Chapter 10, Concerted Pericyclic Reactions Video Solutions, Advanced Organic Chemistry. Part A. Structure and Mechanisms

Problem 1

Show, by construction of both a TS orbital array and an orbital symmetry correlation diagram, which of the following electrocyclizations are allowed.
a. disrotatory cyclization of the pentadienyl cation to the cyclopent-2-enyl cation.
b. disrotatory cyclization of the pentadienyl anion to the 3 -cyclopentenyl anion.
c. disrotatory cyclization of the heptatrienyl anion to the cyclohepta-3,5-dienyl anion.

Tracy Tourville

Numerade Educator

09:09

Problem 4

Offer a mechanistic explanation for each of the following reactions:
a. The 3,5 -dinitrobenzoate esters of the stereoisomeric bicyclo[2.1.0]pentan-2-ols shown below both yield cyclopent-3-enol on hydrolysis in dioxane-water. The relative rates, however, differ by a factor of 10 million! Which is more reactive and why?
b. Optically active 4-A racemizes on heating at $50^{\circ} \mathrm{C}$ with a half-life of $24 \mathrm{~h}$.
c. On being heated to $320^{\circ}-340^{\circ} \mathrm{C}$, compound 4-B produces $1,4-$ dimethoxynaphthalene and 1-acetoxybutadiene. Furthermore, deuterium labeling has shown that the reaction is stereospecific as indicated.
d. It has been found that compounds 4-C and 4-D are opened at $-25^{\circ} \mathrm{C}$ to allylic anions in the presence of strong bases such as lithium $t$-butylamide. In contrast,
e. When the 1,6 -methano- $1,3,5,7,9$-pentaene structure is modified by fusion of two benzene rings as shown in 4-Fa, a valence isomer 4-Fb is the dominant structure.

Ronald Prasad

Numerade Educator

03:50

Problem 5

Suggest a mechanism by which each transformation could occur. More than one step may be involved.

Mikayla Stephens

Numerade Educator

08:09

Problem 6

It has been found that 3-substituted methyl 3-hydroxy-2-methylene alkanoates give rise to a preference for the $Z$-isomer if $\mathrm{R}$ is alkyl, but for the $E$-isomer if $R$ is aryl under the conditions of the thermal orthoester Claisen rearrangement.
Analyze the transition structure for the reaction in terms of steric interactions and suggest a reason for the difference in stereoselectivity.

Nima Gharibi

Numerade Educator

02:29

Problem 7

Give the structure, including stereochemistry, of the product expected for the following reactions:

Colton K

Numerade Educator

01:32

Problem 8

In the series of bicyclic oxepins $\mathbf{1 0}-\mathbf{A}(n=3,4,5)$, only the compound with $n=5$ undergoes rearrangement (at $60^{\circ} \mathrm{C}$ ) to the isomeric oxepin $\mathbf{1 0}-\mathbf{B}$. The other two compounds ( $n=3$ or 4) are stable, even at much higher temperature. When 10-B ( $n=3$ ) was synthesized by another route, it showed no tendency to revert to 10-A $(n=3)$. Offer an explanation for these observations.

Bhumika Jayee

Numerade Educator

05:33

Problem 9

Bromocyclooctatetraene rearranges to $E-\beta$-bromostyrene. The rate of the rearrangement is solvent dependent, with the first-order rate constant increasing from about $10^{-7} \mathrm{~s}^{-1}$ in cyclohexane to about $10^{-3} \mathrm{~s}^{-1}$ in acetonitrile at $80^{\circ} \mathrm{C}$. In the presence of lithium iodide, the product is $E-\beta$-iodostyrene, although $E-\beta$-bromostyrene is unaffected by lithium iodide under the reaction conditions. Suggest a mechanism for the rearrangement.

David Collins

Numerade Educator

04:03

Problem 10

Pyrrolidine derivatives catalyze the formation of 3,5-diaryl-4acetylcyclohexanones from 4-arylbut-3-en-2-ones. A Diels-Alder reaction is believed to be involved. Suggest a mechanism.

Colton K

Numerade Educator

03:42

Problem 11

Propose a transition structure that would account for the stereochemistry observed in each of the following reactions:

Henry R

Numerade Educator

06:49

Problem 12

Classify the following reactions as electrocyclizations, sigmatropic rearrangement, cycloaddition, etc., and give the correct symbolism for the electrons involved in each process. Some of the reactions proceed in two steps.

