An important variation of the DielsAlder reaction is intramolecular, in which the diene

Rank each group of compounds in order of increasing heat of hydrogenation. (a) hexa-1,2-diene; hexa-1,3,5-triene; hexa-1,3-diene; hexa-1,4-diene; hexa-1,5-diene; hexa-2,4-diene (b) (b)

In a strongly acidic solution, cyclohexa-1,4-diene tautomerizes to cyclohexa-1,3-diene. Propose a mechanism for this rearrangement, and explain why it is energetically favorable.

(Review) The central carbon atom of an allene is a member of two double bonds, and it has an interesting orbital arrangement that holds the two ends of the molecule at right angles to each other. (a) Draw an orbital diagram of allene, showing why the two ends are perpendicular. (b) Draw the two enantiomers of penta-2,3-diene. A model may be helpful.

Draw another resonance form for each of the substituted allylic cations shown in the preceding figure, showing how the positive charge is shared by another carbon atom. In each case, state whether your second resonance form is a more important or less important resonance contributor than the first structure. (Which structure places the positive charge on the more-substituted carbon atom?)

When 3-bromo-1-methylcyclohexene undergoes solvolysis in hot ethanol, two products are formed. Propose a mechanism that accounts for both of these products. CH3CH2OH heat CH Br 3 OCH2CH3 OCH2CH3 CH3 CH3

Treatment of an alkyl halide with AgNO3 in alcohol often promotes ionization. Ag+ + RCl AgCl + R+ When 4-chloro-2-methylhex-2-ene reacts with AgNO3 in ethanol, two isomeric ethers are formed. Suggest structures, and propose a mechanism for their formation.

Propose a mechanism for each reaction, showing explicitly how the observed mixtures of products are formed. (a) 3@methylbut@2@en@1@ol + HBr S 1@bromo@3@methylbut@2@ene + 3@bromo@3@methylbut@1@ene (b) 2@methylbut@3@en@2@ol + HBr S 1@bromo@3@methylbut@2@ene + 3@bromo@3@methylbut@1@ene (c) cyclopenta-1,3-diene + Br2 S 3,4-dibromocyclopent-1-ene + 3,5-dibromocyclopent-1-ene (d) 1@chlorobut@2@ene + AgNO3, H2O S but@2@en@1@ol + but@3@en@2@ol (e) 3@chlorobut@1@ene + AgNO3, H2O S but@2@en@1@ol + but@3@en@2@ol

When \(Br_2\) is added to buta-1,3-diene at -\(15^{\circ}C\), the product mixture contains 60% of product A and 40% of product B. When the same reaction takes place at \(60^{\circ}C\), the product ratio is 10% A and 90% B. (a) Propose structures for products A and B. (Hint: In many cases, an allylic carbocation is more stable than a bromonium ion.) (b) Propose a mechanism to account for formation of both A and B. (c) Show why A predominates at -\(15^{\circ}C\) and B predominates at \(60^{\circ}C\). (d) If you had a solution of pure A, and its temperature were raised to \(60^{\circ}C\), what would you expect to happen? Propose a mechanism to support your prediction.

When methylenecyclohexane is treated with a low concentration of bromine under irradiation by a sunlamp, two substitution products are formed. methylenecyclohexane Br2 + two substitution products + HBr hv (a) Propose structures for these two products. (b) Propose a mechanism to account for their formation.

When N-bromosuccinimide is added to hex-1-ene in \(CCl_4\) and a sunlamp is shone on the mixture, three products result. (a) Give the structures of these three products. (b) Propose a mechanism that accounts for the formation of these three products.

Predict the product(s) of light-initiated reaction with NBS in CCl4 for the following starting materials. (a) cyclopentene (b) 2,3-dimethylbut-2-ene (c) CH3 toluene

Addition of 1-bromobut-2-ene to magnesium metal in dry ether results in formation of a Grignard reagent. Addition of water to this Grignard reagent gives a mixture of but-1-ene and but-2-ene (cis and trans). When the Grignard reagent is made using 3-bromobut-1-ene, addition of water produces exactly the same mixture of products in the same ratios. Explain this curious result.

