Ketyl Radicals Generated from Magnesium(I) Compounds: Useful Reagents for C−C Bond Forming Reactions
Christopher A. Rosengarten, K. Yuvaraj, Li Feng Lim, Nicholas J. Cox, Cameron Jones
Abstract
Abstract 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.