Investigations of enantioreversal in both direct and directed enantioselective aldol reactions catalyzed by CuCl2[(−)-sparteine] and NiCl2[(−)-sparteine] complexes

The use of structurally well-defined chiral [CuCl2(sparteine)] 1 and [NiCl2(sparteine)] 2 complexes as catalysts under fluoride anion-promoted double catalytic activation (DCA) conditions cause enantioreversal in the asymmetric Mukaiyama aldol reaction of 1-phenyl-1-trimethylsiloxyethylene with various aromatic aldehydes. A similar enantioreversal also occurs in the direct aldol reaction between methyl vinyl ketone and various aromatic aldehydes under Et3N promoted DCA conditions. We have quantitatively analyzed using group theoretical analysis via symmetry deformation coordinates, the molecular structures of 1 and 2 from the X-ray structural data; the results from the study show that the innate stereochemical differences that are present in their molecular structures, form the basis for the enantioreversal phenomenon in the asymmetric C–C bond-forming aldol reactions.

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(R)-5-Hydroxy-5-(4-nitrophenyl)pent-1-en-3-oneC11H11NO4Ee = 79%[α]D25=+30.2 (c 1, CHCl3)Source of chirality: CuCl2[(−)-sparteine] catalystAbsolute configuration: (5R)

(S)-3-Hydroxy-3-(4-nitrophenyl)-1-phenylpropan-1-oneC15H13NO4Ee = 63%[α]D25=-36.9 (c 1, CHCl3)Source of chirality: NiCl2[(−)-sparteine] catalystAbsolute configuration: (3S)