Review of Alkyl/Alkenyl/Aryl Group Migrations Initiated by Acid-Catalyzed Epoxide Ring-Opening

Acidic Conditions :

• Ring-opening reactions proceed rapidly at low temperatures (usually at or below room temperature).
• The elongated C-O bonds of the protonated, highly strained epoxides are believed to be the origin of high reactivity.
• The result is an SN2-like reaction in terms of stereochemical inversion but at the more substituted Carbon.

Acid-Catalyzed Epoxide Ring-Opening Mechanism:

• Step 1 - Protonate the epoxide.
• Step 2 - The more substituted Carbon has the longer CO bond and is therefore attacked from the opposite side, resulting in specific stereochemistry & regiochemistry.

• The ring opens at the more substituted carbon because this carbon will be more stable with a positive charge.

Alkyl/Alkenyl/ Aryl Group Migrations:

• Migrations will occur if the resulting carbocation is more stable than the former
• Most of the time these groups will migrate so that the most substituted carbon atom has the positive charge
• In this reaction, however, the oxygen can take the positive charge instead which is even more stable than a highly substituted carbocation, so the alkenyl group migrates to form the  more stable carbocation.

Use of Protecting Groups:

• Protecting groups are used to prevent atoms such as oxygen from reacting with reactants when a reaction is desired only at one place on the molecule at a certain stage.
• For example, if the oxygens in this reaction were protonated or deprotonated at the wrong time, the alkenyl group migration and epoxide ring opening would not have occurred. The protecting group bonds to oxygen and keeps this from happening.

at the Mechanism for an Example !!!!

Close