Promblem Set #02 A - Chemistry 436 Professor Paul J....

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Chemistry 436 Professor Paul J. Hergenrother Spring 2009 University of Illinois, Urbana-Champaign Answers to Problem set #2, Due February 6, 2009 1) Triphenylphosphine (Ph 3 P) is the phosphine of choice in the preparation of Wittig reagents. Could trimethylphosphine (Me 3 P) be successfully used? Why or why not? To answer this problem, you need to think about the mechanism of the Wittig reaction. After attack of triphenylphosphine on the alkyl halide, one gets the phosphonium salt: P Br P + phosphonium salt This then gets deprotonated by a strong base to form the ylide: P n-BuLi P phosphonium salt ylide The ylide then reacts with the carbonyl compound. Note that in this ylide formation, there is only one carbon adjacent to phosphorous that has a H on it, and thus only one proton for the base to abstract. Now consider the case with trimethylphosphine: P H 3 C H 3 C n-BuLi phosphonium salt ylide CH 3 P H 3 C H 3 C CH 3 P H 3 C H 3 C CH 2 ylide + In this case, there are multiple carbons adjacent to P that have protons on them,
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Promblem Set #02 A - Chemistry 436 Professor Paul J....

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