Transcript 30. Orbitals and Organic Chemistry: Pericyclic Reactions

30. Orbitals and Organic
Chemistry: Pericyclic
Reactions
Based on McMurry’s Organic Chemistry, 6th
edition
©2003 Ronald Kluger
Department of Chemistry
University of Toronto
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
2
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
3
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
4
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
5
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
6
Example: Electrocyclic Interconversions With
Octatriene
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
7
Example: Electrocyclic Interconversions with
Dimethylcyclobutene
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
8
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
9
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’-oree) opening or closure
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
10
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)
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
11
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 )
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
12
Ring Closure of Conjugated Trienes
 Involves lobes of like sign on the same side of the
molecule and disrotatory ring closure
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
13
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
14
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)
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
15
Rules for Electrocyclic Reactions
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
16
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
17
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
18
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)
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
19
30.7 Stereochemistry of
Cycloadditions
 HOMO of one reactant combines with LUMO of other
 Possible in thermal [4 +2] cycloaddition
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
20
[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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
21
Formation of Four-Membered Rings
 Photochemical [2 + 2] cycloaddition reaction occurs
smoothly
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
22
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
23
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
24
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
25
Stereochemical Rules of Sigmatropic
Rearrangements
Electron Pairs Thermal
Reaction
Even Number Antarafacial
Odd Number Suprafacial
Photochemical
Reaction
Suprafacial
Antarafacial
H
H
H
H
SUPRA
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
ANTARA
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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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
27
Another Example of a Sigmatropic
Rearrangement
 Heating 5,5,5-trideuterio-(1,3Z)-pentadiene causes
scrambling of deuterium between positions 1 and 5
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
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Orbital Picture of a Suprafacial [1,5] H
Shift
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
29
Cope and Claisen Rearrangements are
Sigmatropic
 Cope rearrangement of 1,5-hexadiene
 Claisen rearrangement of an allyl aryl ether
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
30
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
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
31
30.10 A Summary of Rules for
Pericyclic Reactions
Based on McMurry, Organic Chemistry, Chapter
30, 6th edition, (c) 2003
32