Question

Why anti group migrates in beckmann's rearrangement?

Answer 1

The Beckmann rearrangement also involves an alkyl migration. However, this migration is not governed by migratory aptitude. In fact, the which is in anti-position to the hydroxyl group in the oxime migrates, irrespective of its migratory aptitude!

Answer 2

Explanation:

In the Beckmann Rearrangement, the group trans to the -OH group migrates as C=N double bonds can exhibit cis/trans isomerism just like C=C double bonds can. ... For migration to occur, a migrating group has to be able to interact with the a* of the bond to the leaving group, and this is the reason for the specificity here.