1. Conjugated Unsaturated Systems
Chapter 13
Conjugated Unsaturated
Systems
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1. Introduction
A conjugated system involves at least one atom with a p orbital adjacent to at least one p bond
e.g.
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2B. Resonance Description of the Allyl Radical
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3. The Allyl Cation
Relative order of carbocation stability
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In writing resonance structures, we are only allowed to move electrons
resonance structures
not resonance structures
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All of the structures must be proper Lewis structures X
10 electrons!
not a proper
Lewis structure
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All resonance structures must have the same number of unpaired electrons X
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The more stable a structure is (when taken by itself), the greater is its contribution to the hybrid
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4B. How to Estimate the Relative Stability of Contributing Resonance Structures
The more covalent bonds a structure has, the more stable it is
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Structures in which all of the atoms have a complete valence shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid
this carbon has
6 electrons
this carbon has
8 electrons
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Charge separation decreases stability
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5. Alkadienes & Polyunsaturated Hydrocarbons
Alkadienes (“Dienes”)
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Alkatrienes (“Trienes”)
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Alkadiynes (“Diynes”)
Alkenynes (“Enynes”)
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Cumulenes
enantiomers
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Conjugated dienes
Non-conjugated dienes (isolated alkenes)
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6. 1,3-Butadiene: Electron Delocalization
6A. Bond Lengths of 1,3-Butadiene
1.47 Å
1.34 Å sp
sp3
sp3
sp2
sp3
1.54 Å
1.50 Å
1.46 Å
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6B. Conformations of 1,3-Butadiene
trans
single
bond
single
bond
cis
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7. The Stability of Conjugated Dienes
Conjugated alkadienes are thermo-dynamically more stable than isomeric isolated alkadienes
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9. Electrophilic Attack on Conju-gated Dienes: 1,4-Addition
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Mechanism
(a)
(b)
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9A. Kinetic Control versus Thermodynamic Control of a Chemical Reaction
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10. The Diels–Alder Reaction: A 1,4-Cycloaddition Reaction of Dienes
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e.g.
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10A. Factors Favoring the Diels–Alder Reaction
Type A and Type B are normal Diels-Alder reactions
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Type C and Type D are Inverse Demand Diels-Alder reactions
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Relative rate
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Relative rate
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Steric effects
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10B. Stereochemistry of the Diels–Alder Reaction
The Diels–Alder reaction is stereospecific: the reaction is a syn addition, and the configuration of the dienophile is retained in the product
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The diene, of necessity, reacts in the s-cis rather than in the s-trans conformation X
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e.g.
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Cyclic dienes in which the double bonds are held in the s-cis conformation are usually highly reactive in the Diels–Alder reaction
Relative rates:
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The Diels–Alder reaction occurs primarily in an endo, rather than an exo, fashion when the reaction is kinetically controlled
R is exo
longest bridge
R is endo
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Alder-Endo Rule
If a dienophile contains activating groups (group X) with p bonds they will prefer an ENDO orientation in the transition state
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e.g.
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Stereospecific reaction
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Stereospecific reaction
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Examples
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Diene A reacts 103 times faster than diene B even though diene B has two electron-donating methyl groups
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Examples
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Examples
Rate of Diene C > Diene D (27 times), but Diene D >> Diene E
In Diene C, tBu group  electron donating group  increase rate
In Diene E, 2 tBu group  steric effect, cannot adopt s-cis conformation
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10C. How To Predict the Products of a Diels–Alder Reaction
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10D. How to Use a Diels–Alder Reaction in a Retrosynthetic Analysis
Retrosynthesis
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Solution
Note: use EWG’s on
dienophile to ensure a
facile D.A. reaction at
low temperature; using
methyl groups directly
would be a more
difficult reaction
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