Catalytic 1,2-dihydronaphthalene and <i>E</i>-aryl-diene synthesis <i>via</i> Co^III–Carbene radical and <i>o</i>-quinodimethane intermediates

Abstract

Catalytic synthesis of substituted 1,2-dihydronaphthalenes <i>via</i> metalloradical activation of <i>o</i>-styryl <i>N</i>-tosyl hydrazones ((<i>E</i>)-2-(prop-1-en-1-yl)benzene-<i>N</i>-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 <i>R</i>¹ substituents at the aromatic ring, producing the desired ring products in good to excellent isolated yields for substrates with an <i>R</i>² = COOEt substituent at the vinylic position (∼70-90%). Changing the <i>R</i>² 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 <i>via ortho</i>-quinodimethane (<i>o</i>-QDM) intermediates. Furthermore, substrates with an alkyl substituent on the allylic position reacted to form <i>E</i>-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 <i>o</i>-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 <i>E</i>-aryl-dienes. Trapping experiments using TEMPO confirm the involvement of cobalt(iii)-carbene radical intermediates. EPR spectroscopic spin-trapping experiments using phenyl <i>N-tert</i>-butylnitrone (PBN) confirm the radical nature of the catalytic reaction.