Regional Electrophilic and Nucleophilic Fukui Functions Efficiently Highlight the Lewis Acidic/Basic Regions in Ionic Liquids

Abstract

The origin of catalysis and selectivity induced by room temperature ionic liquids in several organic reactions has putatively been associated with the concept of cation effect (hydrogen bond donor ability of the ionic liquids) or anion effect (hydrogen bond accepting ability of the ionic liquids). We show that there may be cases where this a priori classification may not be correctly assigned. Cations may concentrate both Lewis acidity and basicity functions in one fragment of the ionic liquid: an effect we tentatively call bifunctional distribution of the molecular Lewis acidity/basicity. Bifunctionality on the cation is however anion dependent through electronic polarization effects. The molecular distribution of the Lewis acidity/basicity may simply be assessed by evaluating the regional Fukui function within a reference ion pair structure. The model is tested for a set of nine ionic liquids based on the 1-butyl-3-methylimidazolium cation commonly used as solvent to run organic reactions.

Gutmann's Donor Numbers Correctly Assess the Effect of the Solvent on the Kinetics of SNAr Reactions in Ionic Liquids

“…These results confirm the hypothesis proposed by Harper et al in the sense that the structure of the IL maybecome adeterminant factor affecting the S N Ar reactivity. [31] Scheme1.Chemical structures for the series of cations and anions of the ionic liquidsu sed in this study. [16][17][18][19][20][21][22][23][24][25] It is based on amultivariate empirical equationt hat includes the followingp arameters:h ydrogen bond acidity (a), measuring the ability of the IL to donate aH B to the substrate;h ydrogen bond basicity (b), measuring the ability of the IL to accept aH Bf rom the substrate;a nd ap arameter p*, measuring the dipolar polarizability of the IL.…”

“…This simplistic assignation has been recently put into jeopardy,b ecause in [BMIM] + based ILs, both the HB acidity and HB basicity may be located at the cation, ane ffect that is, however,a nion-dependent through polarization effects. [31] Scheme1.Chemical structures for the series of cations and anions of the ionic liquidsu sed in this study. With this set of data at hand, we attempted to perform am ultivariate regressionp rocedure of the form shown in Equation (1), ap rocedure that systematically failed.…”

“…To settle the origin of the solvent effect of the ILs on the model S N Ar reaction, we considered as econd alternative model based on the donor (DN) and acceptor( AN) numbers proposed by ViktorG utmann, [26,27] which have been recently re-formulated to describe acid/base solvent properties in ILs by Schmeisser et al [28] According to the original definition proposed by Gutmann, DN and AN descriptors are aq uantitative measureo fL ewis basicity and acidity of as olvent (generally an on-aqueous one), respectively.T hese numbersc an be measured by using two different techniques, namely,calorimetrically or by using the chemical shift in 31 PNMR spectra. These parameters are widely used in inorganic chemistry to describe the ability of solvents to donate or accept electron pairs or at least electron density to the substrate.…”

“…Gutmann defined the DN as the positive DH value of the reaction of ad onor solvent with the acceptor halide antimony(V) chloride in an inert medium, such as 1,2-dichloroethane,a nd this parameter represents am easure for the donor properties of as olvent. On the other hand, ANs are derived from 31 PNMR measurements of tri-ethyl-phosphine oxide dissolved in the respective solvents. The AN is ameasureofthe electrophilic properties of asolvent.…”

Expression of Cyclooxygenase-2, Nitric Oxide Synthase 2 and Heme Oxygenase-1 mRNA Induced by Bis-Eugenol in RAW264.7 Cells and their Antioxidant Activity Determined Using the Induction Period Method

“…Therefore, the most relevant frontier orbital for electrophiles is the LUMO, whereas the HOMO is most important for nucleophiles (26). Softness is defined as the ease with which electron redistribution takes place during covalent bonding, and thus the softer the electrophile (more negative LUMO energy and higher σ value), the more readily it will form an adduct by accepting an outer shell electron from a soft nucleophile such as a sulfur atom.…”