Ketyl Radicals Generated from Magnesium(I) Compounds: Useful Reagents for C-C Bond Forming Reactions

The aryl ketones, 9-fluorenone (fluor), 9-xanthenone (xanth) and 9,10-anthraquinone (anth), were reacted with β-diketiminato dimagnesium(I) compounds, [{(^Ar Nacnac)Mg}₂ ] (^Ar Nacnac=[HC(MeCNAr)₂ ]^- , Ar=mesityl (Mes) or 2,6-diisopropylphenyl (Dip)). This gave stable magnesium ketyl complexes which are monomeric, [(^Ar Nacnac)(DMAP)Mg(fluor⋅)] (Ar=Mes or Dip, DMAP=4-dimethylaminopyridine) and [(^Mes Nacnac)Mg(xanth⋅)(xanth)]; dimeric, [{(^Mes Nacnac)Mg(μ-fluor⋅)}₂ ], or tetrameric, [{(^Dip Nacnac)Mg(μ-anth⋅)}₄ ]. In contrast, di-2-pyridylketone (OCPy₂ ) is doubly reduced with [{(^Xyl Nacnac)Mg}₂ ] (Xyl=xylyl) to give a diamagnetic alkoxy/amido complex, [{(^Xyl Nacnac)Mg}₂ (μ-OCPy₂ )]. These complexes have been characterized by X-ray crystallography, and in three cases, EPR spectroscopy. Regioselective C-C hetero-coupling reactions between magnesium ketyls and phenanthroline (phen) have yielded the alkoxy compounds, [(^Mes Nacnac)Mg(OCR₂ -2-phen)] (OCR₂ =xanth, OCPh₂ or OC(Ph)(2-Me-Ph)). In addition, homo- C-C couplings of the enones, chalcone (chalc) and dibenziylideneactetone (DBA), using magnesium(I) reducing agents, have afforded dimagnesium enolates, [{(^Ar Nacnac)(THF)Mg}₂ (μ-chalc₂ )] (Ar=Mes or Xyl) and [{(^Dip Nacnac)(THF)Mg}₂ (μ-DBA₂ )]. A pinacol coupling reaction between [{(^Dip Nacnac)Mg}₂ ] and 2-adamantanone (OAd) yielded [{(^Dip Nacnac)(OAd)Mg}₂ (μ-OAd₂ )], presumably via a ketyl intermediate. This study further highlights the utility magnesium(I) compounds have as selective reducing agents in organic transformations.