Synlett 2004(13): 2437-2439  
DOI: 10.1055/s-2004-832830
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© Georg Thieme Verlag Stuttgart · New York

Novel Lewis Acid-Assisted Chiral Lewis Acid (LLA) System: Development of Boroxin-Ti-BINOL-Catalyzed Asymmetric Allylation of Aldehydes

Guoyao Xia, Kazutaka Shibatomi, Hisashi Yamamoto*
Department of Chemistry, University of Chicago, 5735 South Ellis Avenue, Chicago, IL 60637, USA
Fax: +1(773)7020805; e-Mail: yamamoto@uchicago.edu;
Further Information

Publication History

Received 16 March 2004
Publication Date:
24 September 2004 (online)

Abstract

The formation of hetero multimetallic complex having B-O-Ti bond could increase the Lewis acidity of titanium. A novel LLA system made up of Ti-BINOL and 4-(trifluoromethyl)phenylboroxin dramatically accelerated the allylation reaction of aldehyde with high enantioselectivity.

8

Keck has also reported that the use of Ti(Oi-Pr)4 and BINOL (1:1) catalyze the allylation of benzaldehyde very slowly at -20 °C even in the presence of activated molecular sieves [10 mol% (Ti), 70 h, 88% yield, 95% ee]. See ref. [7c]

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A referee has argued that an ionic complex made up of borane-isopropoxide ate complex and cationic titanium(IV) triisopropoxide could be formed and act as catalyst.

10

Maruoka has reported bis[binaphthoxy(isopropoxy)tita-nium] oxide having Ti-O-Ti bond. In this paper, they assumed this type activation system (LLA) as one of the possible intermediates for the reaction. See ref. [7s] . Another possibility in our system is that the monomer of aryl boroxin would coordinate to an oxygen atom on Ti-BINOL complex and enhance the Lewis acidity of titanium in the same fashion, which seem to be also an example of LLA activation.