Charge‐Transfer Cycloaddition of Homobenzvalene with Tetracyanoethylene

The transient yellow color observed inthe cycloaddition of homobenzvalene (HB) with tetracyanoethylen (TCNE) is associated with the charge‐transfer complex [HB, TCNE]. The deliberate photoexcitation of [HB, TCNE] affords a mixture of charge‐transfer cycloadducts (1, 2, 3) that differs from that obtained in thermal cycloaddition. The relationship of [hb ⁺ TCNE.] radical‐io pai (as the critical reactive intermediate in charge‐transfer cycloaddition) to the activation process for thermal cycloaddition is discussed.