Show how you might synthesize the following compounds starting with bromobenzene, and alkyl or alkenyl halides of four carbon atoms or fewer. (a) 3-phenylprop-1-ene (b) 5-methylhex-2-ene *(c) dec-5-ene

Predict the products of the following proposed DielsAlder reactions. (a) + (a) CHO (b) CH3 + O O (b) (c) + C C C C O OCH3 O OCH3 (c) (d) + C C NC CN NC CN (d) (e) C C C C O OCH3 O OCH3 O + (e) (f) + CN CH3O CN

What dienes and dienophiles would react to give the following Diels–Alder products?

Predict the major product for each proposed DielsAlder reaction. Include stereochemistry where appropriate. (a) + C C H H H (a) C N (b) + CH3 O CH3O O (b) (c) + O O O Ph Ph C C

In Solved Problem 15-2, we simply predicted that the products would have a 1,2- or 1,4-relationship of the proper substituents. Draw the charge-separated resonance forms of the reactants to support these predictions.

Predict the products of the following DielsAlder reactions. CH3O + H COOCH3 C C (a) O + OCH3 (b) CH3O CHO + (c) *(d) CN CH3 CH3 +

Show that the [4 + 2] DielsAlder reaction is photochemically forbidden.

(a) Show that the [4 + 4] cycloaddition of two butadiene molecules to give cycloocta 1,5-diene is thermally forbidden but photochemically allowed. (b) There is a different, thermally allowed cycloaddition of two butadiene molecules. Show this reaction, and explain why it is thermally allowed. (Hint: Consider the dimerization of cyclopentadiene.)

One milligram of a compound of molecular weight 160 is dissolved in 10 mL of ethanol, and the solution is poured into a 1-cm UV cell. The UV spectrum is taken, and there is an absorption at \(\lambda_{max}\) = 247 nm. The maximum absorbance at 247 nm is 0.50. Calculate the value of e for this absorption.

Using the examples in Table 15-2 to guide you, match four of the following UV absorption maxima (lmax) with the corresponding compounds: (1) 232 nm; (2) 256 nm; (3) 273 nm; (4) 292 nm; (5) 313 nm; (6) 353 nm. (a) (a) (b) (b) (c) (c) (d)

Phenolphthalein is an acid–base indicator that is colorless below pH 8 and red above pH 8. Explain briefly why the first structure is colorless and the second structure is colored.

Show how to construct the molecular orbitals of ethylene, butadiene, and the allylic system. Show the electronic configurations of ethylene, butadiene, and the allyl cation, radical, and anion. (Problems 15-36 and 37)

Explain which reactions are enhanced by resonance stabilization of the intermediates, such as free-radical reactions and cationic reactions. Propose mechanisms to explain the enhanced rates and the observed products, and draw resonance forms of the stabilized intermediates.

Predict the products of Diels–Alder reactions, including the orientation of cycloaddition with unsymmetrical reagents and the stereochemistry of the products. (Problems 15-29, 30, 31, 33, 36, 39, and 41)

Predict which cycloadditions are thermally allowed and which are photochemically allowed by comparing the symmetry of the molecular orbitals of the reactants.

Use values of \(\lambda_{\max }\) from UV–visible spectra to estimate the length of conjugated systems, and to distinguish between compounds that differ in their conjugated systems.

