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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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5 Stereochemistry of Diels-Alder Reactions. Thermodynamic vs. 276
Kinetic Control
6 Effect of Conformation on Rates of Diels-Alder Reactions 277
7 Cope and Claisen Rearrangements 278
8 Ene Reaction. Kinetic Isotope Effects 279
energy profile for rotation about the central carbon-carbon
bond in 2,2-hexa-2,4-diene (see problem 6)
Electrocyclic Reactions
This is an example of an electrocyclic reaction, and involves rotation of
the terminal methylene groups either in the same way ("conrotatory") or
in opposite ways ("disrotatory").
Ii
hr H YC EA I i-a ?
conrotatory disrotatory
N
HOMO of cis-1,3,5-hcxatricnc anticipates the preferred direction of ring
closure.
C/'s-l,3,5-hexatriene readily undergoes ring closure to give
1,3-cyclohexadiene.
Woodward and Hoffmann speculated that the preferred motion was that which
involved constructive (bonding) overlap between the terminal lobes of the
highest-occupied molecular orbital (HOMO).
Display the HOMO for cis-l,3,5-hexatriene. Which motion (conrotatory or
disrotatory) insures bonding overlap? Examine the geometry of the
transition state for ring closure (hexatriene to cyclohexadiene). Is it
consistent with the anticipated (conrotatory or disrotatory) motion of
the terminal methylenes?
What should be the kinetic product of ring closure of the
dimethylhexatriene shown below?
Is this also the thermodynamic product? (Compare energies of cis and
trans-5,6-dimethylcyclohexa-l,3-diene.)
Repeat your analysis for ring closure of butadiene to cyclobutene. (Start
by examining the HOMO of cis-1,3-butadiene.) What should be the preferred
product of ring closure of the dimethylbutadiene shown below?
Is your predicted product also the thermodynamic product? Energies for
cis and trans-3,4-dimethyl-cyclobutene are available.
CH
272 Chapter 21 Pericyclic Reactions
The Diels-Alder Reaction. A Symmetry Allowed
Process d
The most common and synthetically most useful Diels-Alder reactions
involve the addition of an electron-rich diene and an electron-poor
dienophile, e.g.
MeO
diene dienophile para meta
Woodward and Hoffmann pointed out that the Diels-Alder reaction involved
bonding overlap of the highest-occupied molecular orbital (HOMO) on the
diene and the lowest-unoccupied molecular orbital (LUMO) on the
dienophile. Display the HOMO for 2-methoxybutadiene. Where is it
localized? Display the LUMO for acrylonitrile. Where is it localized?
Orient the two fragments such that the HOMO and LUMO best overlap (A
clearer picture is provided by examining- the HOMO map for 2-
methoxybutadiene and the LUMO map for acrylonitrile.) Which product
should result?
Examine transition states leading to para and meta adducts (2-
methoxybutadiene+acrylonitrile para and meta) as well as the adducts
themselves (l-methoxy-4-cyanocyclohexene and l-methoxy-5-
cyanocyclohexene). What should the kinetic product be? Is this in line
with your expectations based on orbital interactions? Is this also the
thermodynamic product?
Finally, examine the geometry of the lower-energy transition state.
Measure all CC bond lengths. Draw a Lewis structure representing partial
bonds in terms of
dashed lines (... and ). How many (carbon-carbon)
bonds are broken (partially broken) in the transition state? How many
bonds are formed (partially formed)?
"
I#
The Diels-Alder
reaction involves overlap of the HOMO of the diene and the LUMO of the
dienophile.
HOMO map for 2-
methoxybutadiene reveals (in blue) where the HOMO is concentrated on the
exposed electron density surface.

LUMO map for acrylonitrile reveals (in blue) where the LUMO is
concentrated on the exposed electron density surface.
Chapter 21 Pericyclic Reactions 273
Electron Flow in Diels-Alder Reactions
Electrostatic potential map for transition state for Diels-Alder reaction
of cyclopentadiene and acrylonitrile shows negatively-charged regions (in
red) and positively-charged regions (in blue).
Experimental relative rates of
Diels-Alder reactions involving cyclopentadiene
dienophile relative rate
ethene 6xl0'fi
acrylonitrile 1
tetracyanoethylene 4xl07
Electrostatic interactions play a significant role in determining the
rates of Diels-Alder reactions.
Compare electrostatic potential maps for the following Diels-Alder
transition states: cyclopentadiene+ethene, cyclopentadiene+acrylonitrile
and cyclopentadiene+ tetracyanoethylene, with those of reactants:
cyclopentadiene, ethene, acrylonitrile and tetracyanoethylene. Are
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