Elsevier

Journal of Catalysis

Volume 259, Issue 1, 1 October 2008, Pages 26-35
Journal of Catalysis

Stabilization and recovery of gold catalysts in the cyclopropanation of alkenes within ionic liquids

https://doi.org/10.1016/j.jcat.2008.07.009Get rights and content

Abstract

Au(I) and Au(III) salts (NaAuCl4 and KAuCN2 respectively) were found to be active catalysts for the cyclopropanation of alkenes with ethyldiazoacetate, in many cases affording high yields of cyclopropanecarboxylates. But these gold salts rapidly decomposed and agglomerated when working with organic solvents, resulting in less active and less selective gold metal particles. With ionic liquids (ILs) as solvents, the gold catalysts were stabilized, especially Au(I), and products and catalyst separation and recycling could be achieved. In both cases, NaAuCl4 and KAuCN2 underwent reductive transformation to Au(0) to afford gold nanoparticles, which remained stabilized in the IL, behaving as an authentic metal nanoparticle reservoir. It was found that cyclopropanation and aromatic addition were sensitive to gold particle size, whereas the formation of fumarate and maleate esters was insensitive to gold particle size.

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.

Section snippets

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) <10 mg/kg, was supplied by Solvent Innovation. [BMIM] BF4, with purity >98.5%, water content <0.02%, and anion traces (bromide 25 mg/kg; chloride 25 mg/kg;

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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