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exception to this is the reaction of Z,Z-hexa-2,4-diene with
tetracyanoethylene (TCNE), which is actually slower than the
corresponding addition involving E-penta-l,3-diene.
Step through the sequence of structures depicting rotation around the
central carbon-carbon bond in E-penta-1,3-diene. Plot energy (vertical
axis) vs. CiC2C3C4 dihedral angle (horizontal axis). Identify the lowest-
energy conformer, and calculate how much energy is needed to "twist" this
conformer into a conformer that approximates the geometry in the
transition state (E-penta-1,3-diene+TCNE).
Repeat your analysis for Z,Z-hexa-2,4-diene, and again calculate the
energy to twist the diene into the same conformation as seen in the
Diels-Alder transition state (,Z,Z-hexa-2,4-diene+TCNE). Compare the two
"twisting energies", and rationalize the observed relative rates for the
two cycloaddition reactions.
Examine conformational energy profiles for Z-penta-1,3-diene and E,E-
hexa-2,4-diene together with transition-state geometries for
cycloadditions with TCNE (Z-penta-
1,3-diene+TCNE and E,E-hexa-2,4-diene+TCNE, respectively). Predict the
rates of Diels-Alder reactions involving these two dienes, relative to
that for cycloaddition of E-penta-l,3-diene with TCNE.
Chapter 21 Pericyclic Reactions 277
Cope and Claisen Rearrangements
Cope rearrangement of 1,5-hexadiene.
Claisen rearrangement of allyl vinyl ether.
The Cope and Claisen rearrangements are markedly similar reactions,
although they differ in thermodynamic driving force. Whereas the Cope
1,5-hexadiene is thermoneutral (reactant and product are the same), the
analogous Claisen rearrangement of allyl vinyl ether is exothermic. Do
thermodynamic differences lead to differences in transition state
Step through the sequence of structures depicting Cope rearrangement of
1,5-hexadiene. Plot energy (vertical axis) vs. the length of either the
carbon-carbon bond being formed or that being broken (horizontal axis).
Locate the transition state. Measure all CC bond distances at the
transition state, and draw a structural formula for it
representing partial bonds in terms of dashed lines (.
and ). How many carbon-carbon bonds are broken (partially broken) at the
transition state? How many carbon-carbon bonds are formed (partially
formed)? Are all bonds broken or formed to roughly the same extent?
Repeat the procedure for the sequence of structures depicting the Claisen
rearrangement of allyl vinyl ether to 4-pentenal. Plot energy (vertical
axis) vs. the length of either the carbon-oxygen bond being broken or the
carbon-carbon bond being formed (horizontal axis). Locate the transition
state. Measure all relevant bond distances and draw a structural formula
for it representing partial bonds
in terms of dashed lines (.and ). How many carbon-
carbon and carbon-oxygen bonds are broken (partially broken) at the
transition state? How many bonds are formed (partially formed)? Are all
bonds broken or formed to roughly the same extent?
Would you describe the transition state for the Claisen rearrangement as
"early" (like reactants), "late" (like products) or in between? Given the
overall thermodynamics of reaction, do you conclude that the Hammond
Postulate applies? Explain.
278 Chapter 21 Pericyclic Reactions
Ene Reaction. Kinetic Isotope Effects
The ene reaction involves addition of an electrophilic double bond to an
alkene with an all.ylic hydrogen. The allylic hydrogen is transferred and
a new carbon-carbon bond is formed, e.g., the addition of maleic
anhydride and propene.
Compare the geometry of maleic anhydride+propene, the ene transition
state, to those of the reactants (maleic anhydride and propene). Is bond
making and breaking occurring at once? In particular, is the "migrating
hydrogen" partially bonded to two carbons (rather than being fully bonded
to one carbon")? Draw a Lewis structure to represent the transition
state. Use dashed lines (... and =^) to represent partial bonds.
Any change in bonding from reactants to transition state leads to the
possibility of rate changes in response to mass changes of the atoms
immediately involve, i.e., "kinetic isotope effects". In particular,
replacement of the "migrating hydrogen" by deuterium should lead to a
sizable deuterium isotope effect. Use equation (1) to calculate the
isotope effect kH/kD. (Zero-point energies are provided at the right.)
Which reacts faster, propene or deuteropropene?
Compare atomic charges and electrostatic potential maps between reactants
and transition state. Is there charge transfer from one of the reactants
to the other (count the migrating hydrogen as part of propene)? If so,
what is the direction of the transfer? Why? (See also Chapter 21, Problem
Zero-Point Energies (ZPE) (au)
maleic anhydride 0.0606
propene (H) 0.0858
propene (D) 0.0825