Intermolecular oxidative dehydrogenative 3,3[prime or minute]-coupling of benzo[b]furans and benzo[b]thiophenes promoted by DDQ/H+: total synthesis of shandougenine B

By Wirtanen, T.; Muuronen, M.; Hurmalainen, J.; Tuononen, H. M.; Nieger, M.; Helaja, J.
Published in Org. Chem. Front. The Royal Society of Chemistry 2016

Abstract

With an excess of a strong acid, 2,3-dichloro-5,6-dicyano-1,4-quinone (DDQ) is shown to promote metal-free intermolecular oxidative dehydrogenative (ODH) 3,3[prime or minute]-coupling of 2-aryl-benzo[b]furans and 2-aryl-benzo[b]thiophenes up to 92% yield as demonstrated with 9 substrates. Based on the analysis of oxidation potentials and molecular orbitals combined with EPR, NMR and UV-Vis observations, the studied reaction is initiated by a DDQ-substrate charge transfer complex and presumably proceeds via oxidation of the substrate into an electrophilic radical cation that further reacts with another molecule of a neutral substrate. The coupling reactivity can easily be predicted from the oxidation potential of the substrate and the morphology of its frontier molecular orbitals. The intermolecular ODH coupling reaction allowed a concise total synthesis of the natural product shandougenine B.

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