Outline for Today
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Review MO construction for Conjugated Systems
Discuss Diels-Alder Reaction
Chapter 14 – Aromaticity
Tie in Aromaticity to Diels-Alder
Further Reading:
Structure and Mechanism in Organic Chemistry
By Felix A. Carrol
Chapter 4.1-2 – MO theory/aromaticity
Chapter 11.4 Cycloadditions
Lectures available online at
http://perceco2.chem.upenn.edu/~percec/classes.html
Until further notice.
MO’s for Ethene
MO’s for Allyl Systems
MO’s for Butadiene
MO’s for Hexatriene
Diels-Alder Reaction:
The Basics
S-cis diene required,
s-trans does not work
Concerted reaction
Bond made and broken
simultaneously
A Simple Example: 2+2
Cycloadditons
Out of phase
In Phase
Suprafacial vs. Antarafacial
Cycloaddition
Molecular Orbitals of Diels
Alder Cycloaddition
Normal Electron Demand/
Thermal Diels-Alder
Inverse-Electron Demand/
Photochemical Diels-Alder
Normal versus Inverse Electron
Demand
Normal
Inverse
Woodward Hoffman Rules for
Cycloadditons
“A Reaction occurs when the bonding electrons of a product can be transferred,
without a symmetry imposed barrier, to the bonding orbitals of the product.”
Last Chapter of “The Conservation of Orbital Symmetry” – Exceptions?
“There are none.”
Overall Diels Alder Transition
State
Stereochemistry of Diels-Alder
Reactions: Effect of Dienophile
Structure
Stereochemistry of Diels-Alder
Reactions: Effect of Diene
Structure
Diels Alder Approaches: Regio
and Stereochemistry
Endo-Selectivity: Secondary
Orbital Overlap
Controlling Regioselectivity of
Diels Alder Reactions
Interacting Orbitals
Asymmetrically Amplified
to create regioselectivit
Other types of Diels-Alder
Reactions
Lecture 2: Aromatic
Compounds
Chapter 14 in Solomons 9/e
History of the Benzene Structure
Example of Aromatic
Compounds: Motivation
Diels-Transition State
Fullerenes
Brief Note on Benzene
Nomenclature
Aromatic Stabilization:
Resonance Stabilization
Benzene Immune to Many
Standard De-aromitizing
Reactions
Note: There is an error in the diagram on page 605 of Solomons.
MO Description of Benzene
E=α+2β
E=α+β
E=α-β
E=α-2β
Overall stabilization=8β, compared to 6β for 3 ethenes or 7β for hexatriene
Hückel’s Rule/Frost Circles
4n+2 π – electrons = high stabilization due to ideal filling of bonding orbitals
More stable than linear polyene equivalents, closed shell configuration
4n π electron = anti-aromatic
1,3-cyclobutadiene
Less stable than linear butadiene – open shell configuration
Does not exist under non stabilized conditions
Aromatic and Nonaromatic
Annulenes: Application of 4n and
4n+2 rules
Polycyclic Aromatics
Benzene NMR Aromatic Ring
Currents
1H
NMR for aromatic
Hydrogens δ: 6.0-9.5
ppm
13C
NMR for aromatic
Carbon δ:100-170 ppm
The Allotropes of Carbons
b,d,e,f,h all aromatic
Cylcopentadienyl Cations and
Anions
Anti Aromatic
Aromatic
Heterocyclic Aromatics
Heterocyclic Aromatics
Protonation of Pyrolles and
Pyridines
Biochemically Relevant
Aromatics
Amino Acids
Biologically Relevant
Aromatics
Nicotinamide adeine dinucleotide, the biolgical hydrogenator
NADH
NAD+
Diels-Alder and Hückel Theory
Transition state has 6=4n+2 where n=1 electrons, therefore is aromatic
and low in energy, despite high entropic cost.
Note: If Diels-Alder Substrate is Aromatic to begin with will often not participate.