Active catalysts, ionic tagging

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). 

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). 

Another comparative study of the CM reaction catalyzed by a second generation Grubbs carbene complex (without imidazolium tag) has been recently presented by Tang and coworkers [264]. They compared the catalytic transformation of styrene in [BMIM][BF4] and [BMIM PFs] with the same reaction in CH2Q2 (3 h, 45 °C) and obtained comparable yields of the CM product in all solvents in the first cycle. In the case of the two ionic liquids the product was extracted with diethyl ether and the remaining ionic catalyst solution could be recycled four times with only a small drop in catalytic activity (product yield 75% in the fourth cycle compared to 85% in the first cycle in [BMIMjfPFs]). The scope of the study also included the successful... 

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