The Diels-Alder Reaction - ??pericyclic reaction ... What is the product of the Diels-Alder reaction that ... Orbitals and Chemical ReactionsPublished in: Science 1984Authors: M J Goldstein

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  • The The DielsDiels-Alder Reaction-Alder Reaction

    Synthetic method for preparing Synthetic method for preparing compounds containing a compounds containing a cyclohexene cyclohexene ringring conjugated conjugated

    dienedienealkene alkene

    ((dienophiledienophile))cyclohexenecyclohexene

    ++

    In general...In general...

    transition statetransition state

    viavia

    concerted mechanismconcerted mechanism

    [4+2] cycloadditioncycloaddition

    pericyclic pericyclic reactionreaction

    a concerted reaction that proceeds a concerted reaction that proceeds through a cyclic transition statethrough a cyclic transition state

    Mechanistic featuresMechanistic features

    Only the s-cis conformation of the dienecan react

    +

    diene dienophile transitionstate

    new bond

    new bond

  • What makes a reactive What makes a reactive dienophiledienophile??

    The most reactive The most reactive dienophiles dienophiles have anhave anelectron-withdrawing group (electron-withdrawing group (EWGEWG) directly) directlyattached to the double bond.attached to the double bond.

    Typical Typical EWGs EWGs

    CC OO

    CC NN

    CC CC

    EWGEWG

    ++

    100100CC

    (100%)(100%)

    HH22CC CHCHCHCH CHCH22 HH22CC CHCH CHCH

    OO

    CHCH

    OO

    ExampleExample

    ++

    100100CC

    (100%)(100%)

    HH22CC CHCHCHCH CHCH22 HH22CC CHCH CHCH

    OO

    CHCH

    OO

    ExampleExample

    CHCH

    OOvia:via:

    ++

    100100CC

    (100%)(100%)

    OO

    OO

    OO

    ExampleExample

    HH22CC CHCCHC CHCH22

    CHCH33

    HH33CCOO

    OO

    OO

  • ++

    benzenebenzene 100100CC

    (100%)(100%)

    OO

    OO

    OO

    ExampleExample

    HH22CC CHCCHC CHCH22

    CHCH33

    HH33CCOO

    OO

    OO

    via:via:

    HH33CC

    OO

    OO

    OO

    ++

    100100CC

    (98%)(98%)

    HH22CC CHCHCHCH CHCH22

    Acetylenic DienophileAcetylenic Dienophile

    OO

    CCOCHCCOCH22CHCH33CHCH33CHCH22OCCOCC

    OO

    COCHCOCH22CHCH33

    COCHCOCH22CHCH33

    OO

    OO

    Stereospecific syn Stereospecific syn additionaddition

    cis-trans relationship of cis-trans relationship of substituents substituents of of alkene alkene is retained in the productis retained in the product

    DielsDiels-Alder Reaction-Alder Reaction

    The most reactive The most reactive dienophiles dienophiles have anhave anelectron-withdrawing group (electron-withdrawing group (EWGEWG) directly) directlyattached to the double bond.attached to the double bond.

    The most reactive The most reactive dienes dienes have an electron-have an electron-donating (releasing) group (donating (releasing) group (ERGERG) directly) directlyattached to the double bond.attached to the double bond. Eg Eg. . -OR -OR

    Only product.Only product.But, is it But, is it enantiomericenantiomeric??

    ++HH22CC CHCHCHCH CHCH22 CC CC

    CC66HH55 COHCOH

    HH HH

    OO

    HH

    CC66HH55

    HH

    COHCOH

    OO

    Only the s-cisconformation ofthe diene canparticipate in a

    DielsAlderreaction

    Cis Cis Cis Cis

    ((racemicracemic))

    QuestionQuestion

    Rank the compounds below in order ofRank the compounds below in order ofincreasing rate of the increasing rate of the DielsDiels-Alder reaction.-Alder reaction.

    A)A) 1 < 2 < 31 < 2 < 3 B)B) 3 < 1 < 23 < 1 < 2

    C)C) 3 < 2 < 13 < 2 < 1 D)D) 2 < 3 < 12 < 3 < 1only productonly product

    ((racemicracemic))

    ++HH22CC CHCHCHCH CHCH22

    Trans TransTrans Trans

    CC CC

    CC66HH55

    COHCOHHH

    HH

    OO

    HH

    CC66HH55

    HH

    COHCOH

    OO

  • QuestionQuestion

    What is the product of the What is the product of the DielsDiels-Alder reaction that-Alder reaction thatoccurs between the reactants shown here?occurs between the reactants shown here?

