ame: CEM 633: Advanced rganic Chem: ysical Problem Set 5 Due 11/10/17 Please do not look up references until after you turn in the problem set unless otherwise noted. For the following problems, please use Excel (or another graphing program), when necessary. Please submit your graphs with your problem set. 1. Consider the following theoretical reaction coordinate diagrams, describing the transformation of starting material A into product C via intermediate. For each reaction coordinate diagram, please answer the following questions: (a) Is there a possibility that intermediate may be observable using standard spectroscopic methods, such as M spectroscopy? (b) If there is the possibility that may be observable, what is the maximum possible energy difference between A and to allow observation of by M spectroscopy? Please explain your reasoning. (a) (a) Can be observed? G A (b) Maximum possible energy difference? reaction coordinate C (b) (a) Can be observed? G A (b) Maximum possible energy difference? reaction coordinate C 1
2. The temperature-dependent rates of rearrangement of allyl vinyl ether (eq 1) was studied in the gas phase. (1) Temp (K) k (s 1 ) 469.1 2.875 x 10 3 469.4 3.021 x 10 3 473.7 3.838 x 10 3 427.7 0.120 x 10 3 456.7 1.166 x 10 3 451.6 0.788 x 10 3 440.2 0.341 x 10 3 a. Provide a depiction of the transition structure for this transformation. b. Draw a reaction coordinate diagram for this transformation, clearly labeling all intermediates and transition states. c. Determine and S from the data (in kcal/mol and eu, respectively). d. Explain whether your proposed transition structure is consistent with the experimentally measured value of S. 2
3. Consider two possible mechanistic sequences for an acid-catalyzed aldol reaction, (i) and (ii). (i) (ii) Me K 1 Me Me K 1 Me A A k 2 Me k 2 k 2 Me k 2 C C k 3 Me Me C P k 3 Me Me C P (a) Derive a rate expression for sequence (i), using the steady state approximation where appropriate. Your final equation should only contain terms that are experimentally quantifiable (i.e., A, ). You may assume that the initial carbonyl protonation is a fast equilibrium and that the overall reaction is irreversible. (b) Derive a rate expression for sequence (ii), using the steady state approximation where appropriate. Your final equation should only contain terms that are experimentally quantifiable. You may assume that the initial carbonyl protonation is a fast equilibrium and that the overall reaction is irreversible. 3
(2 continued) (c) Show that the two sequences are kinetically distinguishable when the final step (i.e., C C bond formation) is ratelimiting. (d) Show that the two sequences are kinetically indistinguishable when the last step is rapid compared with the initial steps. 4
4. ne proposed mechanistic sequence for the aylis-illman reaction is shown below (e.g., J. rg. Chem. 2003, 68, 692). This reaction has recently been studied using the initial rates method (rg. Lett, 2005, 7, 147). You may assume that no observable intermediates accumulate during the course of the reaction. k 1 Me Me k 1 M D I A k 2 Me fast DAC, proton transfer (DAC) (a) Derive a rate expression for this sequence, using the steady-state approximation where appropriate. Me (b) Show that the steady-state approximation and the pre-equilibrium approximation (for saturation kinetics) are equivalent when k 1 is large compared with k 2. (c) Initial rate experiments showed that the initial rate is first order in [D], first order in [M], and second order in [A]. Evaluate the plausibility of the proposed mechanistic sequence on the basis of this initial rate data. ote: You will see kinetic isotope data for this reaction in the next problem set and be asked to propose an alternative mechanism then. 5
5. Please draw a Woodward offmann molecular orbital correlation diagram for the following reaction and use it to predict the stereochemistry of the product (stereocenters are starred). * * 6. Please propose an arrow-pushing mechanism for the following transformation. Please predict the stereochemical outcome (What is the stereochemistry of the starred carbons?) and explain your prediction. * * 6
7. (a) Please draw a reasonable arrow-pushing mechanism for the following reaction. exo favored endo disfavored (b) Please explain why the product named exo above is favored. 8. Please explain the following trend in reaction rates based on substitution of the styrene dienophile: p-me 2 p-me p-cl m- 2 p- 2 k (x 10 6 mol 1 s 1) 338 102 73 78 79 88 7
9. Using Dewar Zimmerman theory, please explain why Diels Alder cycloadditions proceed suprafacially on both the diene and dienophile. From Grossman, Chapter 4 13. Draw the product of each of the following [3,3] rearrangements, including its stereochemistry. (a) 3 C C 3 (c) 3 C C 3 C 2 C 3 (b) C 3 (d) 8
14. (a) Please propose a reasonable arrow-pushing mechanism for the following transformation (Jung JACS 1980, 102, 2463). Me Me MgX A a 75% * * * * Me Me (b) Predict the relative stereochemistry at each of the starred carbons and explain your prediction using clearly drawn structures. 9