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  • Organic Chemistry OnLine 2000

    2000, Paul R. YoungUniversity of Illinois at Chicago, All Rights Reserved

    Conjugated Dienes &Cycloadditions

  • Organic Chemistry OnLine 2000

    CH3CH3

    CH3

    OH

    Vitamin A: Retinol

    A conjugated system consists of a series ofadjacent sp or sp2 centers such that there can

    be overlap of -electrons.

  • Organic Chemistry OnLine 2000

    Vitamin A: Retinol

    CH3CH3

    CH3

    OH

    A conjugated system consists of a series ofadjacent sp or sp2 centers such that there can

    be overlap of -electrons.

  • Organic Chemistry OnLine 2000

    Conjugated systems poses a series ofadjacent sp2 or sp centers

    Simple Conjugated Systems

  • Organic Chemistry OnLine 2000

    Alkene p-orbitals overlapto form a system

    Simple Conjugated Systems

  • Organic Chemistry OnLine 2000

    Conjugated double bondsoverlap to form a

    continuous system

    Alkene p-orbitals overlapto form a system

    Simple Conjugated Systems

  • Organic Chemistry OnLine 2000

    Conjugated double bondsoverlap to form a

    continuous system

    The overlap presents anenhanced barrier to

    rotation around singlebonds in conjugated

    systems.

    Simple Conjugated Systems

  • Organic Chemistry OnLine 2000

    NBS

    CCl4

    Br

    (CH3)3CO-

    (CH3)3COH

    Synthesis of Conjugated Dienes

  • Organic Chemistry OnLine 2000

    (CH3)3CO-

    (CH3)3COH

    Br

    NBS

    CCl4

    H

    Br

    Synthesis of Conjugated Dienes

  • Organic Chemistry OnLine 2000

    (CH3)3CO-

    (CH3)3COH

    Br

    NBS

    CCl4

    Br H

    Synthesis of Conjugated Dienes

  • Organic Chemistry OnLine 2000

    (CH3)3CO-

    (CH3)3COH

    Br

    NBS

    CCl4

    Synthesis of Conjugated Dienes

  • Organic Chemistry OnLine 2000

    (CH3)3CO-

    (CH3)3COH

    Br

    NBS

    CCl4

    Synthesis of Conjugated Dienes

  • Organic Chemistry OnLine 2000

    (CH3)3CO-

    (CH3)3COH

    Br

    NBS

    CCl4

    Br

    Synthesis of Conjugated Dienes

  • Organic Chemistry OnLine 2000

    Ionic Addition Reactions of ConjugatedDienes

    + HBr

    Br

    Br

    1,2 addition

    1,4 addition

    12

    3

    4

  • Organic Chemistry OnLine 2000

    + HBr

    Br

    Br

    1,2 addition

    1,4 addition

    12

    3

    4

    Ionic Addition Reactions of ConjugatedDienes

  • Organic Chemistry OnLine 2000

    Protonation on the terminal carbon generates the allylic carbocationwith cationic character on both carbons #1 and 3

    1,4 addition

    CH2

    H

    Br-

    CH2

    H

    allylic carbocation

    Br

    CH2

    H

    + HBr

  • Organic Chemistry OnLine 2000

    1,2 & 1,4 Ionic Addition Reactions ofConjugated Dienes

    The 1,2- addition product forms rapidly at lowtemperatures.

  • Organic Chemistry OnLine 2000

    1,2 & 1,4 Ionic Addition Reactions ofConjugated Dienes

    The 1,2- addition product forms rapidly at lowtemperatures.

    The 1,4-addition product is predominant at highertemperatures.

  • Organic Chemistry OnLine 2000

    1,2 & 1,4 Ionic Addition Reactions ofConjugated Dienes

    The 1,2- addition product forms rapidly at lowtemperatures.

    The 1,4-addition product is predominant at highertemperatures.

    Even at low temperatures, 1,4-addition products willpredominate if given enough time.

  • Organic Chemistry OnLine 2000

    1,2 & 1,4 Ionic Addition Reactions ofConjugated Dienes

    The 1,2- addition product forms rapidly at lowtemperatures.

    The 1,4-addition product is predominant at highertemperatures.

    Even at low temperatures, 1,4-addition products willpredominate if given enough time.

    The addition of HBr to butadiene is reversible and isolated1,2-addition product will convert to the 1,4-product athigher temperatures or at longer times.

