Stereoselective C,C-bond formation. Cyclizations of biradicals*

Abstract: The formation of C,C-bonds via photolytically generated biradicals can occur with high stereoselectivity. If triplet biradicals are involved, chiral induction is highly likely. Syntheses that occur only via singlet biradicals have a good chance to show a memory effect of chirality. It has been known for about 20 years that radicals are versatile intermediates in organic synthesis [1], and 5–10 years ago it became clear that the rules for stereoselectivity can be applied also for reactions between radicals and nonradicals (Fig. 1) [2]. However, for reactions between two radicals, which occur with diffusion-controlled rates, one cannot expect that substituents induce stereoselectivity. We have now shown that this changes in reac-tions of photolytically generated biradicals: Both the singlet and the triplet biradicals can react with high stereoselectivity, but the outcome of the reactions depends upon the multiplicity of the biradical