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

Irradiation of 1,2,4,5-benzenetetracarbonitrile (TCB) in acetonitrile in the presence of 1-hexene leads to two isomeric tetrahydroisoquinoli nes through a 2 + 2 + 2 cycloaddition between TCB, the alkene, and MeC N. The process occurs with moderate quantum yield via a strongly polar ized exciplex. With di- and polysubstituted alkenes no cycloaddition t akes place, the only reaction observed being substitution of a cyano g roup by an allyl radical. This is a low quantum yield process occurrin g via deprotonation of the alkene radical cation and TCB(-)-allyl rad ical coupling. The selectivity in the attack on the allyl radical depe nds on steric hindering. Alternatively, the alkene radical cation can be trapped by methanol, yielding a (beta-methoxyethyl)benzenetricarbon itrile. In the presence of water, the corresponding alcohols are not i solated, since they rapidly fragment to yield alkylbenzenetricarbonitr iles. The mechanism is discussed on the basis of the competition of ch emical reactions and back electron transfer.