Relative nucleophilicities, ionic liquids

The fact that ionic liquids with weakly coordinating anions can combine, in a unique manner, relatively high polarity with low nucleophilicity allows biphasic catalysis with highly electrophilic, cationic Ni-complexes to be carried out for the first time [26]. 

Some theoretical aspects of thiophene reactivity and structure have also been discussed, for example the kinetics of proton transfer from 2,3-dihydrobenzo[6]thiophenc-2-onc <06JOC8203>, the configuration of imines derived from thiophenecarbaldehydes <06JOC7165>, and the relative stability of benzo[c]thiophene <06T12204>. The kinetics of nucleophilic aromatic substitution of some 2-substituted-5-nitrothiophenes in room temperature ionic liquids have also been investigated <06JOC5144>. 

The nature of the solvent also determines the chemoselective outcome in the reaction products. Products arising from the incorporation of one solvent molecule are formed (besides dibromides) in alcohols, acetic acid and acetonitrile (Id-e), whereas dibromo derivatives are formed exclusively in chlorinated solvents, nitromethane and in ionic liquids. (9) Chemoselectivity depends on the relative nucleophilicity of the solvent and the counterion, although it is affected also by other phenomena (ion pairing, and ion dissociation) in methanol the addition process gives quasi-exclusively bromo-methoxy adducts, whereas in acetic acid dibromides are the main products, formed in addition to smaller amounts of the bromo-acetoxy derivatives. (70)... 

Several reports on the effect of ionic liquids on Sn reactions have been published. The rate constants of the Sn2 reactions of several anions with n-hexyl and n-octyl mesylates in two ionic liquids, [hexmim]C104 and [hexmim]PF6 containing 2000 ppm of water, were compared with the reactivity in chlorobenzene, DMSO, and MeOH.74 The results indicated that the relative nucleophilicities in the ionic liquids were similar to those in the other solvents. The rates in the ionic liquids were generally faster than those in MeOH but slower than those in DMSO or chlorobenzene. 

SN2 reactions between methyl p-nitrobenzenesulfonate and halide ions and amines in several different ionic liquids have been investigated.76 The rates of reaction can be understood using the Hughes-Ingold rules77 if one considers the ion-ion interactions and ion-dipole interactions that occur with the ionic liquid solvent. Changing the ionic liquid solvent can reverse the relative reactivity of Cl-, Br-, and I- nucleophiles. 

By using competitive experiments, the relative efficiency of different co-catalysts wasdeterminedtobeBMICF3C02>BMI BF4>BMIPFG, which was the opposite of that observed by Afonso ef al. [8] in solution. Based on the interception of these unprecedented supramolecular species, Eberlin and coworkers propose (Scheme 3.4) that 1,3-dialkylimidazolium ionic liquids function as efficient co-catalysts for the MBH reaction by (i) activating the aldehyde toward nucleophilic attack by BMI coordination (species 8 ) and (ii) stabilizing the zwitterionic species that act as the main MBH intermediates through supramolecular coordination (species 9", lO, 11 +, and 13 + ). 

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