Reductions of Arenes using a Magnesium-Dinitrogen Complex

In this contribution, we present “Birch-type”, and other reductions of simple arenes by the potassium salt of an anionic magnesium dinitrogen complex, [{K(^TCHPNON)Mg}₂(μ-N₂)] (^TCHPNON=4,5-bis(2,4,6-tricyclohexylanilido)-2,7-diethyl-9,9-dimethyl-xanthene), which acts as a masked dimagnesium(I) diradical in these reactions. This reagent is non-hazardous, easy-to-handle, and in some cases provides access to 1,4-cyclohexadiene reduction products under relatively mild reaction conditions. This system works effectively to reduce benzene, naphthalene and anthracene through magnesium-bound “Birch-type” reduction intermediates. Cyclohexadiene products can be subsequently released from the magnesium centres by protonolysis with methanol. In contrast, the reduction of substituted arenes is less selective and involves competing reaction pathways. For toluene and 1,3,5-triphenylbenzene, the structural authentication of “Birch-type” reduction intermediates is conclusive, although the formation of corresponding 1,4-cyclohexadiene derivatives was low yielding. Reduction of anisole did not yield an isolable “Birch-type” intermediate, but instead gave a C−O activation product. Treating triphenylphosphine with [{K(^TCHPNON)Mg}₂(μ-N₂)] resulted in the extrusion of both biphenyl and dinitrogen to afford a magnesium(II) phosphanide [{K(^TCHPNON)Mg(μ-PPh₂)}₂]. Reduction of fluorobenzene proceeded via C−F activation of the arene, and isolation of the magnesium(II) fluoride [{K(^TCHPNON)Mg(μ-F)}₂]. Finally, the two-electron reduction of 1,3,5,7-cyclooctatetraene (COT) with [{K(^TCHPNON)Mg}₂(μ-N₂)] yielded a complex, [{K(^TCHPNON)Mg}₂(μ-COT)], incorporating the aromatic dianion (COT²⁻).