Early work showed that allylic alcohols give facial selectivity when using meta-chloroperoxybenzoic acid (m-CPBA) as an oxidant.
This finding leads to the conclusion that hydrogen bonding played a key role in selectivity and the following model was proposed.
[2] However, it was found that for metal catalyzed systems such as those based on vanadium, reaction rates were accelerated when the hydroxyl group was in the axial position by a factor of 34.
This geometry allows for the peroxide to be properly positioned, as well as to allow minimal donation from the C-C pi into the C-O sigma star.
In systems that hydrogen bond, A1,3 strain plays a larger role because the required geometry forces any allylic substituents to have severe A1,3 interactions, but avoids A1,2.
Homoallylic alcohols are effective directing groups for epoxidations in both cyclic and acyclic systems for substrates which show hydrogen bonding.
While hydrogen bonding substrates give the same type of selectivity in allylic and homoallylic cases, the opposite is true of vanadium catalysts.
A transition state proposed by Mihelich shows that for these reactions, the driving force for selectivity is minimizing A1,3 strain in a pseudo chair structure.
Although it is the least reactive metal catalyst for epoxidations, vanadium is highly selective for alkenes with allylic alcohols.