Application of rapidly generated bidentate ligand libraries to zinc catalyzed reductions

Rise of the Zinc Age in Homogeneous Catalysis?

“…Pioneering work in the field was reported by the groups of Noyori and Mimoun , for the reduction of aldehydes and ketones applying silanes as reducing reagents. Later on, several research groups enhanced this method for increased functional group tolerance, in situ deprotection of the silyl ether, and the application of PMHS (polymethylhydrosiloxane) as inexpensive hydride source or incorporation of chirality. − In more detail, for transferring chirality to the substrate zinc complexes modified by ligands based on the chiral motifs 1,2-diaminocyclohexane or 1,2-diphenylethylenediamine have been found to be superior. For instance highest enantioselectivity of 91% enantiomeric excess ( ee ) for the reduction of the benchmark substrate acetophenone was observed for the in situ system composed of ZnEt 2 and ligand 1 (Scheme a) .…”

“…For a better understanding of the zinc catalyzed reductions and for a higher level of improvement a deeper understanding of the underlying reaction mechanism can be a useful tool. During the course of studies in hydrosilylation of carbonyl compounds contrary types of reaction mechanisms have been proposed. − On the one hand zinc can act as a Lewis acid by activation of the carbonyl function and on the other hand via a Zinc-hydride intermediate. More recently, the group of Kwit carried out an experimental and theoretical study to shed light on the discussion of this controversy and found that the Zn-hydride pathway appears energetically most favorable for the investigated zinc-diamine-diol system .…”

First application of chiral phosphotriesters in asymmetric metal catalysis: enantioselective Zn-catalyzed hydrosilylation of ketones in the presence of BINOL-derived phosphates

“…use of Zn/chiral secondary diamine complexes for the reduction of ketones in the presence of hydrosilanes, which enable them to reach 80% ee [11]. Since then, other systems still based on N -containing ligands have been reported [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31], incorporating either diversified diamines or diimines, N ,S-or N ,Ochelating ligands, or Pybox, and excellent conversions and enantiomeric excesses on a wide range of substrates were thus achieved.…”