Isomerization of epoxides to allylic alcohols under strongly basic conditions proceeds by a β-elimination process. A model has been advanced that invokes an initial complex between the lithium amide base and epoxide. Concerted C–O bond cleavage and deprotonation proceeds via a syn transition state to give an allylic alkoxide, which is protonated upon workup. Deprotonation typically occurs at the exist in the transition state for cis double bond formation.
Other processes may take place competitively under basic conditions, particularly when β-elimination is slow or not possible. These pathways likely begin with lithiation of a carbon in the epoxide ring, followed by α-elimination to afford a carbene intermediate. 1,2-hydrogen migration leads to ketones, while intramolecular C–H insertion affords cyclic alcohols with the formation of a new carbon-carbon bond.
In many cases when hexamethylphosphoramide (HMPA) is used as an additive with lithium amide bases, selectivity for the formation of allylic alcohols increases. These reactions are believed to proceed through E2 elimination.