An abbreviated mechanistic description for DCC-mediated amide bond formation is

Question

An abbreviated mechanistic description for DCC-mediated amide bond formation is illustrated as Path A. An alternative pathway for this process has been proposed as well, illustrated in Path B. In this alternative pathway, a second molecule of acid 1 out-competes the amine 4 for the pivotal activated acyl derivative 3, leading to a transient symmetrical anhydride 7 (with discharge of 6). In Path B, anhydride 7 then is the active electrophile that reacts with amine 4 to give the observed amide product 5 with regeneration of one molecule of acid 1 Assuming that only Path B were operational, what amide product(s) would you anticipate if the reaction were run with 1 equiv. of the achiral acid 8, 1 equiv. of the racemic amine 9, and a limiting amount (0.1 equiv.) of the chiral diimide 10? Select one: Chiral amide 11a only. Chiral amide 11b only. An equal mix of 11a and 11b (= racemic mix). An unequal mix of 11a and 11b.

Explanation / Answer

Answer is C. An equal mix of 11a and 11b (= Racemic mixture)

Because reaction proceeds via path B. In path B chiral diimide makes the acid to anhydride where no chirality will play the role since resuting anhydride is symmetric. Followed by also racemic amine will attack the anhydride in either way to give racemic mixture of the product. The conclusion is the chirality of diimide is not used anywhere in the course of reaction. Hence Product will be observed in reaction mixture.

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