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Pericyclic Reactions
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Transcript of Pericyclic Reactions - libvolume6.xyzlibvolume6.xyz/.../pericyclicreactionstutorial2.pdf · Three...

  • Pericyclic Reactions

  • Class test on 10/02/07 (3:30 PM to 4:30 PM)Venue will be intimated laterAll four topics of Organic

  • Three Classes of Organic Reactions

    Polar reactions

    Radical reactions

    CH3CH2 Cl Cl CH3CH2Cl Cl+ +

    OH

    OH

    OH+

    OH

    OH

    OH

    O

    OH

    OHH

    O

    OH2

    OH

    O

    O H

    O O

    ....

    Y R X R Y X+ +R = aliphatic as well as aromatic

    Pericyclic reactions

    This reaction occurs as a result of a cyclic reorganization of electrons

  • Three most common types of pericyclic reactionsElcetrocyclic reactions

    new bond

    Cycloaddition reactions

    +

    new bond

    new bondSigmatropic reactions

    HH3C H3CCH2H

    new bond is formed

    H3C H3C

    bond is broken in the middle of the system

    Diels-Alder reaction

    bond broken at the end of the -system

  • Certain common features of pericyclic reactions

    They are all concerted reactions, electron reorganization takes place in single step. Therefore, there is one TS and no intermediates

    As the reactions are concerted, they are highlystereoselective

    The reactions are generally not effected by catalysts or by a change in solvent

    Product configuration depends onConfiguration of the reactant The number of double bonds in the reactantWhether the reaction is photochemical or thermal

  • Some pericyclic reactions are induced thermally or photochemically

    [4+2]-cycloaddition (Diels-Alder reaction)

    X

    Y

    h +X

    YNo reaction

    [2+2]-cycloaddition

    Ph

    Ph

    Ph

    Ph Ph

    Ph

    Ph

    Ph+h No reaction

  • Some pericyclic reactions are induced thermally and photochemically

    Me

    Me Me

    Me

    Me

    Me

    h

    Electrocyclic reaction

    Me

    Me

    Me

    Me

    hMe

    Me

  • Conservation of Orbital Symmetry Theory

    R. B. Woodward and Roald Hoffmann (1965)

    In phase orbitals overlap during the course of a pericyclic reaction

    Based on Frontier orbital theory (1954, Fukui)

    (HOMO, LUMO)

    Molecular orbital symmetry controls Weather or not compound undergoes a reaction under particular conditions What product will be formed

  • A molecular orbital description (ethene)

    (zero probability of finding electrons)

    Overlap of in-phase p orbitals gives a bonding molecular orbital that is lower in energy than the p atomic orbitals.

    Overlap of out-of-phase p atomic orbitals gives an antibonding molecular orbital that is higher in energy than the p atomic orbitals.

  • 1,3-butadiene

    Asymmetric

    Nodes are more than no. of bonding interactions

    Symmetric

    Asymmetric

    bonding interactions are more than no. of nodes

    Symmetric

    Four p atomic orbitals overlap to give the four molecular orbitals

  • Electrocyclic Reactions

    These reactions are defined as involving the cyclization of an n pi-electronsystem to an (n-2)pi + 2sigma-electron system or the reverse process.

    n() n-2 () + 2 ( )

    The reactions are reversible.

    Observance of ring opening and ring closure depends upon the thermodynamic stability of the open and closed forms.

  • Two distinct modes are possible.

    -system

    BA A B

    B B

    A A

    -system

    disrotatoryring closure

    -system

    BA A B

    B A

    A B

    -system

    conrotatoryring closure

  • HOMO(2E,4Z)-hexadiene

    HOMO(2E,4E)-hexadiene

  • Elcetrocyclic reactions

    CH3

    HCH3

    H

    CH3

    HCH3

    Hh H

    CH3CH3

    HCH3

    HH

    CH3

    CH3

    HH

    CH3

    hCH3

    HCH3

    H

    Con

    Dis

    Con

    Dis

  • Selection rules for electrocyclic reactions

    Woodward-Hoffmann rules

    No. of -electrons

    Thermally allowed, photochemically forbidden

    Thermally forbidden, photochemically

    allowed4n Conrotatory Disrotatory

    4n+2 Disrotatory Conrotatory

    n = integer

  • CH3

    HH

    CH3

    CH3

    CH3h

    disrotatio

    n

    conrotation

    CH3

    CH3

    CH3

    HCH3

    H

    CH3

    CH3h

    disrotat

    ion

    conrotation

    CH3

    CH3

  • 1,3,5-hexatriene

    Six p atomic orbitals overlap to give the six molecular orbitals

  • HOMO

    LUMO

  • Q. Have the following reactions proceeded in the conrotatory or disrotatorymanner? Should they proceed under thermal or photochemical influence?

    H HH H

    * disrotatory

    *

    CH3

    CH3

    HH

    CH3HH

    CH3

    Q. Show that the cyclobutane below open by two alternative conrotarory procsses.What is the product in each instance? Do you expect them to be formed in equalamounts?

    CH3

    HH

    CH3

    H

    CH3

    H

    CH3

    CH3

    H

    CH3

    H

    anticlockwise clockwise

    conrotatory

  • A

    B100 C

    h-20 C

    A' B'25 C

    H

    H

    Disrotatory Conrotatory

    H

    H

    Disrotatory

    A & A are stereoisomersB & B are stereoisomers

  • Sigmatropic reactionsThese reactions are defined as involving migration of a bond that

    is flanked by one or more conjugated systems to a new position within the system

    etc

    etc

    migrating bond

    The reaction is termed [ i, j ] sigmatropic shift when the bond migrates from position [1,1] to position [i,j].

