e reagents, they do not alter the electron count of the complex (Section 2.3 ). The resulting mechanistic complexity and unreliable selectivity makes electrophilic attack far less controllable and less useful than nucleophilic attack. Polysubstitution is also more common in the electrophilic case [17 ].

  Addition to the Metal

  This happens during OA by the SN 2 or ionic mechanisms where initial Electrophilic Addition to the metal (Eq. (8.30 ) and Sections 6.3 and 6.5 ) is followed by substitution:

  8.30

  8.31

  Without the second step, the reaction becomes a pure Electrophilic Addition. For example, the highly nucleophilic Co(I) anion, [Co(dmg)2 py]− reacts with an alkyl triflate with inversion at carbon (Eq. (8.31 )).

  The addition of any zero‐electron ligand to the metal is also an Electrophilic Addition: AlMe3 , BF3 , HgCl2 , Cu+ , and even CO2 , when it binds η1 via carbon, all act in this way. Each of these reagents has an empty orbital by which it can accept a dπ electron pair from the metal.

  Addition to a Metal–Ligand Bond

  Protonation at L is common – for example, protonation of L

  n

  M–H can give a dihydrogen complex[18 ] [L

  n

  M–(H2 )]+ . Early metal alkyls such as Cp2 TiMe2

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  Organic Reaction Mechanisms 2015

  An annual survey covering the literature dated January to December 2015

  • A. C. Knipe(Author)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  CHAPTER 10 Addition Reactions: Polar Additions A. C. Knipe School of Biomedical Sciences, Ulster University, Coleraine, Northern Ireland Reviews . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430 Electrophilic Additions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430 Halogenation and Related Reactions . . . . . . . . . . . . . . . . . . . . . . . 430 Additions of Electrophilic S and Se . . . . . . . . . . . . . . . . . . . . . . . . 437 Additions of Brønsted Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . 440 Additions of Electrophilic Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . 441 Additions Initiated by Metal Ions as Electrophiles . . . . . . . . . . . . . . . . 442 (i) Boron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442 (ii) Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444 (iii) Iron and Ruthenium . . . . . . . . . . . . . . . . . . . . . . . . . . 447 (iv) Cobalt, Rhodium, and Iridium . . . . . . . . . . . . . . . . . . . . 448 (v) Nickel, Palladium, and Platinum . . . . . . . . . . . . . . . . . . . 455 (vi) Copper, Silver, and Gold . . . . . . . . . . . . . . . . . . . . . . . 463 (vii) Miscellaneous metal catalysts . . . . . . . . . . . . . . . . . . . . 471 Miscellaneous Electrophilic Additions . . . . . . . . . . . . . . . . . . . . . . 472 Nucleophilic Additions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 Additions of Multiple Bonds Conjugated with C=O . . . . . . . . . . . . . . . 473 (i) Boron nucleophiles . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 (ii) Sulfur nucleophiles . . . . . . . . . . . . . . . . . . . . . . . . . . 473 (iii) Oxygen nucleophiles . . . . . . . . . . . . . . . . . . . . . . . . . 473 (iv) Phosphorus nucleophiles . . . . . . . . . . . . . . . . . . . . . . . 473 (v) Nitrogen nucleophiles .

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