Ch16: SN1 type reactions of epoxides

Chapter 16: Ethers, Epoxides and Sulfides

S_N1 type Reactions of Epoxides

SN1 like ring opening reactions of epoxides

Reaction type: Nucleophilic Substitution (S_N1 like)

Before weaker nucleophiles react with epoxides the epoxide must first be protonated so the reaction conditions are acidic.
Protonation makes the epoxide more electrophilic and creates a better leaving group.
Examples of nucleophiles requiring this approach : H₂O, ROH etc.
The leaving group is the protonated oxygen atom of the epoxide in the form of a neutral alcohol.
Typically the nucleophile is then deprotonated to give a neutral product.
For non-symmetrical epoxides, there is a question of regioselectivity :

Scenario 1: The Nu attacks the more substituted C of the epoxide at 180^o to the C-O bond that breaks.
Scenario 2: The Nu attacks the least hindered end of the epoxide at 180^o to the C-O bond that breaks.

Experimental results show that scenario 1 is typically observed with these weaker nucleophiles such as water or alcohols.
Recall that S_N1 reactions of alkyl halides follow the order 3^o > 2^o > 1^o(i.e. favours the more stable carbocation)
In order to appreciate this type of reaction we should consider what happens if we break each of the C-O bonds....

If the more stable carbocation is favoured then the nucleophile attacks the more substituted end of the epoxide.
This results in the formation of the less highly substituted alcohol.
Note that although the carbocation does not usually form the carbocation character is important.

In reality it is usually that the epoxide C-O bond to the more substituted center that is weaker resulting in carbocation character at that more substituted C and the nucleophile attacks there....

Stereochemistry ? Because the C-O bond is not totally broken when the nucleophile attacks, the nucleophile still attacks at 180^o with respect to the leaving group so there is still inversion at that center.

This is best described as an S_N1 type of reaction since carbocation character is involved.

Summary

Under acidic conditions, epoxides open in an "S_N1 like" fashion with the nucleophile attacking the more substituted end and with inversion of stereochemistry at that center.