Hardest SNAr Regioselectivity Prediction

What position is kinetically favored for substitution?

  • 3-Cl

    Votes: 0 0.0%
  • 6-F

    Votes: 0 0.0%

  • Total voters
    1
  • Poll closed .
Sep 2019
5
0
North America
During one of my internship in a big pharma in San Francisco, I had to prepare a new core by SNAr on a tetrahalopyrimidine. Link to Reddit post

Predicting the regioselectivity of the second SNAr may seem obvious to an experienced eye, but take your time and explain. Will the substitution occur at the 3rd, 5th or 6th position?


I have the experimental result in my head and will reveal it after some posting.
 
Last edited:
May 2015
34
1
New Delhi -India
Best leaving group

We need to check best leaving group and carbon which is most electron deficient here.
 
Sep 2019
5
0
North America
It doesn't matter which is best leaving group. The first step is irreversible in basic conditions (formation of the anionic sigma-complex). We just need to know the lowest energy barrier that should be correlated to the intermediate's stability (Hammond's postulate for a endergonic step with a late transition state). We have to look at the most stabilized sigma-complex and look at steric hindrance for its formation.
 
Sep 2019
5
0
North America


We basically have to find out which of these two intermediates is more stable.
 
Feb 2020
2
0
India
It doesn't matter which is best leaving group. The first step is irreversible in basic conditions (formation of the anionic sigma-complex). We just need to know the lowest energy barrier that should be correlated to the intermediate's stability (Hammond's postulate for a endergonic step with a late transition state).
 
Feb 2020
2
0
India
I think second diagram is more stable.

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