30. Orbitals and Organic Chemistry: Pericyclic ReactionsBased on McMurrys Organic Chemistry, 6th edition

Pericyclic Reactions What Are?Involves several simultaneous bond-making breaking process with a cyclic transition state involving delocalized electronsThe combination of steps is called a concerted process where intermediates are skipped

30.1 Molecular Orbitals of Conjugated SystemsA conjugated diene or polyene has alternating double and single bondsBonding MOs are lower in energy than the isolated p atomic orbitals and have the fewest nodes Antibonding MOs are higher in energySee Figure 30.1 for a diagram

1,3,5-HexatrieneThree double bonds and six MOsOnly bonding orbitals, 1, 2, and 3, are filled in the ground stateOn irradiation with ultraviolet light an electron is promoted from 3 to the lowest-energy unfilled orbital (4*)This is the first (lowest energy) excited stateSee the diagram in Figure 30.2

30.2 Molecular Orbitals and Pericyclic ReactionsIf the symmetries of both reactant and product orbitals match the reaction is said to be symmetry allowed under the Woodward-Hoffmann Rules (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 no low energy concerted pathsFukuis approach: we need to consider only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the frontier orbitals

30.3 Electrocyclic ReactionsThese are pericyclic processes that involves the cyclization of a conjugated polyeneOne bond is broken, the other bonds change position, a new bond is formed, and a cyclic compound resultsGives specific stereoisomeric outcomes related to the stereochemistry and orbitals of the reactants

Example: Electrocyclic Interconversions With Octatriene

Example: Electrocyclic Interconversions with Dimethylcyclobutene

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 RotationIf two lobes of like sign are on the same side of the molecule, the two orbitals must rotate in opposite directionsone clockwise, and one counterclockwiseWoodward called this a disrotatory (dis-roh-tate-or-ee) opening or closure

Conrotatory Orbital RotationIf lobes of like sign are on opposite sides of the molecule: both orbitals must rotate in the same direction, clockwise or counterclockwiseWoodward called this motion conrotatory (con-roh-tate-or-ee)

30.4 Stereochemistry of Thermal Electrocyclic ReactionsDetermined by the symmetry of the polyene HOMOThe ground-state electronic configuration is used to identify the HOMO (Photochemical reactions go through the excited-state electronic configuration )

Ring Closure of Conjugated TrienesInvolves lobes of like sign on the same side of the molecule and disrotatory ring closure

Contrast: Electrocyclic Opening to DieneConjugated dienes and conjugated trienes react with opposite stereochemistryDifferent symmetries of the diene and triene HOMOsDienes open and close by a conrotatory pathTrienes open and close by a disrotatory path

30.5 Photochemical Electrocyclic ReactionsIrradiation of a polyene excites one electron from HOMO to LUMOThis causes the old LUMO to become the new HOMO, with changed symmetryThis changes the reaction stereochemistry (symmetries of thermal and photochemical electrocylic reactions are always opposite)

Rules for Electrocyclic Reactions

30.6 Cycloaddition ReactionsTwo unsaturated molecules add to one another, yielding a cyclic productThe DielsAlder 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 CycloadditionsThe terminal lobes of the two reactants must have the correct symmetry for bonding to occurSuprafacial 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 CycloadditionsThese 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)

30.7 Stereochemistry of CycloadditionsHOMO of one reactant combines with LUMO of otherPossible in thermal [4 +2] cycloaddition

[2+2] CylcoadditionsOnly 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 RingsPhotochemical [2 + 2] cycloaddition reaction occurs smoothly

30.8 Sigmatropic RearrangementsA s -bonded substituent atom or group migrates across a p electron system from one position to anotherA s bond is broken in the reactant, the p bonds move, and a new s bond is formed in the product

Sigmatropic NotationNumbers in brackets refer to the two groups connected by the s bond and designate the positions in those groups to which migration occursIn 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 groupIn 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 AntarafacialMigration of a group across the same face of the system is a suprafacial rearrangementMigration of a group from one face of the system to the other face is called an antarafacial rearrangement

Stereochemical Rules of Sigmatropic RearrangementsHH

30.9 Some Examples of Sigmatropic RearrangementsA [1,5] sigmatropic rearrangement involves three electron pairs (two bonds and one s bond)Orbital-symmetry rules predict a suprafacial reaction5-methylcyclopentadiene rapidly rearranges at room temperature

Another Example of a Sigmatropic RearrangementHeating 5,5,5-trideuterio-(1,3Z)-pentadiene causes scrambling of deuterium between positions 1 and 5

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

Cope and Claisen Rearrangements are SigmatropicCope rearrangement of 1,5-hexadieneClaisen rearrangement of an allyl aryl ether

Suprafacial [3,3] Cope and Claisen RearrangementsBoth involve reorganization of an odd number of electron pairs (two bonds and one s bond)Both react by suprafacial pathways

30.10 A Summary of Rules for Pericyclic Reactions