Transcript Chemdraw B&W - Pennsylvania State University

30. Orbitals and Organic
Chemistry: Pericyclic Reactions
Based on McMurry’s Organic Chemistry, 6th edition
Pericyclic Reactions – What
Are?
• 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
30.1 Molecular Orbitals 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
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-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 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
30.3 Electrocyclic Reactions
• These are pericyclic processes that involves 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
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 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-rohtate’-or-ee)
30.4 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
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
30.5 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)
Rules for Electrocyclic
Reactions
30.6 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)
30.7 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
30.8 Sigmatropic
Rearrangements
• A s -bonded substituent atom or group migrates
across a p electron system from one position to
another
• A s bond is broken in the reactant, the p 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 s 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
Stereochemical Rules of
Sigmatropic Rearrangements
Electron Pairs
Thermal Reaction
Even Number
Odd Number
Antarafacial
Suprafacial
H
H
SUPRA
Photochemical
Reaction
Suprafacial
Antarafacial
H
H
ANTARA
30.9 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
Orbital Picture of a Suprafacial [1,5] H
Shift
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
30.10 A Summary of Rules for
Pericyclic Reactions