Asymmetric catalytic formation of quaternary carbons by iminium ion trapping of radicals
<p> An important goal of modern organic chemistry is to develop new catalytic strategies for enantioselective carbon–carbon bond formation that can be used to generate quaternary stereogenic centres. Whereas considerable advances have been achieved by exploiting polar reactivity<s...
| Autores: | , , , , |
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| Tipo de recurso: | artículo |
| Fecha de publicación: | 2016 |
| País: | España |
| Institución: | Varias* (Consorci de Biblioteques Universitáries de Catalunya, Centre de Serveis Científics i Acadèmics de Catalunya) |
| Repositorio: | Recercat. Dipósit de la Recerca de Catalunya |
| OAI Identifier: | oai:recercat.cat:2072/305756 |
| Acceso en línea: | http://hdl.handle.net/2072/305756 https://doi.org/10.1038/nature17438 |
| Access Level: | acceso abierto |
| Sumario: | <p> An important goal of modern organic chemistry is to develop new catalytic strategies for enantioselective carbon–carbon bond formation that can be used to generate quaternary stereogenic centres. Whereas considerable advances have been achieved by exploiting polar reactivity<sup><a href="http://www.nature.com/nature/journal/v532/n7598/full/nature17438.html#ref1" id="ref-link-1" title="Quasdorf, K. W. & Overman, L. E. Catalytic enantioselective synthesis of quaternary carbon stereocentres. Nature 516, 181–191 (2014)">1</a></sup>, radical transformations have been far less successful<sup><a href="http://www.nature.com/nature/journal/v532/n7598/full/nature17438.html#ref2" id="ref-link-2" title="Murakata, M., Jono, T., Mizuno, Y. & Hoshino, O. Construction of chiral quaternary carbon centers by catalytic enantioselective radical-mediated allylation of α-iodolactones using allyltributyltin in the presence of a chiral Lewis acid. J. Am. Chem. Soc. 119, 11713–11714 (1997)">2</a></sup>. This is despite the fact that open-shell intermediates are intrinsically primed for connecting structurally congested carbons, as their reactivity is only marginally affected by steric factors<sup><a href="http://www.nature.com/nature/journal/v532/n7598/full/nature17438.html#ref3" id="ref-link-3" title="Fischer, H. & Radom, L. Factors controlling the addition of carbon-centered radicals to alkenes — an experimental and theoretical perspective. Angew. Chem. Int. Ed. 40, 1340–1371 (2001)">3</a></sup>. Here we show how the combination of photoredox<sup><a href="http://www.nature.com/nature/journal/v532/n7598/full/nature17438.html#ref4" id="ref-link-4" title="Schultz, D. M. & Yoon, T. P. Solar synthesis: prospects in visible light photocatalysis. Science 343, 1239176 (2014)">4</a></sup> and asymmetric organic catalysis<sup><a href="http://www.nature.com/nature/journal/v532/n7598/full/nature17438.html#ref5" id="ref-link-5" title="MacMillan, D. W. C. The advent and development of organocatalysis. Nature 455, 304–308 (2008)">5</a></sup> enables enantioselective radical conjugate additions to β,β-disubstituted cyclic enones to obtain quaternary carbon stereocentres with high fidelity. Critical to our success was the design of a chiral organic catalyst, containing a redox-active carbazole moiety, that drives the formation of iminium ions and the stereoselective trapping of photochemically generated carbon-centred radicals by means of an electron-relay mechanism. We demonstrate the generality of this organocatalytic radical-trapping strategy with two sets of open-shell intermediates, formed through unrelated light-triggered pathways from readily available substrates and photoredox catalysts—this method represents the application of iminium ion activation<sup><a href="http://www.nature.com/nature/journal/v532/n7598/full/nature17438.html#ref6" id="ref-link-6" title="Lelais, G. & MacMillan, D. W. C. Modern strategies in organic catalysis: the advent and development of iminium activation. Aldrichim. Acta 39, 79–87 (2006)">6</a></sup> (a successful catalytic strategy for enantioselective polar chemistry) within the realm of radical reactivity.</p> |
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