Chemdraw B&W - Pennsylvania State University
Download
Report
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