Enantioselective ethylation of aromatic aldehydes catalysed by titanium(IV)-bis-BINOLate-2′,2-propylethercomplexes: An inside view of the catalytic active species

A series of bis-BINOL-2',2 ''-propyl ethers with different substituents at the propyl ether bridge, were investigated in the asymmetric titanium catalysed ethylation of arylaldehydes with Et2Zn, with conversions up to 99% and enantiomeric excesses up to 80%. Semiempirical PM6 calculations indicate that the higher ability of the unsubstituted ligand to form chelated titanium complexes could be related to its higher enantioselectivity. Catalytic experiments with partially optically enriched ligand put in evidence a negative non-linear effect (-)-NLE that suggest the presence of two ligand molecule in the active titanium species. Further catalytic data, together with H-1 NMR and circular dichroism (CD) titrations of the ligand with Ti(iPrO)(4), as well as ESI-MS experiments, allow to propose a trinuclear species [Ti3L2((OPr)-Pr-i)(8)] (L= dianion of (1R,1'R)-2',2 ''-(propane-1,3-diy1)bis(oxy)di-1,1'-binaphthyl-2-01) as responsible for the catalytic asymmetric addition of Et2Zn to aldehydes. This catalytic species is only formed with great Ti((OPr)-Pr-i)(4)/ligand molar ratios (ca. 8). (C) 2010 Elsevier B.V. All rights reserved.