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

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.

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

References and Notes

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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).

<A NAME="RG41609ST-16">16</A>

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.