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

Abstract

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.