The direction of rearrangement depends on the nucleophile
The important difference between the following two reactions is the nucleophile used in the reaction. Both reactions go through a very similar aziridinium intermediates, so the difference must be due to the regioselectivity with which the aziridinium opens in each case.
- Supporting information
- Hydroxide nucleophile
- Water nucleophile
The arizidium ion is a compound that contains two alternative leaving groups–one from a primary centre and one from a secondary one. Primary centres can take part in fast SN2 reactions but cannot undergo SN1. Secondary centres can undergo either SN1 or SN2. Hydroxide is a good nucleophile and can therefore do fast SN2 reactions at primary centres, whereas water is a poor nucleophile and cannot. Water waits until the leaving group has left of its own accord, to give a cation, which rapidly grabs any nucleophile. This can only happen at the secondary centre as the primary cation is too unstable to form.
Hydroxide opens the aziridinium at the less hindered end, so the amine migrates from the primary to the secondary position.
Water opens the aziridinium at the more hindered end, so the amine migrates from the secondary to the primary position.