Co^III–Carbene Radical Approach to Substituted 1H‑Indenes

A new strategy for the catalytic synthesis of substituted 1H-indenes via metalloradical activation of o-cinnamyl N-tosyl hydrazones is presented, taking advantage of the intrinsic reactivity of a Co^III carbene radical intermediate. The reaction uses readily available starting materials and is operationally simple, thus representing a practical method for the construction of functionalized 1H-indene derivatives. The cheap and easy to prepare low spin cobalt­(II) complex [Co^II(MeTAA)] (MeTAA = tetramethyltetraaza[14]­annulene) proved to be the most active catalyst among those investigated, which demonstrates catalytic carbene radical reactivity for a nonporphyrin cobalt­(II) complex, and for the first time catalytic activity of [Co^II(MeTAA)] in general. The methodology has been successfully applied to a broad range of substrates, producing 1H-indenes in good to excellent yields. The metallo-radical catalyzed indene synthesis in this paper represents a unique example of a net (formal) intramolecular carbene insertion reaction into a vinylic C­(sp²)–H bond, made possible by a controlled radical ring-closure process of the carbene radical intermediate involved. The mechanism was investigated computationally, and the results were confirmed by a series of supporting experimental reactions. Density functional theory calculations reveal a stepwise process involving activation of the diazo compound leading to formation of a Co^III-carbene radical, followed by radical ring-closure to produce an indanyl/benzyl radical intermediate. Subsequent indene product elimination involving a 1,2-hydrogen transfer step regenerates the catalyst. Trapping experiments using 2,2,6,6-tetra-methylpiperidine-1-oxyl (TEMPO) radical or dibenzoylperoxide (DBPO) confirm the involvement of cobalt­(III) carbene radical intermediates. Electron paramagnetic resonance spectroscopic spin-trapping experiments using phenyl N-tert-butylnitrone (PBN) reveal the radical nature of the reaction.