Lucia Brinchi

Ionic liquids as reaction media for esterification of carboxylate sodium salts with alkyl halides

Abstract Ionic liquids based on 1,3-dialkylimidazolinium methanesulfonate have been used as effective reusable reaction media in the esterification of several carboxylate sodium salts with different alkyl halides. Products are easily isolated by extraction with ether, and the protocol is mild and green, compared to the existing methods based on toxic solvents. Proper ‘design’ of the ionic liquid allows us to obtain esters always in quantitative yields.

Effect of head group size, temperature and counterion specificity on cationic micelles.

The critical micelle concentration (cmc) and ionisation degree (α), of micelles of cetyltrimethylammonium bromide (CTABr), cetyltrimethylammonium chloride (CTACl), cetyltripropylammonium bromide (CTPABr) and cetyltripropylammonium chloride (CTPACl) have been measured over a narrow temperature range at 2 degree intervals using electrical conductivity. CTPACl and CTPABr are very soluble in water and were measured in the temperature range 275.15-323.15K. The Krafft temperatures for CTABr and for CTACl are 293.15K and 284.15K, respectively and established a lower temperature limit for our studies on these two surfactants. The cmc vs temperature curves have a smooth minimum near room temperature and α linearly increases with temperature. The changes of cmc and α with temperature are smaller than those associated with the modification of head group size or counterion nature. Using these results, basic thermodynamic quantities associated with the phenomena of micellization have been evaluated. Thermodynamic properties of the surfactant solutions were discussed in terms of temperature dependence of the free energy, enthalpy and entropy of micellization. A close similarity between the effects of change in temperature on protein folding and micellization process appears from the data.

Viscoelastic solutions formed by worm-like micelles of amine oxide surfactant

Formation and properties of viscoelastic wormlike aqueous micellar solutions of the zwitterionic surfactant p-dodecyloxybenzyldimethylamine oxide (pDoAO) were studied. Semi-dilute aqueous solutions of pDoAO show a sharp increase in viscosity, which exceeds 160 cST for concentrations >50 mM, leading to viscoelastic solutions. Viscoelasticity relates to the surfactant charge type. In fact this viscoelastic system reverses to fluid when acid is added (pH<2), which changes the system to cationic. Under acidic conditions the system resembles solutions of the similar cationic surfactant p-dodecyloxybenzyltrimethylammonium bromide, (pDoTABr) in terms of viscosity. Properties of aqueous solutions of pDoAO were investigated by dynamic light scattering (DLS), rheology and small angle neutron scattering (SANS). Data support the idea that small micelles grow in length (wormlike or threadlike micelles) as surfactant concentration increases and viscoelastic solutions form as micelles become entangled. The micellar diameter as calculated by different techniques is about 5 nm.

Efficient esterification of carboxylic acids with alkyl halides catalyzed by fluoride ions in ionic liquids

Ionic liquids based on 1,3-dialkylimidazolinium methanesulfonate have been used as efficient reusable reaction media in the esterification of several carboxylic acids with alkyl halides catalyzed by fluoride ions. The method has wide applicability, and it is mild and green; it is useful for the protection of acids, via ester formation, for alkali labile molecules.

Reductions of α,β-Unsaturated Ketones by NaBH4 or NaBH4 + CoCl2: Selectivity Control by Water or by Aqueous Micellar Solutions

Operationally simple and environmentally benign procedures have been developed to selectively reduce different α,β-unsaturated ketones, 4,4-dimethylcyclohex-2-ene-1-one (1), isophorone (2), benzylideneacetone (3), chalcone (4) by NaBH4 or by the system NaBH4 + CoCl2. Alternative reaction media to the extensively used MeOH have been explored, and new procedures take advantage of the acceleration and chemoselectivity induced by water or by aqueous micellar solutions. It was possible to selectively and quantitatively afford pure products of 1,2 and of 1,4 reduction as well as the totally reduced compounds (yield and selectivity > 90%) by simple changes in the experimental conditions.

