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Catalytic 1,2-Dihydronaphthalene and E-Aryl-Diene Synthesis via CoIII-Carbene Radical and o-Quinodimethane Intermediates

Abstract

Catalytic synthesis of substituted 1,2-dihydronaphthalenes via metalloradical activation of o-styryl N-tosyl hydrazones ((E)-2-(prop-1-en-1-yl)benzene-N-tosyl hydrazones) is presented, taking advantage of the intrinsic reactivity of a cobalt(III)-carbene radical intermediate. The method has been successfully applied to a broad range of substrates with various R1 substituents at the aromatic ring, producing the desired ring products in good to excellent isolated yields for substrates with an R2 = COOEt substituent at the vinylic position (~70-90% ). Changing the R2 moiety from an ester to other substituents has a surprisingly large influence on the (isolated) yields. This behaviour is unexpected for a radical rebound ring-closure mechanism, and points to a mechanism proceeding via ortho-quinodimethane (o-QDM) intermediates. Furthermore, substrates with an alkyl substituent on the allylic position reacted to form E-aryl-dienes in excellent yields, rather than the expected 1,2-dihydronaphthalenes. This result, combined with the outcome of supporting DFT calculations, strongly points to the release of reactive o-QDM intermediates from the metal centre in all cases, which either undergo a 6π-cyclisation step to form the 1,2-dihydronaphthalenes, or a [1,7]-hydride shift to produce the E-aryl-dienes. Trapping experiments using TEMPO confirm the involvement of cobalt(III)-carbene radical intermediates. EPR spectroscopic spin-trapping experiments using phenyl N-tert-butylnitrone (PBN) confirm the radical nature of the catalytic reaction.

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Publication details

The article was received on 06 Sep 2017, accepted on 11 Oct 2017 and first published on 12 Oct 2017


Article type: Edge Article
DOI: 10.1039/C7SC03909C
Citation: Chem. Sci., 2017, Accepted Manuscript
  • Open access: Creative Commons BY-NC license
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    Catalytic 1,2-Dihydronaphthalene and E-Aryl-Diene Synthesis via CoIII-Carbene Radical and o-Quinodimethane Intermediates

    C. te Grotenhuis, B. G. Das, P. F. Kuijpers, W. Hageman, M. Trouwborst and B. de Bruin, Chem. Sci., 2017, Accepted Manuscript , DOI: 10.1039/C7SC03909C

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