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?

Hum, R0饵R. + GMtHQ O H2N-R 1 CyHN︿ NHCy Path A O ON R' \OH Cy ,N=C=N- R" Path B OO O O R/ \OH R, R 4

Select one:

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.

Answer 1

Answer C. An equal mix of 11 a and 11 b (racemic mixture).

Here reaction is passing through mechanism Path B. In bath B we are getting achiral anhydride 7(no chirality in 7). It is reactiong with racemic mixture. So it will give racemic mixture.

If reaction is going path A. you are getting compound 3 which is chiral in nature. chiral compound react with racemic mixture with different rates. So you will get unequal mix of 11 a and 11b.