Recent Advances in Ene Reactions with Carbon Enophiles

Ene reactions are inter- or intramolecular addition reactions between a four-electron "ene" component and a two-electron enophile functionality, in which two π-bonds and a C–H (or metal–C) σ-bond are rearranged into one π-bond and two σ-bonds. One of the newly formed σ-bonds is a carbon–carbon bond, while the other is the rearranged a C–H (or metal–C) bond. This transformation enables the formation of structurally and functionally complex ene products in a single step. Originally, ene reactions were discovered as concerted and stepwise pericyclic elementary processes. However, increasing mechanistic insight—gained through both experimental and computational studies—along with extensive empirical methodological work, has paved the way to a plethora of ene-type reactions involving carbon enophiles. Since 2012, the already well-established transition metal-catalyzed variants of ene reactions have significantly expanded. Moreover, highly reactive aryne intermediates—generated in situ either by fluoride induced elimination from ortho-silyl aryltriflates at room temperature, or by hexadehydro-Diels–Alder reaction of a 1,3-diyne with an alkyne (both inter- and intramolecularly)—find increasing application in novel ene-type sequences. Propargyl ene reactions also gain traction, particularly because the resulting ene-allenes serve as highly valuable intermediates in the design of domino sequences that lead to complex polycyclic fused structures. Finally, the implementation of ene reactions in polymer chemistry for the synthesis of functionalized polymers and renewable unsaturated raw materials has emerged as a promising alternative to the use of purely petrochemistry based substrates.