abstract
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 Et(2)Zn, 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 (1)H 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 [Ti(3)L(2)(O(i)Pr)(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 Et(2)Zn to aldehydes. This catalytic species is only formed with great Ti(O(i)Pr)(4)/ligand molar ratios (ca. 8). (C) 2010 Elsevier B.V. All rights reserved.
keywords
BETA-AMINO ALCOHOLS; TITANIUM(IV) COMPLEXES; ASYMMETRIC CATALYSIS; SEMIEMPIRICAL METHODS; ALKYL-GROUPS; DIETHYLZINC; LIGAND; VALIDATION; MECHANISM; ACIDS
subject category
Chemistry
authors
Abreu, AR; Lourenco, M; Peral, D; Rosado, MTS; Eusebio, MES; Palacios, O; Bayon, JC; Pereira, MM
our authors
Mirtha A. O. Lourenço
Researcheracknowledgements
The authors thank financial support from Portuguese FCT through project PTDC/QUI/66015/2006, Merquinsa (Barcelona, Spain), and the Spanish DG1 through project B102006-14420-CO2-02. ARA and DC thanks for a PhD grant to FCT (PhD grant SFRH/BD1213141 2005) and UAB, respectively.