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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 Based on McMurry’s Organic Chemistry, 7th edition
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
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
1,3,5-Hexatriene • 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
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
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
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
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
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)
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 )
Ring Closure of Conjugated Trienes • Involves lobes of like sign on the same side of the molecule and disrotatory ring closure
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
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)
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
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
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)
30.6 Stereochemistry of Cycloadditions • HOMO of one reactant combines with LUMO of other • Possible in thermal [4 +2] cycloaddition
[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
Formation of Four-Membered Rings • Photochemical [2 + 2] cycloaddition reaction occurs smoothly
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
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
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
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
Another Example of a Sigmatropic Rearrangement • Heating 5,5,5-trideuterio-(1,3Z)-pentadiene causes scrambling of deuterium between positions 1 and 5
Cope and Claisen Rearrangements are Sigmatropic • Cope rearrangement of 1,5-hexadiene • Claisen rearrangement of an allyl aryl ether
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