Abstract
Suitably functionalised carboxylic acids undergo a previously unknown photoredox reaction when irradiated with UVA in the presence of maleimide. Maleimide was found to synergistically act as a radical generating photoxidant and as a radical acceptor, negating the need for an extrinsic photoredox catalyst. Modest to excellent yields of the product chromenopyrroledione, thiochromenopyrroledione and pyrroloquinolinedione derivatives were obtained in thirteen preparative photolyses. In situ NMR spectroscopy was used to study each reaction. Reactant decay and product build-up were monitored, enabling reaction profiles to be plotted. A plausible mechanism, whereby photo-excited maleimide acts as an oxidant to generate a radical ion pair, has been postulated and is supported by UV/Vis. spectroscopy and DFT computations. The radical-cation reactive intermediates were also characterised in solution by EPR spectroscopy.
Highlights
Radical methodology has developed into an important tool for synthetic organic chemists by virtue of the fact that highly reactive species can be generated under mild and neutral conditions, often without the need to employ protecting groups.[1,2,3,4,5,6,7,8,9,10]
The generation of carbon-centred radicals has traditionally been dominated by reductive chain processes mediated by group 14 metal hydrides, tin.[11]
We recently investigated the photoredox reactions of phenoxyacetic acids with various acceptor alkenes mediated by photo-excited TiO2.[43]. We have found that suitably functionalised carboxylic acids, in combination with maleimide, will take part in a novel reaction sequence when irradiated with mild UVA and in the complete absence of any extrinsic photo
Summary
Radical methodology has developed into an important tool for synthetic organic chemists by virtue of the fact that highly reactive species can be generated under mild and neutral conditions, often without the need to employ protecting groups.[1,2,3,4,5,6,7,8,9,10] The generation of carbon-centred radicals has traditionally been dominated by reductive chain processes mediated by group 14 metal hydrides, tin.[11]. Carboxylic acids are well known to homolytically decarboxylate upon photolysis, generating carbon centred radicals in many cases.[20,21,22] The carboxylate moiety, generally needs to be irradiated with short wavelength Disadvantages include the need to remove these agents following reaction, commercial availability and health concerns in several instances.
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