Tagged catalysts

Fig. 1. Fluorous biphasic catalysis part I hydroformylation, hydro-boration, and transesterification with fluorous tagged catalysts.

Suzuki coupling reactions (solvent DMF, temp. 80 °C) and compared to the unsupported perfluoro-tagged catalyst at different catalyst concentrations. The yields using the perfluoro-tagged catalyst are significantly lower compared to the reactions were the supramolecular dendritic catalyst assembly... 

This supramolecular dendritic assembly serves as a valuable soluble model for the interaction of perfluoro-tagged catalysts with insoluble supports such as fluorous silica gel and clearly reveals the ligand diffusion from the complex at elevated temperatures. This behavior can also explain the high catalytic activity of the heterogeneous FPS system. 

A different approach has used a basic tagged ruthenium complex for facile recovery and immobilization using PS-S03H [161,162]. A chemically tagged catalyst was immobilized to the solid phase (37), and the catalyst was activated because of the ammonium ion s electron-withdrawing properties (Scheme 4.74). The catalyst was used for ring closing metathesis in a continuously recirculated system (5 mol% Ru)... 

Perfluorinated molecules are prepared from their hydrocarbon analogues by electrochemical fluorination or by fluorination using cobalt trifluoride. Functional perfluorinated molecules are then used to prepare the tagged catalysts and reagents (Figure 7.4). Therefore, in terms of life cycle analysis, fluorous solvents are not as green as a solvent that does not need to be prepared, e.g. water, or a solvent that requires little substrate modification, e.g. a renewable VOC. However, the ability of FBSs to perform efficient separations often reduces the overall amount of solvent that is required in a process and therefore they are considered green alternative solvents. 

In the next two sections we limit our analysis to a few recent examples of the use of water or ionic liquids in metathesis reactions using new technical solutions or ionically tagged catalysts. 

By such an SPE, products can be separated from perfluoro-tagged catalysts, byproducts or reagents. One example is a Mitsunobu reaction ulilizirig fluorous aza-dicarboxylate and triphenylphosphine [28]. Many more examples of the use of this... 

Bannwarth and co-workers have recently developed new protocols for the separation and recycling of perfluoro-tagged catalysts without the need for fluorous solvents [25]. They have employed Pd complexes 11c, lid, and 17, immobilized by adsorption on fluorous reversed-phase silica gels (FRPSG) 18 and 19, and demonstrated the application to Suzuki couplings in organic solvents. Coarse-grained... 

Cyclodimerizations of conjugated enynes 22a-22e in the presence of perfluoro-tagged Pd catalyst were reported by Saito et al. [Eq. (6)] [28]. Reactions of enynes 22a-22e were carried out in toluene/hexane/perfluorodecalin with 1 mol% of Pd2(dba)j as palladium source and 8 mol% of perfluoro-tagged phosphane 1 giving the desired products (23a-23e) in moderate to good yield (43-78%). Recovery of the perfluoro-tagged catalyst was possible up to four times. However, IPC atomic emission analysis of the fluorous phase indicated that the concentration of the palladium species decreased significantly. 

The halogen-bonded adduct DABCO-(CgFi7l)2 has been developed as a recyclable, fluorous-tagged catalyst for the MBH reaction that can be recovered by precipitation/filtration. ... 

Schneider and Bannwarth [25] have prepared three new perfluoro-tagged catalysts, which can be used in fluorous biphasic systems these can be recycled up to six times without significant reduction in yield. Several years ago, Mathey, Regitz, and coworkers [26] reported on the use of palladium complexes of a 10-membered tetraphosphane, which they suggested to be more cost effective than the... 

Figure 5.33 Examples of fluorous-tagged catalysts for use in fluorous, biphasic media.

Recently, various Hoveyda-type carbenes were attached to different resins or insoluble inorganic supports preferentially via the 2-alkoxybenzylidene fragment. An alternative method involves the use of tagged catalysts, which can be easily separated from untagged products by a phase-transfer event (precipitation or hquid-hquid partition). One particularly innovative method involves a catalyst immobilization in ionic liquids (IL). Because of their interesting properties such as ease of reuse, nonvolatility, and good ability to dissolve... 

Figure 8. Comparative recyclability of ionic-tagged catalysts 14a, 15. and 16 in ionic media Determined by H-NMR spectroscopic analysis at 400 MHz. Reaction performed at 40°C.

To evaluate a real effect of the pyridinium function in terms of activity and recyclability, we then prepared the complex 16. This pyridinium-containing catalyst, equivalent to 14a, possesses the same C3 alkyl linker between the ionic tag and the benzenylidene pattern. The direct comparison between the imidazolium-tagged catalyst 14a, and its pyridinium-containing equivalent 16 showed similar conversions and reuses over six cycles (Figure 8, entry 3). 

Scheme 4. Various approaches towards 1,2,3-triazolium ionic liquid tagged catalysts...

The direct application of unmodified aldehydes in catalytic Michael additions can be severely hindered due to the presence of undesirable intermolecular self-aldol reactions (Hagiwara, Komatsubara et al. 2001 Hagiwara, Okabe et al. 2001). Barbas and co-workers achieved the first direct catalytic asymmetric Michael reaction between unmodified aldehydes and nitroolefins. The usage of an (S)-2-(morphohnomethyl) pyrrolidine catalyst in 20 % furnished the Michael addition products in 72 % enantioselectivity, 12 1 diastereoselectivity and 78 % yield (Betancort and Barbas 2001 Betancort, Sakthivel et aL 2004 Mosse, Andrey et al. 2006). The utilization of the ionic hquid tagged catalysts 25 and 26 in the Michael reactions of frans- -nitrostyrenes to aldehydes resulted in high yields but... 

Scheme 7. Synthesis of a,a-diphenylprolinol derived ionic liquid tagged catalyst...

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