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Synlett 2010(6): 962-966  
DOI: 10.1055/s-0029-1219534
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

BINAP-AgSbF6 vs. BINAP-AgClO4 Complexes as Catalysts for the Enantioselective 1,3-Dipolar Cycloaddition of Azomethine Ylides and Alkenes

María Martín-Rodrígueza, Carmen Nájeraa, José M. Sansano*a, Paulo R. R. Costab, Evanoel Crizanto de Limab, Ayres G. Diasb
a Departamento de Química Orgánica e Instituto de Síntesis Orgánica (ISO), Facultad de Ciencias, Universidad de Alicante, 03080 Alicante, Spain
Fax: +34(96)5903549; e-Mail: jmsansano@ua.es;
b Laboratório de Química Biorgânica, Núcleo de Pesquisas de Produtos Naturais, Centro de Ciencias da Saúde, Universidade Federal do Rio de Janeiro, Cidade Universitária, 21941-590 Rio de Janeiro, Brazil
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Publication History

Received 30 December 2009
Publication Date:
23 February 2010 (online)

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Abstract

The employment of AgSbF6 and BINAP ligands has been evaluated in the catalyzed enantioselective 1,3-dipolar cycloadditions between azomethine ylides and electrophilic alkenes. The results are compared with the analogous ones obtained when using AgClO4. The cycloaddition with maleimides and trans-1,2-bis(phenylsulfonyl)ethylene are clearly improved by the AgSbF6-derived catalyst, and its efficiency is crucial for ensure good yields and excellent ee in the three-component reaction.

Key words

asymmetric catalysis - prolines - silver salts - azomethine ylides - multicomponent reaction

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The BINAP-AgSbF6 (2:1) chiral cluster was not characterized because it afforded lower endo/exo diastereoselectivity, such as occurred in the previously reported chiral BINAP-AgClO4-catalyzed 1,3-DC (ref. 7).

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General Procedure for the Catalytic Enantioselective 1,3-DC Using Silver Salts
A solution of the imino ester (1 mmol) and dipolarophile
(1 mmol) in toluene (5 mL) was added to a suspension containing (R)- or (S)-BINAP (0.05 mmol, 31 mg) and silver(I) salt (0.05 mmol) in toluene (5 mL). To the resulting suspension Et3N (0.05 mmol, 7 µL) was added and the mixture stirred at r.t. and in the absence of the light for 16-48 h (see main text). The reaction was filtered, the organic filtrate was directly evaporated, and the residue was purified by recrystallization or by flash chromatography yielding pure endo cycloadducts.
General Procedure for the Three-Component Catalytic Enantioselective 1,3-DC Using Silver Salts
To a suspension containing (R)- or (S)-BINAP (0.05 mmol, 31 mg) and AgSbF6 (0.05 mmol, 17 mg) in toluene (10 mL) was added the freshly distilled aldehyde (1 mmol), glycine methyl ester hydrochloride (1 mmol), the dipolarophile (1 mmol), and Et3N (140 µL, 1.05 mmol). The mixture was stirred at r.t. and in the absence of the light for 16-48 h (see main text). Following the same workup as that described above, final cycloadducts 3aa and 8c were obtained.