Other Palladium Catalysis in Ionic Liquids

Another interesting carbonylation that has been carried out in ionic liquids is that of 3-alkyn-l-ols and l-alkyn-4-ols by [Pd(OAc)2]/ PyPPh2(PyPPh2=2-(diphenylphosphino)pyridine) to give evo-a-methylene y- and 8-lactones, respectively.60 The products were isolated in almost quantitative yields by distillation under reduced pressure or by solvent extraction from the ionic liquid with diethyl ether. However, when recycling of the ionic liquid solution was attempted for the carbonylation of 3-butyn-l-ol the yield fell from 99 to 37% after only two runs. 

The reaction of butadiene with methanol leads to the formation of the telomers l-methoxy-2,7-octadiene (trans- and cis-), 3-methoxy-l,7-octadiene and the dimers 1,3,7-octatriene and 4-vinylcyclohexene (as minor products). Using palladium(II) acetate with either PPh3 or TPPMS in a biphasic system with ionic liquid leads to an inactive system when [C2mim]+ and [C4mim]+ 

The formation of NHC complexes is clearly going to be important to the palladium chemistry seen in ionic liquids. The formation of such complexes has been seen to be advantageous (as in the Suzuki reaction) or disadvantageous (as in the telomerization of butadiene with methanol), depending on the particular system being investigated. What is yet to be seen is if the ionic liquid can be manipulated to generate NHC s with particular useful properties. 

Finally, the author would like to make some general points. When an area of chemistry has such potential for application and becomes fashionable discussion around it often polarizes. That seems very much to be the case with ionic liquids. Ionic liquids are not a panacea for all of the problems of chemical industries, but that equally well does not mean that they are a waste of time. It is just over a decade since the modern generation of ionic liquids were first discovered.83 This is a remarkably short period of time and it is not surprising that large-scale processes that use these ionic liquids are not yet in place. However, in this short time, huge advances have been made in understanding how they might be useful. 

Finally, much of the contention that seems to arise around ionic liquids derives from statements of the kind ionic liquids are... . No one would ever consider making the blanket statement molecular liquids are... . 

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