Immobilization systems Heck reactions

Recently, the high activity of palladium/NHC complexes in the Heck reaction was combined with an efficient recyclability process [63]. Bis-carbene pincer complexes of palladium(II) were immobilized on montmorillonite K-10. The catalytic activity of the heterogeneous system is similar to that displayed by their homogeneous counterparts. The stability of the catalyst was tested in the reaction of phenyl iodide and styrene. The product yield decreases from 99 to 79% after ten cycles. 

The concept of SI L catalyst has been developed quickly in the last decade. Holderich et al. [4] added acidic chloroaluminate ILs to various types of supports, and the catalytic activities of the immobilized ILs were found to be higher than those of the conventional catalysts under the same conditions. Inspired by this work, SIL catalysts have been widely used in the coupling reactions for olefin hydroformylation [5], olefin metathesis [6], Heck reactions [7], and hydroamination [8], and so on. SIL catalytic systems have also been reported for some other reactions, such as water-gas shift reaction [9], dihydroxylation of olefins [10], and hydrogenation [1 Ij. The solid supports used include magnetic NPs [12], mesoporous molecular sieves [13], soluble organic ions [14], noncovalently solid-phase [15], IL-functionalized carbon nanotubes [16], polymer cocktail [17], and so on. 

A combination of a Mizoroki-Heck reaction and a C—N coupling on solid phase was reported by Yamazaki et al. [105] in the synthesis of indole carboxylate 197 employing immobilized A-acetyldehydroalanine (194) and a bifunctionalized arene 195. In these transformations, 196 is the intermediate. The best results were obtained with o-dibromoarenes such as 195b and 195d using the Pd2(dba)3/P-r-Bu3/Cy2NMe catalytic system developed by Littke and Fu [106]. The protocol was also extended to the synthesis of isoquinolines [107] (Scheme 8.50). 

The Heck reaction was performed on immobilized aryl halides, mostly iodides, or iodonium salts with soluble alkenes (Table 1) or on immobilized alkenes with soluble aryl halides (Table 2). When performed on the same type of resin and with the same catalyst system, the immobilization of aryl iodides appears to be more beneficial than that of... 

This catalytic system allows three independent transformations to occur in sequence the Heck reaction, N-oxidation and asymmetric dihydroxylation (AD). The mechanism of the Heck reaction is discussed in the previous section. Here we take a closer look at the last two steps. They are coupled processes, based on the Sharpless asymmetric dihydroxylation reaction [22, 23]. Several recent reviews on Sharpless asymmetric dihydroxylation cover the general synthetic aspects [24-27], together with methods for immobilization of the osmium catalysts [28]. 

The Heck reaction is another Pdcarbon-carbon bond forming process that is widely employed in organic synthesis and can occur in water. A recent example reported by Cacchi and coworkers was applied to the chemoenzymatic synthesis of (f ) -Rhododendrol (34) and other chiral alcohols (Scheme 4.17) (67]. To aid this work, perfluoro-tagged palladium nanoparticles (Pd p) immobilized on fluorous silica gel or through covalent bonding to silica were used as the catalytic systems. The Heck coupled product could be further treated with (R)-selective LbADH and 2-propanol to address the synthesis of (R)-Rhododendrol in 90% conversion and with 99% ee. 

Gerritsma, D. A., Robertson, A., McNulty, J. Capretta, A. (2004). Heck reactions of aryl halides in phosphonium salt ionic liquids library screening and applications. Tetrahedron Letters, 45,41, 7629-7631, ISSN 0040-4039 Giiell, R., Antico, E., Salvado, V. Fontas, C. (2008). Efficient hollow fiber supported liquid membrane system for the removal and preconcentration of Cr(VI) at trace levels. Separation and Purification Technology, 62,2,389-393, ISSN 1383-5866 Guibal, E., Vincent, T. Jouannin, C. (2009). Immobilization of extractants in biopolymer capsules for the synthesis of new resins a focus on the encapsulation of tetraalkyl phosphonium ionic liquids. Journal of Materials Chemistry, 19, 45, 8515-8527, ISSN 0959-9428... 

A different example of triphasic catalysis for the Heck, Stille and Suzuki reactions relied on a three-phase microemulsion/sol-gel transport system. Gelation of an z-octyl(triethoxy)silane, tetramethoxysilane and Pd(OAc)2 mixture in a H2O/CH2CI2 system led to a hydrophobicitized sol-gel matrix that entrapped a phosphine-free Pd(ii) precatalyst. The immobilized precatalyst was added to a preformed microemulsion obtained by mixing the hydrophobic components of a cross coupling reaction with water, sodium dodecyl sulfate and a co-surfactant, typically zz-propanol or butanol. This immobilized palladium catalyst was leach proof and easily recyclable. 

As ionic liquids are stiU expensive in comparison with conventional molecular solvents, their efficient recycling is an important issue that addresses the economics of their use, especially in large-scale applications. The most studied examples are the 1,3-dialkylimidazolium-based ionic liquids where the ionic solvent is usually recycled through several cycles of the reaction. Numerous examples describe the immobilization of a transition metal catalyst in the ionic liquid phase of a biphasic system. A variety of palladium coupling reactions, such as the Heck [27], Suzuki... 

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