    A) A) B)B) C) C) D)D)

    Stereospecific, concerted, syn addition:

    Predict the reaction products:

    1. Consider the alignment of the reactants

    2. Consider the charge distribution in each of the reactants

    QuestionQuestion

    Select the correct starting materials to makeSelect the correct starting materials to makethe compound at the right through thethe compound at the right through the

    DielsDiels-Alder reaction.-Alder reaction.

    A) A) B)B)

    C)C) D)D)

  • Cyclic Cyclic dienes dienes yield bridged yield bridged bicyclicbicyclicDielsDiels-Alder adducts.-Alder adducts.

    Since only cis dienes can participate in DielsAlderreactions: 5- & 6- membered rings are ideal

    ++

    CC CC

    COCHCOCH33HH

    HH

    OO

    CHCH33OCOC

    OO HH

    HH

    COCHCOCH33

    OO

    COCHCOCH33

    OO

    is theis thesame assame as

    HH

    HH

    COCHCOCH33

    OO

    COCHCOCH33

    OO

    HH

    HH

    COCHCOCH33

    OO

    COCHCOCH33

    OOThe The Molecular Molecular OrbitalsOrbitals

    ofof

    Ethylene and 1,3-ButadieneEthylene and 1,3-Butadiene

  • Orbitals Orbitals and Chemical Reactionsand Chemical Reactions

    A deeper understanding of chemical reactivityA deeper understanding of chemical reactivitycan be modeled using can be modeled using frontier frontier orbitals orbitals of theof thereactants.reactants.

    Electrons flow from the highest occupiedElectrons flow from the highest occupiedmolecular orbital (HOMO) of one reactant tomolecular orbital (HOMO) of one reactant tothe lowest unoccupied molecular orbitalthe lowest unoccupied molecular orbital(LUMO) of the other.(LUMO) of the other.

    We can illustrate HOMO-LUMO interactions byWe can illustrate HOMO-LUMO interactions byway of the way of the DielsDiels-Alder reaction between-Alder reaction betweenethylene and 1,3-butadiene.ethylene and 1,3-butadiene.

    We need only consider only the We need only consider only the electrons of electrons ofethylene and 1,3-butadiene. We can ignoreethylene and 1,3-butadiene. We can ignorethe framework of the framework of bonds in each molecule. bonds in each molecule.

    Orbitals Orbitals and Chemical Reactionsand Chemical Reactions

    Reactants HOMO and LUMO

    LUMO of ethylene (LUMO of ethylene (dienophiledienophile))

    HOMO of 1,3-butadieneHOMO of 1,3-butadiene

    HOMO of 1,3-butadieneHOMO of 1,3-butadieneand LUMO of ethyleneand LUMO of ethyleneare in phase with oneare in phase with oneanotheranother

    allows allows bond formation bond formationbetween the between the alkene alkene andandthe the dienediene

    MO Analysis of MO Analysis of DielsDiels-Alder Reaction-Alder Reaction

    Two Possible Configurations of BridgedBicyclic Compounds

    Secondary orbital overlap favors the endo productformation

  • A A Molecular Orbital Analysis Molecular Orbital Analysis

    of theof the

    DielsDiels-Alder Reaction-Alder Reaction

    MO Analysis of MO Analysis of DielsDiels-Alder Reaction-Alder Reaction

    Inasmuch as electron-withdrawing groupsInasmuch as electron-withdrawing groupsincrease the reactivity of a increase the reactivity of a dienophiledienophile, we, weassume electrons flow from the HOMO of theassume electrons flow from the HOMO of thediene diene to the LUMO of the to the LUMO of the dienophiledienophile..

    LUMO of ethylene (LUMO of ethylene (dienophiledienophile))

    HOMO of 1,3-butadieneHOMO of 1,3-butadiene

    MO Analysis of MO Analysis of DielsDiels-Alder Reaction-Alder Reaction A "forbidden" reactionA "forbidden" reaction

    The The dimerization dimerization of ethylene to give of ethylene to give cyclobutanecyclobutanedoes not occur under conditions of typicaldoes not occur under conditions of typicalDielsDiels-Alder reactions.-Alder reactions.

    HH22CC CHCH22

    HH22CC CHCH22

    ++

    A "forbidden" reactionA "forbidden" reaction

    HH22CC CHCH22

    HH22CC CHCH22

    ++

    HOMO of HOMO of one ethyleneone ethylenemoleculemolecule

    LUMO of LUMO of other ethyleneother ethylenemoleculemolecule

    HOMO-LUMOHOMO-LUMOmismatch of twomismatch of twoethylene moleculesethylene moleculesprecludes single-stepprecludes single-stepformation of two newformation of two new bondsbonds

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