Ionic liquids nucleophilic reactivities

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 rates for the Sn2 reactions of seven different anionic nucleophiles (Cl-, Br-, I-, OAc-, CN-, SCN- and trifluoromethylacetate-) with methyl / -nitrobenzenesulfonate have been determined in CH2CI2, MeOH, DMSO, and three ionic liquid solvents.122 The reactivity was not correlated with the dielectric constant for the solvents as predicted by the Hughes-Ingold rules and a different nucleophilic order was found in... 

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. 

Crowhurst L, Falcone R, Lancaster NL et al. (2006) Using Kamlet-Taft solvent descriptors to explain the reactivity of anionic nucleophiles in ionic liquids. J Org Chem 71 8847-8853... 

Details about ILs properties are covered in this book in the contributions by Seddon, Chiappe and Scott. However, two features deserve a comment for their possible consequences on reactivity and catalysis. First, depending on a delicate balance of entropie and enthalpic factors, including the polarity of the transition state structures with respect to regents, a reaction can be either speeded up or decelerated when carried out in an ionic liquid medium compared to a molecular solvent. An elegant study by Welton shows that in S-,2 reactions, primary, secondary and tertiary amines are more reactive as nucleophiles in ionic liquids, while halides react faster in conventional molecular solvents such as CH2CI2. In particular in a series of [Bmim] salts the order of nucleophilicity of halides is determined by the anion partner. To the same direction moves a kinetic study by Dyson on a cationic Ru(II) complex-catalysed hydrogenation of styrene in ILs, where it is clearly demonstrated that both the cation and the anion of the IL can inhibit or accelerate the formation of the active catalytic species. ... 

Betti C, Landini D, Maia A (2008) Reactivity of tuiionic nucleophiles in ionic liquids and molecular solvents. Tetrahedron 64 1689-1695... 

Recently, the reactivity (base, nucleophile, catalyst) and the role in modem organic synthesis of N-heterocyclic carbenes (cychc carbenes bearing at least one amino substituent) has been set-up by Ender et al. [60,61]. The possible utilization of NHCs as ligands for transition-metal catalyst [62,63] and as organocatalyst has been emphasized [61]. The inversion of the normal reactivity (umpolung) induced via NHCs has been extensively discussed. Classical carbon-carbon-bond-forming reactions (benzoin condensation, Stetter reaction, etc.) have been re-examined using ionic liquids as pre-catalysts in the presence of bases. 

In agreement with the behavior of amines, the reactivity of other neutral nucleophiles, such as water and methanol, towards primary and secondary haloalkanes in ionic liquids is generally higher than in molecular solvents and lower amounts of elimination products are formed. Water and methanol are more nucleophilic but less basic in ionic liquids than in molecular solvents [41]. 

As expected by the application of the Hughes-lngold rules, the reactions of Cr with dimethyl-4-nitrophenylsulfonium salts [4-N02CeH4S(CH3)2]X, Scheme 10.4) in ionic liquids were considerably slower than the same reactions in most molecular solvents. However, these reactions also followed a different reaction mechanism in the ionic liquids to those seen in molecular solvents [15]. In molecular solvents, the reactions follow a mechanism that is initiated by a fast ion metathesis to form the reactive ion pairs, which go on to give the products via a nucleophilic substitution reaction fScheme 10.51. 

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