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

Question

An abbreviated mechanistic description for DCC-mediated amide bond formation is illustrated C 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. H2N-R1 4 CyHN NHCy Path A O N Cy NEC N OH NHCy Cy Path B O H2N-R OH OH Assuming that only Path Awere 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? Ph Ph (10, chiral, 0.1 equiv) Ph NH2 Ph NH2 Ph OH Ph P 8(1 equiv) 9 (racemic mix, 1.0 equiv of 11a 11b amine in total)

Explanation / Answer

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

This can be explained as follows. Since the acid and the amine and carbodiimide all are chiral in nature, the rate of the reaction with each of the isomer would differ from the other and therefore, the intermediate (activated intermediate) with unequal amount of the chiral compounds that would again react different rate of reaction with chiral amines as well. Hence the products that are going to be formed are un equal mxture of 11a and 11b

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