Photochemically Activated Dimagnesium(I) Compounds: Reagents for the Reduction and Selective C–H Bond Activation of Inert Arenes

The photochemical activation of dimagnesium(I) compounds, and subsequent high yielding, regioselective reactions with inert arenes are reported. Irradiating benzene solutions of [{(Ar Nacnac)Mg}2 ] (Ar Nacnac=[HC(MeCNAr)2 ]- ; Ar=2,6-diisopropylphenyl (Dip) or 2,4,6-tricyclohexylphenyl (TCHP)) with blue or UV light, leads to double reduction of benzene and formation of the "Birch-like" cyclohexadienediyl bridged compounds, [{(Ar Nacnac)Mg}2 (μ-C6 H6 )]. Irradiation of [{(Dip Nacnac)Mg}2 ] in toluene, and each of the three isomers of xylene, promoted completely regio- and chemo-selective C-H bond activations, and formation of [(Dip Nacnac)Mg(Ar')] (Ar'=meta-tolyl; 2,3-, 3,5- or 2,5-dimethylphenyl), and [{(Dip Nacnac)Mg(μ-H)}2 ]. Fluorobenzene was cleanly defluorinated by photoactivated [{(Dip Nacnac)Mg}2 ], leading to biphenyl and [{(Dip Nacnac)Mg(μ-F)}2 ]. Computational studies suggest all reactions proceed via photochemically generated magnesium(I) radicals, which reduce the arene substrate, before C-H or C-F bond activation processes occur.