Issue 12, 2017

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.

Graphical abstract: Catalytic 1,2-dihydronaphthalene and E-aryl-diene synthesis via CoIII–Carbene radical and o-quinodimethane intermediates

Supplementary files

Article information

Article type
Edge Article
Submitted
06 9 2017
Accepted
11 10 2017
First published
12 10 2017
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2017,8, 8221-8230

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, 8, 8221 DOI: 10.1039/C7SC03909C

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