Elimination in sulfobetaine micelles: effects of head group bulk

Zwitterionic micelles of tetradecyldimethylammoniopropanesulfonate and tetradecyldibutylammoniopropanesulfonate, SB3-14 and SBBu3-14, respectively, accelerate the E2 reaction of OH− with 4-nitrophenethyl bromide (1b), but the reaction of phenethyl bromide (1a) is inhibited by SB3-14 and is only slightly accelerated by SBBu3-14. Analysis of these data allows the separation of the association constants for transfer of reactants from water to micelles from the second-order rate constants in the micelles. These second-order rate constants are, as in water, larger for 1b than 1a, and for both substrates are higher in SBBu3-14 than in SB3-14, as found for reactions in otherwise similar cationic micelles. Rate constants in micelles, relative to those in water, are consistently higher in E2 than in SN2 reactions. Copyright

Chemoselectivity in SN2-E2 reactions induced by aqueous association colloids

Abstract The effects of cationic and sulfobetaine surfactants and quaternary ammonium salts upon the products of SN2 and E2 reactions of sulfonate esters have been examined in mixtures of bromide and hydroxide ions. Surfactants are: n-C16H33N+R3X−, R = Me, Et, n-Pr, n-Bu, X = Br, OH, OMs; n-C14H29N+Me2(CH2)3SO3−, and salts were R4NBr, R = Me, n-Bu. Without surfactants the reaction of OH− is dominant, but with surfactants the reaction of Br− generates alkyl bromides, and selectivity is largest with n-C16H33N+Bu3Br−. Elimination is favored relative to substitution by OH−. Most reactions were made with 10−3 M substrate, but scale-up to 10−2 M substrate was successful in hexane water with extraction of the products and re-use of the aqueous surfactant.

Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in aqueous cationic micelles: kinetic evidence of microinterface property changes

We studied decarboxylation of 6-nitrobenzisoxazole-3-carboxylate, 1, as a kinetic probe to investigate microinterface properties of aqueous micelles formed by cationic surfactants of increasing head group bulk, i.e., cetyltrialkylammonium bromide, with alkyl=Me (CTABr), Et (CTEABr), n-Pr (CTPABr), n-Bu (CTBABr) and p-octyloxybenzyltrialkylammonium bromide surfactants with alkyl=Me (pOOTABr), n-Pr (pOOTPABr), and n-Bu (pOOTBABr), and the longer p-dodecyloxybenzyltrimethylammonium bromide (pDoTABr) at concentrations higher than 0.05 M. The pseudophase kinetic treatment fails to fit the data that show anomalies with abrupt increases in k(obs) for CTPABr and CTBABr (but not for CTEABr) and with smooth and continuos increase of k(obs) for all p-alkyloxybenzyltrilakylammonium bromides. Abrupt and successive modifications of the micellar interface properties, undergoing only when the polar head or the alkyl chain have some covalent structure, account for the observed kinetic behavior.

Structure of Micellar Head-Groups and the Hydrolysis of Phenyl Chloroformate − The Role of Perchlorate Ion

Spontaneous hydrolysis of phenyl chloroformate is inhibited by micelles, but inhibition by SDS (n-C12H25OSO3Na) micelles is much greater than inhibition by cationic or sulfobetaine micelles (n-C16H33NR3X: CTAX, CTEAX, CTBAX, R = Me, Et, Bu, respectively, X = Cl, Br, OMs; n-C14H29N+R2(CH2)3SO3−: SB3-14, SBP3-14, R = Me, Pr, respectively, and n-C14H29N+Me2(CH2)nSO3−: SB4-14, SB5-14, n = 4 and 5, respectively). − Inhibition by cationic micelles increases with increasing head-group bulk and affinity of the counterion for cationic micelles, while addition of NaClO4 to sulfobetaines inhibits reaction strongly. Values of kobs relative to reactivity in water, for cationic micelles, are 0.64 (CTACl) and 0.32 (CTBABr); sulfobetaines behave similarly, but the corresponding values for SDS and sulfobetaine/ClO4− mixtures are ca. 0.06. These head-group and counterion effects indicate that depletion of water in the interfacial region complements the charge effect in controlling reactivity in micelles.

Convenient Esterification of Carboxylic Acids by SN2 Reaction Promoted by a Protic Ionic-Liquid System Formed in Situ in Solvent-Free Conditions

Abstract The reaction of esterification of benzoic acid with benzyl chloride was chosen as a model reaction to study the esterification by SN2 promoted by tertiary amine as deprotonating agent. The use of ionic liquid (IL) 1,3-dimethylimidazolium methanesulfonate [MMIm][OMs] as reaction medium has proven to give quantitative yield of the ester, but interestingly the reaction does occur even in solvent-free conditions, where the acid + the amine form a liquid system (a protic IL) in situ. This last methodology was extended to several carboxylic acids in conditions of atom economy (i.e., without excess of any reagent), giving moderately good yields of esters (54–78%) recovered by weight in pure form. GRAPHICAL ABSTRACT