[4 + 2]‐Cycloadditions with 1,4‐Bis(N,N‐dimethylamino)‐1,3‐dienes: Stereochemical Studies and Observation of Radical Ions

Electron transfer as a mechanistic alternative in [4 + 2] cycloadditions has been examined in the case of the electron-rich dienes 1–3 reacting with a series of dienophiles of increasing acceptor strength. ESR spectroscopy permits the phenomenon actually to be observed with dienophiles having reduction potentials ≥ −0.5 V (vs. SCE; oxidation potential of the dienes ca. −0.3 V). In the absence of electron transfer, reaction of 1 or 2 with E,Z-isomeric dienophiles is stereospecific; otherwise it is a multistep process, and thus not stereospecific.