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 ß-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.

Enantiopure dimagnesium(i) and magnesium(ii) hydride complexes incorporating chiral amidinate or ß-diketiminate ligands.

A known chiral ß-diketiminate, [HC{MeCN-(S)-(-)-CHMePh}(MeCNDip)]- (Dip = 2,6-diisopropylphenyl) L1, and a new chiral amidinate, [ButC(NAr*){N-(S)-(-)-CHMePh}]- L2 (Ar* = C6H2{C(H)Ph2}2Me-2,6,4) have been utilised in the synthesis of the first examples of enantiopure, dinuclear magnesium(i), [(L1)Mg-Mg(L1)], and magnesium(ii) hydride complexes, [(L2)Mg(µ-H)2Mg(L2)] and [(L2)(THF)Mg(µ-H)2Mg(THF)(L2)]. A related enantiopure ß-diketiminato magnesium hydride cluster compound, [(L1)4Mg5H6], has also been synthesised, presumably forming via a partial redistribution of in situ generated [(L1)Mg(µ-H)2Mg(L1)].