Catalytic 1,2-dihydronaphthalene and -aryl-diene synthesis Co-Carbene radical and -quinodimethane intermediates.

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