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?

a. Chiral amide 11a only.

b. Chiral amide 11b only.

c. An equal mix of 11a and 11b (= racemic mix).

d. An unequal mix of 11a and 11b.

Explanation / Answer

Answer d. An unequal mix of 11 a and 11 b because acid is achiral and it is making bond with chrial dimide will generate complex. Racemic amine has attacks on chiral complex. May one of the face is blocked by chiral dimide . Attack of racemic amine results an unequal mix of 11 a and 11 b.

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