An electron-catalyzed cope cyclization. The structure of the 2,5-dicyano-1,5-hexadiene radical anion in the gas phase

The radical anion of 2,5-dicyano-1,5-hexadiene is shown to undergo Cope cyclization in a flowing afterglow-triple quadrupole apparatus. The cyclic structure of the 2,5-dicyano-1,5-hexadiene radical anion was established by using chemical reactivity. The ion reacts with CO2 and CS2 by addition, whereas the radical anions of closed-shell molecules such as fumaronitrile do not react with these reagents. The ion exhibits reactivity characteristic of a distonic ion in that it sequentially adds CO2 and NO or NO2. It reacts with NO by forming a product at m/z 135 corresponding to addition followed by loss of HCN. The reactivity and CID spectrum of the product ion at m/z 135 agrees with that of oximate ion, which requires a cyclic precursor ion. Attempts to generate radical anions of acrylonitrile and 2,6-dicyano-1,6-heptadiene were unsuccessful, providing additional evidence against a linear structure as a stable structure for 2,5-dicyano-1,5-hexadiene radical anion. The cyclization of the radical anion of the 2,5-dicyano-1,5-hexadiene is the first example of an electron-catalyzed Cope cyclization.