Ene Reactions with Alkenes as Enophiles

Abstract

The thermal reaction of an alkene having an allylic hydrogen (an ‘ene’) with a compound containing a double or triple bond (enophile) to form a new bond with migration of the ene double bond and 1,5-hydrogen shift is referred to as the ene reaction (equation 1). The ene reaction is mechanistically related to the much better known Diels–Alder reaction since both reactions can be concerted, proceeding through cyclic transition states involving six electrons.1–5Stepwise ene reactions proceeding through either a diradical or zwitterionic intermediate are also known. Ene reactions proceed most readily when the enophile, like the dienophile in a Diels–Alder reaction, is electron deficient. Similarly, the ene, like the diene in the Diels–Alder reaction, should be electron rich. Although the exact order depends upon the enophile and reaction conditions, the relative reactivity of alkenes as ene components has typically been found to be 1,1-di- > tri- > tetra- > mono- > 1,2-disubstituted. Unfortunately, ene reactions typically occur at higher temperatures than related Diels–Alder reactions, limiting the synthetic utility of the ene reaction.