Todd:Chem3x11 ToddL7
Chem3x11 Lecture 7
Unfinished Sat May 12.
This lecture is about the opening of epoxides a surprisingly important aspect of organic synthesis.
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Key concepts
- Epoxides are stable and easy to make, but are reactive to nucleophiles
- There are very useful stereoelectronic aspects of the epoxide opening reaction, giving precisely defined product stereochemistry
The Opening of Epoxides
An epoxide is a strained ring system that can be opened by a large range of nucleophiles. Epoxides may easily be made from double bonds. Though reactive, they can be isolated and handled easily.
As you can see, the simple mode of epoxide opening involves a reaction like an SN2 mechanism, except in this case the "leaving group" oxygen is still attached to the molecule. So one of the carbons (where the nucleophile attacks) will be inverted and the other will stay unchanged.
We need to examine two cases. The first is where the epoxide carbons are substituted. The second is when the epoxide is part of a ring.
Direct Nucleophilic Opening of Epoxides
When we combine an epoxide with a reactive nucleophile, and if the two carbons of the epoxide are differently substituted, then the choice of which carbon the nucleophile goes for is typically determined by the same factors as for SN2 reactions. So attack is quicker at primary centres, slower at secondary centres and virtually non-existent at tertiary centres. When there is a preference between different reaction sites on the same molecule that involve similar chemistry we say there is regioselectivity, or that the steric crowding of the substitutents causes regiocontrol. (If there is a choice between two different functional groups we say there is chemoselectivity).
Acid-Mediated Epoxide Opening
Things change when the epoxide is activated by acid.
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