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The Molecular Modeling Workbook for Organic Chemistry - Hehre J.W.

Hehre J.W., Shusterman J.A. The Molecular Modeling Workbook for Organic Chemistry - Wavefunction, 1998. - 307 p.
Download (direct link): molecularmodelingworkbook1998.djvu
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electrons transferred from diene to dienophile in the transition states
(relative to reactants) or vice versa? For which reaction is the transfer
the greatest? The least? Quantify your conclusion by measuring the total
charge on the diene and dienophile components in the three transition
states.
Calculate activation energies for the three Diels-Alder reactions (energy
of transition state - sum of energies of reactants). Which reaction has
the smallest energy barrier? Which has the largest energy barrier? Do
your results parallel the measured relative rates of the same reactions
(see table at left)?
Is there a correlation between activation energy and the magnitude of
charge transfer between diene and dienophile components in the transition
state? Explain.
274 Chapter 21 Pericyclic Reactions
Catalysis of Diels-Alder Reactions
According to Frontier Molecular Orbital (FMO) theory, Diels-Alder
reaction between an electron-rich diene and an electron-poor dienophile
involves interaction between the highest-occupied molecular orbital
(HOMO) on the diene and the lowest-unoccupied molecular orbital (LUMO) on
the dienophile. The better the HOMO/LUMO overlap and the smaller their
energy difference, the more favorable the interaction and the faster the
reaction.
Lewis acids catalyze Diels-Alder reactions. Do they enhance overlap
between diene and dienophile orbitals and/ or do they reduce the
HOMO/LUMO energy difference?
Simultaneously examine the HOMO of 1-methylcyclo-pentadiene (the diene)
and the LUMO of acrylonitrile (the dienophile). Orient the two on screen
such that they are disposed for Diels-Alder addition, i.e.
Similarly, examine the overlap between the HOMO of
1-methylcyclopentadiene and the LUMO of an acrylonitrile BF3 complex.
Does the Lewis acid affect overlap? Would you expect BF3 to enhance,
retard, or leave unchanged the rate of Diels-Alder addition?
LUMO energies for "free" and complexed acrylonitrile are .103 and .089 au
(65 and 56 kcal/mol), respectively. On the basis of orbital energies,
would you expect BF3 to enhance, retard, or leave unchanged the rate of
Diels-Alder cycloaddition?
Check your predictions by calculating activation energies for Diels-Alder
additions. Data for transition states:
1-methylcyclopentadienee-acrylonitrile and 1-methyl-
cyclopentadiene+acrylonitrile BF3 are available.

LUMO for
acrylonitrile receives electrons in a Diels-Alder addition.
Mi
HOMO for
1-methylcyclopentadiene donates electrons in a Diels-Alder addition.
Chapter 21 Pericyclic Reactions 275
3
The thermodynamic product is the lowest energy product, while the kinetic
product is the most easily formed product.
N; is the number of molecules i
E, is the energy of molecule i (in au)
^major _ g "1 060(Emajor

^minor) (2)

Nj is the number of molecules i
Ef is the energy of the transition state leading to molecule i (in au)
Stereochemistry of Diels-Alder Reactions. Thermodynamic vs. Kinetic
Control
Chemical reactions often yield entirely different product distributions
depending on the conditions under which they are carried out. In
particular, high temperatures and long reaction times favor the most
stable ("thermodynamic") products, while low temperatures and short
reaction times favor the most easily formed ("kinetic") products.
Consider Diels-Alder reaction of cyclopentadiene and maleic anhydride,
leading to endo or exo adducts.
+
Display space-filling models of endo adduct and exo adduct. Which appears
to be the less crowded? Identify specific interactions which disfavor the
higher-energy adduct. Next, compare energies of the two adducts. Which is
the more stable? Were the reaction under thermodynamic control, which
would be the major product and what would be the ratio of major to minor
products? Use equation (1).
Compare energies of endo transition state and exo transition state. Which
of the two has the lower energy? Were the reaction under kinetic control,
which would be the major product and what would be the ratio of major to
minor products? Use equation (2).
Are the kinetic and thermodynamic products the same? If not, describe
conditions which will favor the endo adduct. The exo adduct.
276 Chapter 21 Pericyclic Reactions
Effect of Conformation on Rates of Diels-Alder Reactions
Experimentally, the rates of Diels-Alder reactions between electron-rich
dienes and electron-poor dienophiles generally increase with increased
alkyl substitution on the diene. This is because alkyl groups act as
electron donors and lead to buildup of electron density on the diene. An
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