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.