Ionic tag

Another example is the ionic tagging of a ruthenium carbene catalyst for ringclosing alkene metathesis. A [-(CH2)4-MIM] PF6 moiety was built into the structure of the catalyst, which enabled the long-term retention of the catalyst in the [BMIM]PF6 ionic liquid for multiple recycles (188). 

In this case, the ruthenium complex was synthesized with an ionic tag moiety that is fully compatible with the [BMIM]PF6. A minimum amount of this ionic liquid was used for the reaction in a mixture containing 90 vol% CH2CI2. The catalyst (5mol%) was selectively retained in the ionic liquid after 10 repeated uses, without significant loss of activity in tests at 50°C that lasted 3h each (Table VI). 

Reuse of Catalysts in Ring-Closure Metathesis with an Ionic Tagged Ruthenium Carbene Complex (188)... 

Scheme 9.20 Synthesis of a chiral vanadyl salen complex with an ionic tag...

Yao, Q., Zhang, Y. Olefin metathesis in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate using a recyclable Ru catalyst Remarkable effect of a designer ionic tag. Angew. Chem., Int. Ed. Engl. 2003,42, 3395-3398. 

However, the conversion and enantioselectivity were slowly decreased from the fifth run onwards (entries 21-25). Nevertheless, these results clearly demonstrated the effects of ionic tag on the catalyst recycling in ionic liquids. 

The catalyst leaching problem could be solved by incorporating an ionic tag onto the 4-hydroxyproline [82]. In teres lirigly, the imidazolium-tagged organo-... 

Much stronger, however, is the electrostatic interaction of the ion pairs of an IL with a catalyst possessing an ionic tag installed on its scaffold, e.g. 17. In this coulombic environment either acceleration or inhibition can be observed with respect to the use of molecular solvents. In the absence of a robust theoretical... 

We now discuss applications of the ionic tagging strategy to the design of catalysts for the direct asymmetric Michael addition of carbonyl compounds to nitroalkenes. Table 1.3 shows a few examples. 

Table 1.3 Ionic-tagged pyrrolidines for the asymmetric Michael reaction under neat conditions or in water...

A more elaborate molecular design is at the basis of a solvent-less reaction protocol. The ionic-tagging strategy, combined with the installation of a... 

The concept of ionic tagging in aryl phosphine design is made explicit by the structures of ligands collected in Table 1.5. 

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. 

An alternative solution to ionic tagging to confine a catalyst into an IL phase is that of using a cationic metathesis complex, e.g. ruthenium allenylidene salt (86). Treatment of diallyltosylamide at 80 °C for 5h dissolved in [bmi-m][OTf] in the presence of 2.5 mol.% of pre-catalyst 86 led to the... 

Sebesta R, Kmentova 1, Toma S (2008) Catalysts with ionic tag and their use in ionic liquids. Green Chem 10 484-496... 

Mass spectrometry (MS) studies have played a key role in the study of metathesis reactions, particularly in the hands of Chen and coworkers, who have identified intermediates in the catalytic cycle,and probed the energetics of their reactions, using electrospray MS techniques. Species such as 14e ruthenium carbene complexes can be detected by MS in the presence of different alkene substrates, the different carbene products (from CM or ROMP, for example) can be detected. Further, the fragments into which any proposed species can be broken by successively higher lens potentials can be used to check the species structure. In successive and more advanced studies, interpretation of data from the energy-resolved, coUision-induced dissociation cross-section measurements allowed the construction of potential energy surfaces for some steps of the metathesis reaction.Metathesis precatalysts were typically custom-made species, modified with ionic tags, to facilitate detection by MS. 

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.

Capitalizing on our efforts aimed at developing more environment friendly olefin metathesis processes, we have describe recently synthesis of new air-stable pyridinium containing Ru catalysts 15 for which the ionic tag is anchored to the styrenyl-ether benzylidene fragment (Figure 8). 

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). 

These results showed the dramatic influence of the steric hindrance on the aUcoxy substituent. This influence was observed in the original work by Hoveyda [11]. Since the isopropoxy coordinating group seemed to be necessary, we considered the synthesis of a sterically activated catalyst by introducing the ionic tag at the ortho-position of the isopropoxy substituent. Indeed, Blechert showed that the introduction of a substituent at the orthoposition of the isopropoxy substituent dramatically increased the catalyst activity [43]. Thus, catalyst 12 was prepared in six steps from the commercially available 2,3-dihydroxybenzaldehyde (Scheme 15). 

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