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 beta-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 H-1 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 eta(2)-fashion and another sulfonamide bonded to the second Ti metal center in a eta(1)-fashion. The complex 4 is not an effective catalyst for the asymmetric diethylzinc addition reaction. With further addition of Ti(O-i-Pr)(4), the resulting systems become effective with results nearly identical to the in situ-formed catalytic systems. Another interesting feature is that the 1H spectrum of 4 shows the presence of at least four isomeric species in CD2Cl2 at 0 degreesC, and possible structures of these species are discussed.