2+2+2-CYCLOADDITION VS RADICAL-ION CHEMISTRY IN THE PHOTOREACTIONS OF 1,2,4,5-BENZENETETRACARBONITRILE WITH ALKENES IN ACETONITRILE

Irradiation of 1,2,4,5-benzenetetracarbonitrile (TCB) in acetonitrile in the presence of 1-hexene leads to two isomeric tetrahydroisoquinolines through a 2 + 2 + 2 cycloaddition between TCB, the alkene, and MeCN. The process occurs with moderate quantum yield via a strongly polarized exciplex. With di- and polysubstituted alkenes no cycloaddition takes place, the only reaction observed being substitution of a cyano group by an allyl radical. This is a low quantum yield process occurring via deprotonation of the alkene radical cation and TCB*(-)-allyl radical coupling. The selectivity in the attack on the allyl radical depends on steric hindering. Alternatively, the alkene radical cation can be trapped by methanol, yielding a (beta-methoxyethyl)benzenetricarbonitrile. In the presence of water, the corresponding alcohols are not isolated, since they rapidly fragment to yield alkylbenzenetricarbonitriles. The mechanism is discussed on the basis of the competition of chemical reactions and back electron transfer.