ps_2 (2) - Problem Set II M. Movassaghi Chemistry 5.43...

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Unformatted text preview: Problem Set II M. Movassaghi Chemistry 5.43 Tuesday 2.20.2007 Due in class: Monday, Feb. 26th, 2007 at 12:05 pm. 1. The cobalt complexes shown below catalyze the addition of water to propylene oxide. 2. Consider the following reaction (EWG = electron withdrawing group, X = leaving group): HH X EWG N t-Bu O t-Bu t-Bu O OAc t-Bu t-Bu t-Bu 1 a. O Me H N Co O O O Me racemic t-Bu 2 (single enantiomer) + H2O 1 (5 mol%) OH HO + H2O 2 (5 mol%) + B • HX a) Propose two possible mechanisms for this transformation. b) Compare the reaction of A with that of doubly deuterated derivative A-d2. Predict the isotopic effect in each of your proposed mechanisms for part "a" of this question and explain your reasoning, including the use of a free energy diagram. D D Me X EWG A-d2 100% racemic b. OAc t-Bu EWG A H N N Co Base (B) OH HO Me 50% When catalyst 1 is used, complete conversion to the diol is observed. However, when catalyst 2 is used, the addition of water proceeds to 50% (a significant drop in the reaction rate is observed beyond this point). c) If the mono-deuterated derivative A-d1 is used in the above experiment, provide the sturcture and ratio of the expected products in accord with each of your proposed mechanisms. H D X EWG A-d1 Explain these results, provide clear energy diagrams for equations a and b consistent with your explanation, and describe the stereochemical relationship between the products of equations a anb b. 1/2 Problem Set II M. Movassaghi 3. Base-catalyzed isomerization of 3-cyclohexenone to 2cyclohexenone was achieved in H2O and D2O. Chemistry 5.43 4. Treatment of alkoxide 3 with acetaldehyde gives ester 4: The rate of this isomerization in H2O was found to be seven times faster than in D2O. The incorporation of deuterium in the α-position of 3-cyclohexenone was found to be faster than the olefin isomerization. M+ O– O Et O O α equation 1 3 + O NaOH, H2O Me –O Me Me O step-1 M+ H O Et Me Int O Me step-2 M+ O– O Et Me 4 H kH a) Draw a reaction coordinate diagram consistent with step-1 being the rate-determining step. What would this imply about the reversibility of step-1? kH =7 kD O O α equation 2 D NaOD, D2O kD D b) Draw a reaction coordinate diagram consistent with step-2 being the rate-determining step. What would this imply about the reversibility of step-1? a) Provide a mechanism for the transformation depicted in equation 1 including arrow pushing. c) Provide a drawing and a description of the transition state structure for formation of the intermediate (Int) based on the Hammond Postulate consitent with your answer to 4a. b) Draw a reaction coordinate diagram for your answer to 3a. Clearly label all important features of the diagram. d) Consider your answer to 4a. Would you expect the rate to be faster in a polar or in a non-polar solvent? Use a reaction coordinate diagram to explain your reasoning. e) Provide a rate law for your answer to 4a. c) Which is the rate-determining step? Explain your reasoning. 2/2 ...
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