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

Abstract

AbstractThe aryl ketones, 9‐fluorenone (fluor), 9‐xanthenone (xanth) and 9,10‐anthraquinone (anth), were reacted with β‐diketiminato dimagnesium(I) compounds, [{(ArNacnac)Mg}2] (ArNacnac=[HC(MeCNAr)2]−, Ar=mesityl (Mes) or 2,6‐diisopropylphenyl (Dip)). This gave stable magnesium ketyl complexes which are monomeric, [(ArNacnac)(DMAP)Mg(fluor⋅)] (Ar=Mes or Dip, DMAP=4‐dimethylaminopyridine) and [(MesNacnac)Mg(xanth⋅)(xanth)]; dimeric, [{(MesNacnac)Mg(μ‐fluor⋅)}2], or tetrameric, [{(DipNacnac)Mg(μ‐anth⋅)}4]. In contrast, di‐2‐pyridylketone (OCPy2) is doubly reduced with [{(XylNacnac)Mg}2] (Xyl=xylyl) to give a diamagnetic alkoxy/amido complex, [{(XylNacnac)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, [(MesNacnac)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, [{(ArNacnac)(THF)Mg}2(μ‐chalc2)] (Ar=Mes or Xyl) and [{(DipNacnac)(THF)Mg}2(μ‐DBA2)]. A pinacol coupling reaction between [{(DipNacnac)Mg}2] and 2‐adamantanone (OAd) yielded [{(DipNacnac)(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.