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, [{( Nacnac)Mg} ] ( Nacnac=[HC(MeCNAr) ] , Ar=mesityl (Mes) or 2,6-diisopropylphenyl (Dip)). This gave stable magnesium ketyl complexes which are monomeric, [( Nacnac)(DMAP)Mg(fluor⋅)] (Ar=Mes or Dip, DMAP=4-dimethylaminopyridine) and [( Nacnac)Mg(xanth⋅)(xanth)]; dimeric, [{( Nacnac)Mg(μ-fluor⋅)} ], or tetrameric, [{( Nacnac)Mg(μ-anth⋅)} ]. In contrast, di-2-pyridylketone (OCPy ) is doubly reduced with [{( Nacnac)Mg} ] (Xyl=xylyl) to give a diamagnetic alkoxy/amido complex, [{( Nacnac)Mg} (μ-OCPy )]. 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, [( Nacnac)Mg(OCR -2-phen)] (OCR =xanth, OCPh 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, [{( Nacnac)(THF)Mg} (μ-chalc )] (Ar=Mes or Xyl) and [{( Nacnac)(THF)Mg} (μ-DBA )]. A pinacol coupling reaction between [{( Nacnac)Mg} ] and 2-adamantanone (OAd) yielded [{( Nacnac)(OAd)Mg} (μ-OAd )], presumably via a ketyl intermediate. This study further highlights the utility magnesium(I) compounds have as selective reducing agents in organic transformations.