The effect of Al2O3 and ionic liquids in palladium catalyzed arylation of cyclohexene. Interaction of Hg(0) with immobilized palladium
Introduction
The Heck reaction belongs to the most important and intensively studied processes leading to the formation of C
C bonds [1], [2], [3], [4], [5], [6], [7], [8]. This is because the Heck procedure enables the efficient synthesis of arylated olefins, important materials in the chemical and pharmaceutical industries [7]. According to this procedure, aryl halides are coupled with linear [4], [9], [10], [11] and cyclic olefins [12], [13], [14], [15], [16], [17], allyl alcohols [18], [19], unsaturated esters and acids. A particular interest in the arylation of cyclic olefins is related to the possible formation of a stereogenic center in the reaction product [20], [21], [22]. The most extensively studied cyclic olefins are cyclopentene [15], [23], [24], 2,3-dihydrofuran [12], [13], [14], [20], and cyclohexene [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. However only for 2,3-dihydrofuran and dihydropyran has successful enantioselective Heck coupling been reported [20]. For example, the employment of a chiral IL makes it possible to obtain the product of 2,3-dihydrofuran arylation with ee > 99 [13].
Very good results have been obtained in the Heck coupling of cyclic olefins catalyzed by heterogeneous palladium catalysts supported on TiO2 [36], mixed oxides [34], [35], [36], [37], [38], [39], and zeolites [41]. In our previous studies, palladium supported on Al2O3 appeared more efficient in the arylation of cyclohexene than homogeneous catalysts [25].
In this work, we concentrated our efforts on the studies of three-component catalytic systems, containing a palladium precursor, Al2O3, and IL (IL = ionic liquid), in the Heck coupling of cyclohexene. Our concept was to apply Al2O3 as a support for a heterogeneous palladium catalyst and also as an additive to the homogeneous system to form a heterogenized palladium catalyst in situ. By introducing selected IL, we expected to improve the stability and selectivity of palladium catalysts, following a synergistic effect of the support and ILs observed recently in the Suzuki–Miyaura reaction [42].
Section snippets
Results and discussion
The Heck arylation of cyclohexene with iodobenzene is schematically shown in Fig. 1. In this reaction, two groups of products can be expected, namely mono- and diarylated ones [23], [25]. In these studies, the temperature of 100 °C was used in order to increase the selectivity to monoarylated products, aryl-cyclohexenes. Actually, at 140 °C, up to 22% of diarylated products were formed [25], while preliminary studies showed that their formation was totally retarded at 100 °C.
Materials
All reactants were obtained from Aldrich “for synthesis” quality or higher and were used without further purification or drying.
Conclusions
The Heck coupling of iodobenzene with cyclohexene performed at 100 °C with homogeneous and heterogeneous palladium catalysts and small amounts of an IL made it possible to obtain the high conversion of substrates to 4-phenylcyclohexene. A positive influence of Al2O3 used as an additive to the homogeneous reaction was shown. The addition of Al2O3 caused an increase of PhI conversion from 45% to 72% and to 82% when an IL (C) was introduced. Similarly, in the presence of ILs in a reaction catalyzed
Acknowledgements
Financial support of National Science Centre (NCN) with grant 2012/05/B/ST5/00265 is gratefully acknowledged.
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