Classify the following dienes and polyenes as isolated, conjugated, cumulated, or some combination of these classifications. (a) cycloocta-1,4-diene (b) cycloocta-1,3-diene (c) cyclodeca-1,2-diene (d) cycloocta-1,3,5,7-tetraene (e) cyclohexa-1,3,5-triene (benzene) (f) penta-1,2,4-triene

Predict the products of the following reactions. (a) allyl bromide + cyclohexyl magnesium bromide (b) cyclopentadiene + anhydrous HCl (c) 2-methylpropene + NBS, light (d) furan + trans-1,2-dicyanoethylene (e) buta-1,3-diene + bromine water (f) hexa-1,3,5-triene + bromine in \(CCl_4\) (g) 1-(bromomethyl)-2-methylcyclopentene, heated in methanol (h) cyclopentadiene + methyl acrylate, \(CH_2=CH-COOCH_3\)

Show how the reaction of an allylic halide with a Grignard reagent might be used to synthesize the following hydrocarbons. (a) 5-methylhex-1-ene (b) 2,5,5-trimethylhept-2-ene (c) 1-cyclopentylpent-2-ene

Draw the important resonance contributors for the following cations, anions, and radicals. +CH2 (a) CH2 (b) COCH3 O (c) H + H H (d) (e) CH2 C O H O O (f) O CH2 (g) + (h) +CH2 C OCH3 C H H

A solution was prepared using 0.0010 g of an unknown steroid (of molecular weight around 255) in 100 mL of ethanol. Some of this solution was placed in a 1-cm cell, and the UV spectrum was measured. This solution was found to have lmax = 235 nm, with A = 0.74. (a) Compute the value of the molar absorptivity at 235 nm. (b) Which of the following compounds might give this spectrum?

The diene lactone shown in part (a) has one electron-donating group \((-\mathrm{OR})\) and one electron-withdrawing group \((\mathrm{C}=\mathrm{O})\). This diene lactone is sufficiently electron-rich to serve as the diene in a Diels–Alder reaction. (a) What product would you expect to form when this diene reacts with methyl acetylenecarboxylate, a strong dienophile? (b) The Diels–Alder product A is not very stable. Upon mild heating, it reacts to produce \(\mathrm{CO}_{2}\) gas and methyl benzoate \(\left(\mathrm{PhCOOCH}_{3}\right)\), a very stable product. Explain how this strongly exothermic decarboxylation takes place. (Hint: Under the right conditions, the Diels–Alder reaction can be reversible.)

Predict the products of the following Diels–Alder reactions. Include stereochemistry where appropriate.

Predict the products of the following reactions, including stereochemistry where applicable.

A graduate student was following a procedure to make 3-propylcyclohexa-1,4-diene. During the workup procedure, his research adviser called him into her office. By the time the student returned to his bench, the product had warmed to a higher temperature than recommended. He isolated the product, which gave the appropriate =C-H stretch in the IR, but the C=C stretch appeared around \(1630\ cm^{-1}\) as opposed to the literature value of \(1650\ cm^{-1}\) for the desired product. The mass spectrum showed the correct molecular weight, but the base peak was at M-29 rather than at M-43 as expected. Because of the anomalous IR spectrum, he took a UV spectrum that showed \(\lambda_{max}\) at 261 nm. (a) Should he have his IR recalibrated or should he repeat the experiment, watching the temperature more carefully? What does the \(1630\ cm^{-1}\) absorption suggest? (b) Draw the structure of the desired product, and propose a structure for the actual product. (c) Show why he expected the MS base peak to be at M-43, and show how your proposed structure would give an intense peak at M-29.

Show how Diels–Alder reactions might be used to synthesize the following compounds.

Give the structures of the products represented by letters in this synthesis. Part 1 A mCPBA NaOH H3O+ H2O B C excess NaOCl HOAc excess CH3MgBr D E (C8H12) H2SO4 mixture of diastereomers ether Part 2 Part 3 O3 (CH3)2S NaBH4 H3O+ H2SO4 78 C H2O F E (C + 8H12) H (C3H4O2) I (C11H16O2)

Furan and maleimide undergo a DielsAlder reaction at 25 C to give the endo isomer of the product. When the reaction takes place at 90 C, however, the major product is the exo isomer. Further study shows that the endo isomer of the product isomerizes to the exo isomer at 90 C. furan: O maleimide: O O N H (a) Draw and label the endo and exo isomers of the DielsAlder adduct of furan and maleimide. (b) Which isomer of the product would you usually expect from this reaction? Explain why this isomer is usually favored. (c) Examine your answer to (b) and determine whether this answer applies to a reaction that is kinetically controlled or one that is thermodynamically controlled, or both. (d) Explain why the endo isomer predominates when the reaction takes place at 25 C and why the exo isomer predominates at 90 C.

