Synthesis, characterization, and structures of oxovanadium(v) complexes of Schiff bases of β-amino alcohols as tunable catalysts for the asymmetric oxidation of organic sulfides and asymmetric alkynylation of aldehydes

Abstract

Oxovanadium(V) complexes 3a–3k and 4a–4d with general formula VO(L₃*)(OR⁵) were prepared in quantitative yields in alcohol (R⁵OH) from reactions of VO(O-i-Pr)₃ and tridentate Schiff bases of β-amino alcohols having one or two stereogenic centers, (HO)C*(R¹)(R²)C*H(R³)NCH(2-OH-3,5-R⁴₂–C₆H₂) (H₂L₃*). The alkoxy OR⁵ ligand exchanges readily with the alcoholic molecule in the solvent. Crystal structures of 3b, 3f, 3i, and 4a were determined to be five-coordinate square pyramidal monomers. However, ¹H NMR spectra of the complexes reveal two sets of signals, indicating the presence of two isomers in solution. The two isomers are suggested to be the endo/exo pair or the monomer/dimer pair. Asymmetric oxidations of methyl phenyl sulfide catalyzed by catalyst precursors 3 were demonstrated to afford the chiral sulfoxide in yields and ee values similar to those obtained from the in situ-formed catalytic systems of VO(acac)₂ and corresponding Schiff base ligands. Complexes 3 and 4 are also good catalysts for asymmetric alkynyl additions to aldehydes. Structural differences between the oxovanadium complexes, for inducing high stereoselectivities in the asymmetric oxidation of organic sulfides and the asymmetric alkynyl addition to aldehydes, are rationalized.