title: Mechanism of triplet photosensitized Diels-Alder Reaction between indoles and cyclohexadienes: theoretical support for an adiabatic pathway.
abstract: Diels-Alder reactions between indoles (InHs) and 1,3-cyclohexadienes (CHDs) were achieved by using aromatic ketones as photosensitizers. For instance, irradiation of deaerated dichloromethane solutions containing benzoylthiophene (BT, 1 mM), indole (20 mM), and phellandrene (40 mM), in the presence of an acylating agent, led to the N-acetylated Diels-Alder cycloadduct in 46% yield (endo:exo ratio of 1.8:1). Energy transfer from the BT triplet to the CHD gave rise to diene dimers as byproducts. Several combinations of CHDs, InHs, and aromatic ketones were tested; the Diels-Alder reaction was found to be a general process, except when the indole nucleus was substituted at position 2 or 7 and when aromatization of the CHD was favored. Theoretical calculations support a stepwise mechanism involving a triplet ternary complex TC(T1), arising from a nearly barrierless reaction between CHD and the 3(BT...HIn) exciplex. All subsequent steps proceed downhill in the triplet excited state, leading to a triplet cycloadduct-sensitizer CA-BT(T1) radical pair. Attempts to detect this species, which is basically an aminyl/BT ketyl radical pair, were performed by laser excitation of a solution containing BT, phellandrene, and indole. The observed transient absorption spectra could be compatible with the added spectra of the expected components of the radical pair.
authors.affiliation: Instituto de Ciencia Molecular, Departamento de Química Organica, Universidad de Valencia, Polígono La Coma, 46980 Paterna, Valencia, Spain.
authors.forename: María
authors.initials: M
authors.lastname: Gonzalez-Béjar
authors.forename: Salah-Eddine
authors.initials: SE
authors.lastname: Stiriba
authors.forename: Luis R
authors.initials: LR
authors.lastname: Domingo
authors.forename: Julia
authors.initials: J
authors.lastname: Pérez-Prieto
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Miranda