Diels-Alder reaction between indoles and cyclohexadienes photocatalyzed by a (thia)pyrylium salt

Abstract

Thiapyrylium salt 1 is an efficient photocatalyst in the Diels-Alder reaction between indoles (InHs) and 1,3-cyclohexadienes (CHDs). For instance, irradiation of deaerated dichloromethane solutions containing 1 (1 mM), indole (20 mM) and cyclohexadiene (40 mM), in the presence of an acylating agent, led to the Diels-Alder cycloadduct in 62% yield (endo:exo ratio of 1.8:1). Taking into account the very high intersystem crossing yield (Φisc = 0.97) of thiapyrylium salt 1, involvement of its triplet excited state in this Diels-Alder reaction has been studied using steady-state and time-resolved experiments; the results are compared with those previously obtained for pyrylium salts, which exhibit a considerable fluorescence quantum yield and are assumed to operate via electron transfer from the InH to their singlet excited state. ©ARKAT USA, Inc.