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30. Orbitals and Organic Chemistry: Pericyclic Reactions. Based on McMurry’s Organic Chemistry , 7 th edition. Pericyclic Reactions – What Are They?. Involves several simultaneous bond-making breaking process with a cyclic transition state involving delocalized electrons

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30. Orbitals and Organic Chemistry: Pericyclic Reactions

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30 orbitals and organic chemistry pericyclic reactions l.jpg

30. Orbitals and Organic Chemistry: Pericyclic Reactions

Based on McMurry’s Organic Chemistry, 7th edition

Pericyclic reactions what are they l.jpg

Pericyclic Reactions – What Are They?

  • Involves several simultaneous bond-making breaking process with a cyclic transition state involving delocalized electrons

  • The combination of steps is called a concerted process where intermediates are skipped

    Why this chapter?

  • To gain a better understanding of pericyclic reactions

  • Understanding biological pathways where these reactions do occur

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30.1 Molecular Orbitals and Pericyclic Reactions of Conjugated  Systems

  • A conjugated diene or polyene has alternating double and single bonds

  • Bonding MOs are lower in energy than the isolated p atomic orbitals and have the fewest nodes

  • Antibonding MOs are higher in energy

  • See Figure 30.1 for a diagram

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  • Three double bonds and six  MOs

  • Only bonding orbitals, 1, 2, and 3, are filled in the ground state

  • On irradiation with ultraviolet light an electron is promoted from 3 to the lowest-energy unfilled orbital (4*)

  • This is the first (lowest energy) excited state

  • See the diagram in Figure 30.2

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Molecular Orbitals and Pericyclic Reactions

  • If the symmetries of both reactant and product orbitals match the reaction is said to be symmetry allowed under the Woodward-HoffmannRules (these relate the electronic configuration of reactants to the type of pericyclic reaction and its stereochemical imperatives)

  • If the symmetries of reactant and product orbitals do not correlate, the reaction is symmetry-disallowed and there are no low energy concerted paths

  • Fukui’s approach: we need to consider only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the frontier orbitals

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30.2 Electrocyclic Reactions

  • These are pericyclic processes that involve the cyclization of a conjugated polyene

  • One  bond is broken, the other  bonds change position, a new σ bond is formed, and a cyclic compound results

  • Gives specific stereoisomeric outcomes related to the stereochemistry and orbitals of the reactants

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Electrocyclic Interconversions with Octatriene

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Electrocyclic Interconversions with Dimethylcyclobutene

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The Signs on the Outermost Lobes Must Match to Interact

  • The lobes of like sign can be either on the same side or on opposite sides of the molecule.

  • For a bond to form, the outermost  lobes must rotate so that favorable bonding interaction is achieved

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Disrotatory Orbital Rotation

  • If two lobes of like sign are on the same side of the molecule, the two orbitals must rotate in opposite directions—one clockwise, and one counterclockwise

  • Woodward called this a disrotatory (dis-roh-tate’-or-ee) opening or closure

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Conrotatory Orbital Rotation

  • If lobes of like sign are on opposite sides of the molecule: both orbitals must rotate in the same direction, clockwise or counterclockwise

  • Woodward called this motion conrotatory (con-roh-tate’-or-ee)

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30.3 Stereochemistry of Thermal Electrocyclic Reactions

  • Determined by the symmetry of the polyene HOMO

  • The ground-state electronic configuration is used to identify the HOMO

  • (Photochemical reactions go through the excited-state electronic configuration )

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Ring Closure of Conjugated Trienes

  • Involves lobes of like sign on the same side of the molecule and disrotatory ring closure

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Contrast: Electrocyclic Opening to a Diene

  • Conjugated dienes and conjugated trienes react with opposite stereochemistry

  • Different symmetries of the diene and triene HOMOs

  • Dienes open and close by a conrotatory path

  • Trienes open and close by a disrotatory path

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30.4 Photochemical Electrocyclic Reactions

  • Irradiation of a polyene excites one electron from HOMO to LUMO

  • This causes the old LUMO to become the new HOMO, with changed symmetry

  • This changes the reaction stereochemistry (symmetries of thermal and photochemical electrocylic reactions are always opposite)

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Rules for Electrocyclic Reactions

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30.5 Cycloaddition Reactions

  • Two unsaturated molecules add to one another, yielding a cyclic product

  • The Diels–Alder cycloaddition reaction is a pericyclic process that takes place between a diene (four  electrons) and a dienophile (two  electrons) to yield a cyclohexene product Stereospecific with respect to substituents

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Rules for Cylcoadditions - Suprafacial Cycloadditions

  • The terminal  lobes of the two reactants must have the correct symmetry for bonding to occur

  • Suprafacial cycloadditions take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on the same face of the other reactant

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Rules for Cylcoadditions - Antarafacial Cycloadditions

  • These take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on opposite faces of the other reactant (not possible unless a large ring is formed)

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30.6 Stereochemistry of Cycloadditions

  • HOMO of one reactant combines with LUMO of other

  • Possible in thermal [4 +2] cycloaddition

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[2+2] Cylcoadditions

  • Only the excited-state HOMO of one alkene and the LUMO can combine by a suprafacial pathway in the combination of two alkenes

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Formation of Four-Membered Rings

  • Photochemical [2 + 2] cycloaddition reaction occurs smoothly

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30.7 Sigmatropic Rearrangements

  • A s -bonded substituent atom or group migrates across a  electron system from one position to another

  • A s bond is broken in the reactant, the  bonds move, and a new s bond is formed in the product

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Sigmatropic Notation

  • Numbers in brackets refer to the two groups connected by the  bond and designate the positions in those groups to which migration occurs

  • In a [1,5] sigmatropic rearrangement of a diene migration occurs to position 1 of the H group (the only possibility) and to position 5 of the pentadienyl group

  • In a [3,3] Claisen rearrangement migration occurs to position 3 of the allyl group and also to position 3 of the vinylic ether

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Sigmatropic Stereospecificity: Suprafacial and Antarafacial

  • Migration of a group across the same face of the  system is a suprafacial rearrangement

  • Migration of a group from one face of the  system to the other face is called an antarafacial rearrangement

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Stereochemical Rules of Sigmatropic Rearrangements

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30.8 Some Examples of Sigmatropic Rearrangements

  • A [1,5] sigmatropic rearrangement involves three electron pairs (two  bonds and one s bond)

  • Orbital-symmetry rules predict a suprafacial reaction

  • 5-methylcyclopentadiene rapidly rearranges at room temperature

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Another Example of a Sigmatropic Rearrangement

  • Heating 5,5,5-trideuterio-(1,3Z)-pentadiene causes scrambling of deuterium between positions 1 and 5

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Orbital Picture of a Suprafacial [1,5]-H Shift

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Cope and Claisen Rearrangements are Sigmatropic

  • Cope rearrangement of 1,5-hexadiene

  • Claisen rearrangement of an allyl aryl ether

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Suprafacial [3,3] Cope and Claisen Rearrangements

  • Both involve reorganization of an odd number of electron pairs (two  bonds and one s bond)

  • Both react by suprafacial pathways

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30.9 A Summary of Rules for Pericyclic Reactions

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