Question

5. Lithium bis(trimethylsilyl)amide (LIHMDS) is a common base in organic synthesis. Please predict the major and minor products from the reaction with compound 1. Then, sketch a reaction energy diagram showing formation of products A (major) and B (minor). (8 pts) (LIHMDS) THE major (product A) minor (product B) Energy rxn

Answer 1

The given reaction is an example of elimination reaction. Now, generally during elimination reaction, the more substituted alkene becomes the major product, when both the alkyl halide and the base is sterically less-demanding. But, for sterically crowded bases, the hydrogen which is in less crowded, is abstracted to avoid steric crowding in the transition state. In the given reaction, the base is very bulky, so it will abstract less - crowded 10 methyl hydrogen preferentially #. less crowded x hydrogens ci More crowded hydrogen 1 LiHMDS less stable, but I | Produced at a tastes rate More stable, but produced at slower major product (A) Minor Product (B B Ran. Coordinate -

LiHMDS is a bulky base and therefore prefers to abstract the hydrogen from a less crowded site, i.e. from the methyl group. Thus, formation of "A" is associated with lower activation energy than "B".

This is why, less hyperconjugatively stable alkene is the major product in this reaction and more hyperconjugatively stable alkene is minor product.