(a) Sketch the pi molecular orbitals of hexa-1,3,5-triene (Figure 15-25). (b) Show the electronic configuration of the ground state of hexa-1,3,5-triene. (c) Show what product would result from the [6 + 2] cycloaddition of hexa-1,3,5-triene with maleic anhydride. (d) Show that the [6 + 2] cyclization of hexa-1,3,5-triene with maleic anhydride is thermally forbidden but photochemically allowed. (e) Show the Diels–Alder product that would actually result from heating hexa-1,3,5-triene with maleic anhydride.

The pentadienyl radical, \(H_2C=CH-CH=CH-CH_2 \cdot\), has its unpaired electron delocalized over three carbon atoms. (a) Use resonance forms to show which three carbon atoms bear the unpaired electron. (b) How many MOs are there in the molecular orbital picture of the pentadienyl radical? (c) How many nodes are there in the lowest-energy MO of the pentadienyl system? How many in the highest-energy MO? (d) Draw the MOs of the pentadienyl system in order of increasing energy. (e) Show how many electrons are in each MO for the pentadienyl radical (ground state). (f) Show how your molecular orbital picture agrees with the resonance picture showing delocalization of the unpaired electron onto three carbon atoms. (g) Remove the highest-energy electron from the pentadienyl radical to give the pentadienyl cation. Which carbon atoms share the positive charge? Does this picture agree with the resonance picture? (h) Add an electron to the pentadienyl radical to give the pentadienyl anion. Which carbon atoms share the negative charge? Does this picture agree with the resonance picture?

A student was studying terpene synthesis, and she wanted to make the compound shown here. First she converted 3-bromo-6-methylcyclohexene to alcohol A. She heated alcohol A with sulfuric acid and purified one of the components (compound B) from the resulting mixture. Compound B has the correct molecular formula for the desired product. (a) Suggest how 3-bromo-6-methylcyclohexene might be converted to alcohol A. (b) The UV spectrum of compound B shows \(\lambda_{max}\) at 269 nm. Is compound B the correct product? If not, suggest a structure for compound B consistent with these UV data. (c) Propose a mechanism for the dehydration of alcohol A to compound B.

Part of a synthesis by E. J. Corey and David Watt (Harvard University) involves the DielsAlder cycloaddition of the following pyrone and cyclohexenone. The initial reaction gives the endo product, which loses carbon dioxide in a retro-DielsAlder to generate a diene with predictable stereochemistry and functionality. IR and UV spectroscopy of the final product show that it contains a diene conjugated with an ester, and an unconjugated ketone. Determine the structures of the intermediate and the final product, with particular attention to their stereochemistry. 24 h 150 C CO2 + DielsAlder product nal product O O CO2Me O H3C

Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous \(s p^{2}\) hybridized atoms.

An important variation of the DielsAlder reaction is intramolecular, in which the diene and the dienophile are connected. This type of DielsAlder reaction makes two new rings. Draw the compound produced in each of these examples; try to predict stereochemistry (using models will help). In some cases, Lewis acid catalysts are used; that can be ignored for this problem. (a) C N (b) O OSiR3 OR2 (c) O O O O SiMe3 (d) O O C CH Ar2 Ar1 Prof. A. Kirschning, Leibnitz U. Hannover Organic Letters, 2014, 16, 568. (Ar1 and Ar2 are different aryl groups.) Prof. H. Miyaoka, Tokyo U. Synthesis Letters, 2011, 547. (R and R2 are different alkyl groups.) Prof. H. Zhai, Lanzhou U. Organic Letters, 2014, 16, 216.