Stabilization and recovery of gold catalysts in the cyclopropanation of alkenes within ionic liquids
Introduction
Gold has been shown to be an active and selective catalyst for numerous reactions, including C–C [1], [2], [3], [4], [5], C–N [6], [7], [8], [9], [10], C–O [11], [12], C–H [13], [14], [15], and C–Si [16] bond-forming reactions; oxidation [17]; and enantioselective hydrogenation of olefins and imines [18], [19]. In many cases, gold salts have good activity and selectivity, but have drawbacks associated with catalyst recovery and recycling and, even more importantly, in many cases can decompose during the reaction, leading to the formation of large particles of gold metal that are not active or unselective. Thus, it is of interest to find a procedure to stabilize the gold salts and avoid decomposition while achieving easy separation of the reaction products and catalyst recycling.
The transition metal-catalyzed transfer of carbene units from diazo compounds to alkenes provides a powerful organic synthesis tool for obtaining cyclopropane derivatives, which are important intermediates for the construction of relatively complex molecules [20], [21], [22], [23], [24], [25], [26], [27]. Cyclopropanation of alkenes from diazo compounds can be catalyzed by Cu(I) [23]. Because the external electronic structure of Cu(I) is similar to that of Au(I), gold possibly could be a suitable cyclopropanation catalyst. Indeed, the literature reports that a gold(I)-based catalyst also can be an efficient and chemoselective catalyst for this reaction [28]. Here we demonstrate that Au(I) as well as Au(III) salts (KAuCN2 and NaAuCl4) can catalyze the cyclopropanation of alkenes by ethyldiazoacetate in ionic liquids (ILs) as reaction media. We show that when the same gold salt catalysts are used in the presence of the appropriated IL, the Au(III) and Au(I) salts can be stabilized toward the formation of gold metal agglomerates, thereby resulting in higher yields of cyclopropanes than can be achieved using conventional solvents. Moreover, the separation of products and catalyst recovery and recycling is much easier when using ILs.
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Materials and starting reagents
The starting reagents (alkenes and ethyldiazoacetate) and catalysts NaAuCl4⋅2H2O and KAuCN2 were purchased from Aldrich. [MOIM] PF6, with purity of 99.9% and water content of 182.3 ppm, was supplied by Fluka. Halides and sulfates ions were not detected. [BMIM] PF6, with purity >98.5%, water content <0.02%, and halogen content (as chloride) , was supplied by Solvent Innovation. [BMIM] BF4, with purity >98.5%, water content <0.02%, and anion traces (bromide ; chloride ;
Results and discussion
Gold(III)-catalyzed carbene transfer reaction from ethyldiazoacetate (EDA) to styrene was initially carried out in the presence of NaAuCl4 but in the absence of solvent. Under these experimental conditions, the cis/trans-cyclopropanecarboxylates were obtained with low selectivity (see entry 1 in Table 1) due to the formation of relatively large amounts of EDA dimerization products (diethyl fumarate and maleate), together with o-, m-, p-aromatic addition products (Scheme 1).
The formation of
Conclusion
In the present study, ionic gold complexes in different imidazolium ILs were used as recoverable and reusable homogeneous catalysts in the cyclopropanation reaction of alkenes with ethyldiazoacetate to give cis- and trans-cyclopropanes. The following trends were found: With few exceptions, the trans-isomer was the major product of the reaction, and the stability and performance of these gold salts is much higher in ILs than in typical solvents used in this reaction (e.g., 1,2-dichloroethane, CH2
Acknowledgments
Financial support by the Dirección General de Investigación Científica y Técnica of Spain (Project MAT2006-14274-C02-01) is gratefully acknowledged. I.D. and T.R. thank the Consejo Superior de Investigaciones Científicas for I3-P fellowships. The authors thank the referees for their useful comments.
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