STEREOSPECIFIC NUCLEOPHILIC TRAPPING OF ENCOUNTER COMPLEXES BETWEEN PHOTOEXCITED 1-CYANONAPHTHALENE AND NORBORNADIENE

Photo-induced electron donor-acceptor reactions between 1-cyanonaphtha lene (CNN) and norbornadiene (N) generate products of several structur e types. Methanol adducts (1-3) formed in polar solvents are rationali zed via the radical cation, N+., and stereospecific (exo-) nucleophili c attack by methanol. In less polar solvents, CNN and N form [2 + 2]-c ycloadducts, exclusively on the exo-face of N. In non-polar solvents c ontaining methanol, CNN, N and methanol combine to form 1:1:1 adducts, containing the sensitizer on the ende-and the methoxy groups on the e xo-face. The formation of these products is rationalized via the trapp ing of encounter complexes of different geometries. Any rearrangement of the norbornenyl system can be eliminated, since neither tricyclyl n or 7-methoxynorbornenyl structures are formed. Apparently, the alcohol captures an endo-encounter complex of CNN and N by attack from the ex o-face, similar to the attack of methanol on N+.. (C) 1998 John Wiley & Sons, Ltd.