Structural and Electronic Effects in Asymmetric Diethylzinc Addition to Benzaldehyde Catalyzed by Titanium(IV) Complexes of N-Sulfonylated β-Amino Alcohols

A series of N-sulfonylated β-amino alcohols (R,S)-3, (1R,2S)-PhCH(OH)CH(NHSO2R)CH2Ph, was prepared from reaction of various alkyl- or aryl-sulfonyl chlorides with (1R,2S)-2-amino-1,3-diphenylpropanol. A 1H NMR study of these ligands shows a correlation of electron-withdrawing abilities of ligands with electronic properties of R substituents on the sulfonyl group. The asymmetric diethylzinc additions to benzaldehyde catalyzed by titanium(IV) complexes of (R,S)-3 were carried out, and yields and enantioselectivities of the desired (R)-1-phenylpropanol increase with increasing electron-donating abilities of R substituents. A dimeric titanium(IV) complex 4 was prepared, and the structure of 4 shows an interesting feature of inequivalent titanium metal centers with one sulfonamide of the chiral ligand bonded to a Ti metal center in a η2-fashion and another sulfonamide bonded to the second Ti metal center in a η1-fashion. The complex 4 is not an effective catalyst for the asymmetric diethylzinc addition reactio...