Pset_6_Soln - Massachusetts Institute of Technology Organic...

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Massachusetts Institute of Technology Organic Chemistry 5.12 Problem Set 6 Solutions April 28, 2011 By T.A. Phil Hamzik 1. Brønsted acids (e.g., HCl) are not the only catalysts of reversible addition reactions to carbonyl groups. Lewis acids may also be used. For the BF3–catalyzed formation of the ketal shown, write an arrow-pushing mechanism (for the forward direction (left-to-right) only). (FYI: Et2O•BF3, boron trifluoride etherate, is a convenient source of BF3 but itself is not the catalyst of the reaction below. Why?) ANSWER: The boron atom in Et 2 O•BF 3 is coordinatively saturated. Et 2 O must thus dissociate for BF3 to catalyze the reaction: O B F F F BF 3 + O The mechanism for lewis acid catalyzed acetal formation is analogous to the Brønsted acid catalyzed version: O BF 3 O BF 3 HO OH O BF 3 O OH H proton transfer O BF 3 O OH H O OH O O H HO BF 3 O O + BF 3 H 2 O 2. The synthesis below has many, many problems with it. What are they? How would you revise it in order to convert ethyl acetate into phenylacetaldehyde ? Feel free to radically overhaul the synthesis. As long as you start with ethyl acetate and end with phenylacetaldehyde, you need not make all of the intermediate compounds shown. ANSWER: Flaws in the proposed synthesis are detailed below:
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Me OEt O PhLi THF Me O LiAlH 4 H 2 O Me OH NaOEt EtOH cat. H + H 2 O HO K 2 Cr 2 O 7 H 2 SO 4 O H LAH reductions should not be run in protic solvents. H - will act as a base and remove a proton from water,
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Pset_6_Soln - Massachusetts Institute of Technology Organic...

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