Ian Kaigh

Numerade Educator

04:45

Problem 13

The "ene" reaction is a concerted reaction in which addition of an alkene and an electrophilic olefin occurs with transfer of a hydrogen to the electrophile and with a double-bond shift. For example:
Depict the orbital array through which this reaction can occur as a concerted process.

Ian Kaigh

Numerade Educator

05:33

Problem 14

Predict the regiochemistry and stereochemistry of the following cycloaddition reactions and indicate the basis for your prediction.

Niamat Khuda

Numerade Educator

09:12

Problem 15

On treatment with potassium metal, cis-bicyclo[6.1.0]nona-2,4,6-triene gives a monocyclic aromatic dianion. The trans isomer under similar conditions give a bicyclic radical anion that does not undergo further reduction. Explain how the stereochemistry of the ring junction can control the course of these reductions.

Matthew Lueckheide

Numerade Educator

01:04

Problem 16

The following compounds are capable of degenerate rearrangement at the temperature given. Identify reaction processes that are consistent with the temperature and would lead to a degenerate rearrangement. Indicate by an appropriate labeling scheme the carbons and hydrogens that become equivalent as a result of the rearrangement process you have suggested.

Grigoriy Sereda

Numerade Educator

05:25

Problem 17

On heating at $225^{\circ} \mathrm{C}$, 5-allylcyclohexa-1,3-diene, 17-A, undergoes intramolecular cycloaddition to give the tricyclononene $\mathbf{1 7}-\mathbf{B}$. The same product is predicted for both $[2+2]$ and $[2+4]$ cycloaddition. The mechanism of the reaction has been probed by using the deuterium-labeled derivative, as shown. Indicate the position of the deuterium labeling in the product if the reaction proceeds by (a) a [2+2] cycloaddition or (b) a [2+4] cycloaddition.

Ian Kaigh

Numerade Educator

00:40

Problem 18

Computations on the cyclization of pentadienyl cations to cyclopentenyl cations has indicated increasing reactivity in the order $\mathrm{X}=\mathrm{NH}_{2}<\mathrm{OH}<\mathrm{H}Based on these results, indicate which of the following types of groups would be favorable relative to the unsubstituted system: (a) alkyl; (b) $\pi$-conjugated EWGs, e.g., $\mathrm{CN}, \mathrm{CH}=\mathrm{O}$; (c) $\sigma$-EWGs, e.g., $\mathrm{CF}_{3}, \mathrm{CF}_{3} \mathrm{SO}_{2}$.

Mishal Gul

Numerade Educator

02:07

Problem 19

Suggest mechanisms for the following reactions. Classify the orbital symmetry character of the process as completely as you can.

Vasu Makani

Numerade Educator

11:06

Problem 20

Both compounds 20-A and 20-B undergo solvolysis in polar solvents at low temperature. The isolated product is $\mathbf{2 0}$-C. When $\mathbf{2 0}$-B is labeled with deuterium it is found that there is complete scrambling of label equally among all positions in the product.
It has been suggested that the cyclononatetraenyl cation might be an intermediate, and several $\left[\mathrm{C}_{9} \mathrm{H}_{9}\right]^{+}$structures have been compared computationally to determine their relative energy. Structure $\mathbf{3}$ has the lowest energy among the monocations. It has an $E$-configuration at one double bond. Structure $\mathbf{4}$ is also an energy minimum, but it is $21.6 \mathrm{kcal} / \mathrm{mol}$ higher in energy than $\mathbf{3}$. The calculated relative energy and nucleus independent chemical shift (NICS) values are given for several structures, including structure $\mathbf{6}$, which gives rise to the observed product. Formulate a mechanism that is in accord with the experimental observation of label scrambling. Discuss the role of structure $\mathbf{3}$ in the mechanism.

Anish Wadhwa

Numerade Educator

05:51

Problem 21

Reaction of ketene with cyclopentadiene proceeds in a $[2+2]$ rather than a $[2+$ 4] manner. A number of potential TSs have been characterized computationally. The diagrams below show product and TS energy, TS bond orders, and TS NPA charges from MP4SDQ/6-31G* $+$ ZPE computations. Analyze the computational output in order to answer the following questions:
a. In very broad terms, why is the $[2+2]$ product 2 favored over the other possible products? Draw a reaction potential energy diagram to illustrate your conclusion.
b. More specifically, why is product 2 preferred to product 3 ? What structural features account for this preference?
c. What structural features of TS 1 make it less favorable than TS 2 ?

Ian Kaigh

Numerade Educator