Radical Reactions Induced by Visible Light in Dichloromethane Solutions of Huenig's Base: Synthetic Applications and Mechanistic Observations

beta -(3-Iodopropoxy)-substituted alpha , beta -unsaturated lactams, lactones, and cycloalkenones (eight examples) underwent reductive radical reactions in a dichloromethane solution of N,N-diisopropylethylamine (Huenig's base) upon irradiation with visible light ( lambda =419nm). Apart from plain reduction reactions (hydro-de-iodination), a significant degree of cyclization was observed in three cases. In parallel to the conversion of the substrates, the formation of intensely colored by-products was observed. Based on mass spectrometric evidence and upon comparison with known compounds, the by-products were identified as cyanine dyes. Their formation supports the hypothesis that irradiation of dichloromethane solutions of Huenig's base leads to the formation of radicals, which in turn can either initiate a radical reaction or combine with cyanine precursors. It was shown by deuterium-labelling experiments, that one equivalent of dichloromethane is incorporated into the cyanine dyes and that the reductive quenching of radical intermediates is at least partially due to hydrogen abstraction from the solvent. As a consequence, a reductive cyclization of the starting materials is favored in CD sub(2)Cl sub(2) solutions as shown for two beta -(3-iodopropoxy)-substituted tetronates, which underwent in dichloromethane almost exclusive reduction, but gave predominantly the cyclization products in CD sub(2)Cl sub(2). Colored footprint: The visible-light-mediated radical reaction of substrates 1 (X=O, CH sub(2), NH; n=1,2) is triggered by photochemical excitation of an electron-donor-acceptor complex between Huenig's base and CH sub(2)Cl sub(2). Strongly absorbing by-products were observed after irradiation and were identified as cyanine dyes.