Ultra-violet light irradiation of N-benzoylmdole, 1, m the presence of mono-substituted alkenes results in the formation of cyclobutane adducts of potential synthetic utility.2$3 The reaction is regioselectrve but not stereoselecttve, so that stereoisomers of adduct 3 are formed while 2 is either not observed or comprises a minor proportion of the products.
We have shown that the lowest singlet excited state of many N-benzoylindole derivatives exhibits unusual fluorescence4 but is not involved in this cycloadditron reactron. Rather, adduct formation proceeds from the triplet excited state" in competrtron with photo-Fries rearrangement of the N-benzoyl gro~p.~ Kinetic evidence5 and the finding7 that both c.is and trum alkenes yield the similar mixtures of stereoisomeric adducts have led us to propose5 that the triplet excited state of the N-benzoylmdole reacts non-concertedly with the alkene to produce one or more triplet 1,4-biradrcal intermediates 4-7, as shown in Scheme 1. The evidence suggests that following spin inversion these 1,Cbiradrcals either ring close to give the isolated adducts or fragment back to the ground state starting materials. We have shown that this partitronmg