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 (OCPy 2 ) is doubly reduced with [{( Xyl Nacnac)Mg} 2 ] (Xyl=xylyl) to give a diamagnetic alkoxy/amido complex, [{( Xyl Nacnac)Mg} 2 (μ-OCPy 2 )]. 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(OCR 2 -2-phen)] (OCR 2 =xanth, OCPh 2 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-chalc 2 )] (Ar=Mes or Xyl) and [{( Dip Nacnac)(THF)Mg} 2 (μ-DBA 2 )]. A pinacol coupling reaction between [{( Dip Nacnac)Mg} 2 ] and 2-adamantanone (OAd) yielded [{( Dip Nacnac)(OAd)Mg} 2 (μ-OAd 2 )], presumably via a ketyl intermediate. This study further highlights the utility magnesium(I) compounds have as selective reducing agents in organic transformations.