    Examples: 1

    23

    12

    3 1

    1

    2

    2

    3

    3[3,3] sigmatropic shift

    H

    12

    3

    [1,3] sigmatropic shiftH

    12

    3

  • Q. Classify the following sigmatropic reactions of order [i,j]

    H

    D D

    H

    H3C

    H

    H D2C

    H H

    HH

    H

    [1,5]*

    S

    R1 R2S

    R1 R2

    [2,3]*

    CH3H CH3

    [1,5]*

  • A Cope rearrangementC6H5

    CH3

    C6H5

    CH3

    A Claisen rearrangement

    OCH3

    O

    CH3

    Allyl vinyl ether

    Q. Give the product of the following reaction.O

    O[3,3]

    O

    H

    OH

  • Q. Suggest a mechanism

    O

    H3CCH3

    OH

    H3CCH3

    O

    H3C CH3

    O

    H3CCH3

    O

    H3CCH3

    H

  • HO

    [3,3]slow

    HO O

    KH

    O O

    H3O+fast[3,3]

    KH

    HO

    THF

  • A biological reaction involving an electrocyclicreaction and a sigmatropic reaction

    7-dehydrocholesterol, a steriod formed in skin, is converted intoVitamin D3 by two pericyclic reactions

    Deficiency in vitamin D causes rickets. Deficiency in vitamin D can be prevented by getting

    enough sun. Too much vitamin D is also harmful causes calcification of soft tissuesskin pigmentation protects the skin from the suns UV rays (it prevents the synthesis of too much vitamin D3)

  • CH3

    CH3CH3

    H

    HO

    CH3

    H

    H3C CH3

    CH3CH3

    H

    HO

    CH3

    H3C

    provitamin D3

    an electrocyclic reaction

    h

    7-dehydro cholesterol

    CH3

    CH3CH3

    H

    HO

    CH2

    H3C

    vitamin D3

    [1,7] sigmatropic shift

  • An industrial synthesis of citral

    CHO

    citrala key intermediate in the synthesis of vitamin A

    - H2O

    O [3,3]

    Clasienrearrangment

    O O

    [3,3] Coperearrangement

    CHO

    OH

    +

  • Cycloaddition reactions

    H3C CH3H3C CH3

    OO O

    OO

    O

    a [4+2] cycloaddition reaction(Diels-Alder reaction)

    +

    + O

    O

    O

    heat

    UV light O

    O

    O

    a [2+2]cycloaddition reaction

  • Cycloaddition reactions

    Why does maleic anhydride react easily with butadiene,but not at all with ethylene?

    O

    O

    O

    + O

    O

    O

    O

    O

    O

    + O

    O

    O

  • HOMO of one reactant should react with LUMO of the other reactant

    Thermal Photo chemical

    HOMO Excited state HOMO

    LUMOLUMO

    HOMO LUMO

    LUMO HOMO

  • Q. Classify the following as [m+n] cycloaddition reactions

    O

    CH3

    CH2

    CH2 OCH3

    CH3

    O

    OCH3

    +

    +

    +

    *

    *

    *

  • General description of Diels-Alder reaction CHO

    NO2

    NO2

    CHO

    +

    diene dienophile adduct

    adductdiene dienophile

    Dienes permanently in s-cis conformation undergo Diels-Alder reactions with ease

    O

    Dienes permanently in s-trans conformation cannot undergo Diels-Alder reaction

  • Q. Which of the following do not respond to the Diels-Alder reaction as a diene?

    Q. Arrange the following dienes according to their reactivity towards Diels-Alder reaction

    ButBut

    But

    &

  • Q. Following compounds are synthesized by Diels-Alder reaction, find out suitable dienes and dienophiles for their synthesis.

    O

    CN

    CN

    MeCO2Me

    CO2Me

    OH

    H MeO

    O

    O

  • Captan

    (an industrial application of Diels-Alder reaction)

    ClS

    CCl3

    O

    O

    O

    N

    H

    H

    O

    O

    S CCl3

    O

    H

    H

    O

    O

    NH3NH

    H

    H

    O

    O

    +

    Captan(agricultural fungicide)

  • The endo rule for Diels-Alder reaction

    O

    H

    H

    O

    O

    'endo' adduct (formed)

    H

    H

    'endo' adduct

    +

    O+ O

    H

    H

    O

    O

    O

    O

    O

    O

    O

    'exo' adduct (not formed)H

    H

  • Diels-Alder reaction : endo rule

    endorelationshipbetween two alkenes

    Secondary bonding Interactions leading to endo product

    Primary bonding interactions leading to new bonds

    HOMO

    LUMO

  • Reaction of cyclopentadiene with maleic anhydride

    OO O

    HOMO

    LUMO

    primary bonding interaction

    secondary bonding interaction

  • C lC l

    C l

    C l C l

    C l

    A lderin

    C lC l

    C l

    C l C l

    C l

    D ie ldrin

    O

    In 1950's tw o very effective pesticides

    O

    O

    O

    Cl ClCl

    Cl

    Cl

    Cl

    flame retardent

  • Doubts: Room C211, 5.00 to 6.00 PMChemistry Department

    Inorganic classes will be started from next week onwards