  • Organic Chemistry OnLine 2000

    lower activation energyfaster reaction

    high activation energy,slower reaction

    Br

    Br

    Ene

    rgy

    Effect of Temperature and Time on ProductDistribution

  • Organic Chemistry OnLine 2000

    lower activation energyfaster reaction

    high activation energy,slower reaction

    Br

    Br

    Ene

    rgy

    Effect of Temperature and Time on ProductDistribution

  • Organic Chemistry OnLine 2000

    G G

    Ene

    rgy

    Therefore, 1,2 addition isfaster, but forms a less stableproduct, while 1,4 addition is

    slower, but gives a morestable product.

    Effect of Temperature and Time on ProductDistribution

    smaller G, lessfavored at equilibrium,but formed faster

    larger G, morefavored at equilibrium,but formed more slowly

    Br

    Br

  • Organic Chemistry OnLine 2000

    G G smaller G, lessfavored at equilibrium,but formed faster

    larger G, morefavored at equilibrium,but formed more slowly

    Br

    Br

    Ene

    rgy

    ...at equilibrium (ThermodynamicControl) the more stable 1,4

    addition product will be favored,but if you examine the initialproduct distribution, the more

    rapidly formed 1,2 product willpredominate (Kinetic Control).

    Effect of Temperature and Time on ProductDistribution

  • Organic Chemistry OnLine 2000

    + HBrCH3

    kinetic product

    thermodynamic product

  • Organic Chemistry OnLine 2000

    + HBrCH3

    Br

    Br

    kinetic product

    thermodynamic product

  • Organic Chemistry OnLine 2000

    Br

    Br

    H

    H

    3 carbocation initiallyformed; product also has

    less steric hindrance

    2 carbocation initiallyformed

  • Organic Chemistry OnLine 2000

    Cycloaddition Reactions

    The Diels-Alder Reaction

  • Organic Chemistry OnLine 2000

    +heat

    +heat

    +heat

    O

    CH3

    C

    O

    CH3

    4 + 2 Cycloaddition Reactions

    a diene... ...and adienophile

  • Organic Chemistry OnLine 2000

    4 + 2 Cycloaddition Reactions

    a diene... ...and adienophile

    +heat

    +heat

    +heat

    O

    CH3

    C

    O

    CH3

  • Organic Chemistry OnLine 2000

    heat

    O

    CH3

    C

    O

    CH3

    a diene a dieneophile

    Cycloaddition reactions work best with dienes containingelectron donating substituents and dienophiles containing

    electron withdrawing substituents.

  • Organic Chemistry OnLine 2000

    s-cis s-trans

    free rotation

    s-cis stereochemistry is required for a4+2 cycloaddition reaction

  • Organic Chemistry OnLine 2000

    concertedtransition

    state

  • Organic Chemistry OnLine 2000

    a diene

    a dieneophile

    CNNC NC CN

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 1999Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

  • Drawing the cyclohexene as a chair allows thestereochemistry of attached groups to be clearly described;the cyclohexene ring, however, is actually distorted due to

    the 120 bond angles of the sp2 centers.

    Organic Chemistry OnLine 2000

  • Organic Chemistry OnLine 2000

    C

    CH

    HH

    H

    C

    CH

    CH

    H

    H

    O

    C

    CH

    CH

    H

    CH3

    O

    C

    CH

    CH

    H

    OCH3

    O

    C

    C

    H

    H CO

    C

    O

    O

    C

    CC

    CH

    H

    C

    O

    C

    O

    H

    H

    C

    CH

    CH

    H

    NC

    C

    COOCH3

    H

    poor

    Common Dienophiles

  • Organic Chemistry OnLine 2000

    H

    H

    H

    H

    H

    H

    H

    H

    H

    H

    H

    H

    ...benzene can be formed from a 2+2+2 cycloaddition reaction

  • Organic Chemistry OnLine 2000

    CN

    +

    COOCH3+

    CHO

    +

    C

    O

    CH3+

  • Organic Chemistry OnLine 2000

    CN

    +

    COOCH3+

    CHO

    +

    C

    O

    CH3+

    CN

  • Organic Chemistry OnLine 2000

    CN

    +

    COOCH3+

    CHO

    +

    C

    O

    CH3+

    CN

    COOCH3

  • Organic Chemistry OnLine 2000

    CN

    +

    COOCH3+

    CHO

    +

    C

    O

    CH3+

    CN

    COOCH3

    CHO

  • Organic Chemistry OnLine 2000

    CN

    +

    COOCH3+

    CHO

    +

    C

    O

    CH3+

    CN

    COOCH3

    CHO

    C

    O

    CH3

  • Organic Chemistry OnLine 2000

    O

    O

    CN+

    +

    C

    O

    CH3+

  • Organic Chemistry OnLine

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