Chem 221b

Problem Set 2

Chapter 15

Due: Monday, January 31, 2005


See Pericyclic Reactions in the StudyAids for a review on conjugation, cycloadditions, and electrocyclic reactions. Electrocyclic reactions are not covered in the text.

1.
1
Camphorquinone (1; CQ) is used as a photosenitizer by your dentist. CQ absorbs electromagnetic radiation and causes polymerization of a monomer, bis-GMA. These composites have mostly replaced silver amalgams. At the right is a spectrum of CQ.
a) What kind of spectrum is it?
b) What is the λmax of CQ?
c) What concentration (mg/mL) of CQ is required to produce this spectrum [εlog=1.49 at λmax ]? [Assume a 1 cm cell.]
d) What color light does CQ absorb?
e) What color is CQ?
f) What color are the glasses that you wear when the dentist zaps you with the "light gun"? Why are they that color?

2. Draw the structures of the Diels-Alder adducts arising from 1,3-pentadiene and CH3CH=CHCO2CH3 under the conditions described in the Table. Provide a transition state model for example 2. Which Examples give the same racemic product? Explain.

Example
Diene
Dienophile
Transition State

1

E

E

endo

2

E

Z

endo

3

Z

E

endo

4

Z

Z

endo

5

Z

E

exo

 


3. In each of the problems below, provide the missing information, i.e., conditions, product, reactant, etc. Also, explain the reason for your choice.

 

 


4. Compound A is colorless an it is devoid of absorption in the range 200-800 nm. Thermolysis of A affords B, which absorbs at 227 nm. When B is heated with maleic anhydride, compound C is formed, which has five singlets in its broadband decoupled 13C NMR spectrum. Compound C displays vinyl hydrogens in its 1H NMR spectrum. What are the structures A-C? Explain and illustrate.

5. An optically active monoterpene A has λmax = 265 nm (calc. 263 nm). Oxidation of A with warm KMnO4 leads to the isolation of (R)-isopropyl succinic acid. Hydrogenation of A affords B and C. C is less stable than B. Both B and C have 7 singlets in their respective broadband decoupled 13C NMR spectra. Treatment of A with methyl acrylate leads to the formation of an endo Diels-Alder adduct D. What are the structures of A-D. Explain and illustrate.

6. Complete the following problems. Be sure to explain the role of orbital symmetry.

.

7. Explain and illustrate in terms of HOMOs and LUMOs why 1,3-butadiene and allyl cation undergo concerted, thermal cycloaddition but the allyl anion does not.