Böhm Alexander, Bach Thorsten
Department Chemie and Catalysis Research Center (CRC), Technische Universität München, Lichtenbergstraße 4, 85747, Garching, Germany.
Chemistry. 2016 Oct 24;22(44):15921-15928. doi: 10.1002/chem.201603303. Epub 2016 Sep 15.
β-(3-Iodopropoxy)-substituted α,β-unsaturated lactams, lactones, and cycloalkenones (eight examples) underwent reductive radical reactions in a dichloromethane solution of N,N-diisopropylethylamine (Hünig's base) upon irradiation with visible light (λ=419 nm). 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 Hünig'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 Cl solutions as shown for two β-(3-iodopropoxy)-substituted tetronates, which underwent in dichloromethane almost exclusive reduction, but gave predominantly the cyclization products in CD Cl .
β-(3-碘丙氧基)取代的α,β-不饱和内酰胺、内酯和环烯酮（8个实例）在N,N-二异丙基乙胺（胡尼希碱）的二氯甲烷溶液中，于可见光（λ=419 nm）照射下发生还原自由基反应。除了简单的还原反应（氢脱碘反应）外，在3个实例中还观察到了显著程度的环化反应。与底物的转化同时，观察到了颜色很深的副产物的形成。基于质谱证据并与已知化合物进行比较，副产物被鉴定为花青染料。它们的形成支持了这样一种假设，即照射胡尼希碱的二氯甲烷溶液会导致自由基的形成，这些自由基进而可以引发自由基反应或与花青前体结合。氘标记实验表明，一当量的二氯甲烷被并入花青染料中，并且自由基中间体的还原猝灭至少部分是由于从溶剂中夺取氢。因此，在CDCl溶液中起始原料的还原环化反应更有利，如两个β-(3-碘丙氧基)取代的特窗酸酯所示，它们在二氯甲烷中几乎只发生还原反应，但在CDCl中主要生成环化产物。
