Mechanistic studies on the enantioselective BINOLAM/titanium(IV)-catalyzed cyanobenzoylation of aldehydes: Part 1

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Title: Mechanistic studies on the enantioselective BINOLAM/titanium(IV)-catalyzed cyanobenzoylation of aldehydes: Part 1
Authors: Baeza, Alejandro | Nájera, Carmen | Sansano, Jose M. | Saá Rodríguez, José Manuel
Research Group/s: Procesos Catalíticos en Síntesis Orgánica
Center, Department or Service: Universidad de Alicante. Departamento de Química Orgánica | Universidad de las Islas Baleares. Departamento de Química
Keywords: Titanium | Enantioselective
Knowledge Area: Química Orgánica
Date Created: 2011
Issue Date: 30-Jun-2011
Publisher: Elsevier
Citation: BAEZA, Alejandro, et al. "Mechanistic studies on the enantioselective BINOLAM/titanium(IV)-catalyzed cyanobenzoylation of aldehydes: Part 1". Tetrahedron: Asymmetry. Vol. 22, No. 12 (30 June 2011). ISSN 0957-4166, pp. 1282-1291
Abstract: The enantioselective titanium(IV)-catalyzed cyanobenzoylation of aldehydes using 1:1 BINOLAM/Ti(OiPr)4 mixtures as a precatalyst gave O-aroyl cyanohydrins 4 with good enantiomeric excesses. The standard optimization set carried out on the assumption of Curtin–Hammett behavior, led to no amelioration. Extensive experimental and computational studies were carried out with the purpose of identifying the key mechanistic aspects governing enantioselectivity. HCN and isopropyl benzoate were detected in the reacting mixtures. This as well as the reaction response to the presence of an exogenous base, and the failure to react in the presence of Binol/Ti(OiPr)4 mixtures, led us to propose, not a direct but an indirect process involving an enantioselective hydrocyanation step followed by O-benzoylation. Computational work carried out with mononuclear monomeric MM and dinuclear mixed dimer DlMD as catalysts support this mechanistic proposal. On the other hand, cyanobenzoylations carried out with 1:2 or higher 1:n (up to 1:5) BINOLAM/Ti(OiPr)4 mixtures appear to involve a reversal of the enantioselection. This, together with the fact that the benzoylation of the ligated iPrOH is a slow reaction, has led us to conclude that these cyanobenzoylations do not fit within the standard Curtin–Hammett kinetic scheme. Instead, such BINOLAM/Ti(OiPr)4 -catalyzed cyanobenzoylations of aldehydes rather behave as non-Curtin–Hammett kinetic schemes. Further computational analysis is needed in order to make a clear distinction between Curtin–Hammett and non-Curtin–Hammett kinetic frameworks.
Sponsor: This work is supported by MICINN (Spain) through projects CTQ2007-62952/BQU, CTQ2007-62771/BQU, CTQ 2010-17339, CTQ2010-20387, and Consolider INGENIO 2010 CSD2007-00006.
ISSN: 0957-4166 (Print) | 1362-511X (Online)
DOI: 10.1016/j.tetasy.2011.07.006
Language: eng
Type: info:eu-repo/semantics/article
Peer Review: si
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