Chemistry 201: Organic Reaction Mechanisms I. Name: Midterm Exam October 31, 2007

Transcription

1 Chemistry 201: rganic Reaction chanisms I 1 ame: Midterm Exam ctober 31, 2007 Please read the entire exam before starting and answer the questions you understand best first. You should have 7 questions. Write all answers in the space provided. Use clear three dimensional drawings of structures to illustrate your points wherever possible. Your mechanism schemes must be neat and legible. Such organization helps your thought process and my review of your answers. Ambiguous drawings and mechanisms will not be given the benefit of the doubt. You are welcome to use your models and page 9 contains information that may be useful. Scratch paper is attached. Question 1 Score Total /89 = %

2 Question 1. (12 points total). 2 a. The C bond length in trifluoromethoxide is 1.21 Å. (Rablen J. Am. Chem. Soc. 2000, 122, 357.) i. What is a typical C bond length (e.g., in dimethyl ether)? ii. Why is length of the C bond in trifluoromethoxide different from a typical C bond? F F F C bond length = 1.21 Å b. Briefly explain the difference in A-values for a carboxylic acid and a carboxylate ion (3 points). A-value = 1.35 A-value = 1.92 c. Isoxazolidinones may be prepared from enoates and hydroxylamines (Aubé Tetrahedron 1987, 28, 179). Which cyclization, c or d, would you predict to be faster, and why? a c b C3 d

3 Question 2. (12 points) a. Circle the lower energy conformer, and give a brief explanation for your choice. Estimate ΔG o for the interconversion. 3!G o = A B Estimate the equilibrium distribution of the two conformers at room temperature: A: B = b. Circle the major product. Give a brief explanation for your choice. Use clear, threedimensional drawings of structures to illustrate your answer. t-bu 2 CuLi t-bu + t-bu

4 4 Question 3. (11 points) a. Briefly describe the major stabilizing and destabilizing interactions in the structure below, and illustrate your answer. b. Fill in the requested molecular orbitals. Illustrate (draw) and clearly label each M. All energy levels must be in order with respect to each other. S C3! and LUM! and LUM M and LUM LUM

5 Question 4. (12 points) M Theory. 5 + a. The Klopman-Salem equation contains several terms. In English, briefly describe what each term represents (1 phrase or sentence for each term).!e = b. For the Diels-Alder reaction above, which term of the Klopman-Salem equation will make the smallest contribution to ΔE and why? c. Which term is considered in Frontier Molecular rbital Theory? What factors determine the importance of this term?

6 Question 5. (12 points). Consider the following isomerization: 6 3!G = 2.8 kcal/mol (E) (Z) a. Estimate the equilibrium distribution of (E)- to (Z)-ketoimines: b. Provide an arrow-pushing mechanism for the isomerization. c. Reduction of this ketoimine provides one diastereomer of Simpkins base. Provide a rational for the formation of the major diastereomer. " " (E) Simpkins' base

7 7 Question 6. Suaveolindole is a natural product with activity against gram-positive bacteria (see: Danishefsky J. Am. Chem. Soc. 2007, 129, 10640). 2 C a. (6 points) Provide a clear 3-D drawing of the lowest energy conformation of suaveolindole. Illustrate the important factors that control the ground state conformation. You can abbreviate the structure: suaveolindole R Ar b. (14 points) Consider rotation around the C16 C17 bond. For all three conformers, provide ewman projections looking down the C16 C17 bond. Illustrate any important destabilizing factors. Circle the most stable conformer. Draw the most stable conformer using sawhorse notation C

8 Question 7. (10 points) Provide an arrow-pushing mechanism. Illustrate the relevant M- LUM interactions for one elementary step (Coates J. Am. Chem. Soc. 2003, 68, 6935). Don t worry about stereochemistry. 8 3

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