Captodative olefins in normal and inverse Diels-Alder Reactions

Olefins with captodative substitution are reactive dienophiles in Diels‐Alder reactions with normal and inverse electron demand. This is shown for reactions of 2‐(tert‐butylthio)acrylonitrile (1) with various dienes and heterodienes, e.g. 1,3‐cyclohexadiene, hexachloro‐1, 3‐cyclopentadiene, acrolein, methacrolein, and methyl vinyl ketone (Schemes 2 and 3). In case of the hetrodienes, 3,4‐dihydro‐2H‐pyrans are formed beside small amounts of tetrahydrothiophenes; however, with methyl vinyl ketone, both reaction pathways are equally followed. The high reactivity of captodative olefins in Diels‐Alder reactions are rationalized on the basis of Sustmann's FMO model under consideration of Viehe's concept of captodative substitution of alkenes.