PATERNO-BUCHI REACTIONS VIA CORRELATED RADICAL-ION PAIRS - A CIDNP INVESTIGATION

The Paterno-Buchi photocycloadditions of electron-deficient carbonyl c ompounds (quinones) with donor olefins (anetholes) in polar solvent (a cetonitrile) were studied by measurements of chemically induced dynami c nuclear polarization (CIDNP). Contrary to mechanistic hypotheses put forward in the literature for Paterno-Buchi reactions of related syst ems, the cycloadditions were found to occur via radical ion pairs (RIP s), the key step being combination of spin-correlated RIPs in triplet states to give triplet biradicals (3)BR. Direct formation of (3)BR wit hout the intervention of RIPs could be excluded by CIDNP experiments w ith photoinduced electron transfer sensitization. This novel cycloaddi tion mechanism may be of some generality for donor-acceptor photocyclo additions in polar media.