József Rábai

Facile catalyst separation without water : fluorous biphase hydroformylation of olefins

A novel concept for performing stoichiometric and catalytic chemical transformations has been developed that is based on the limited miscibility of partially or fully fluorinated compounds with nonfluorinated compounds. A fluorous biphase system (FBS) consists of a fluorous phase containing a dissolved reagent or catalyst and another phase, which could be any common organic or nonorganic solvent with limited or no solubility in the fluorous phase. The fluorous phase is defined as the fluorocarbon (mostly perfluorinated alkanes, ethers, and tertiary amines)—rich phase of a biphase system. An FBS compatible reagent or catalyst contains enough fluorous moieties that it will be soluble only or preferentially in the fluorous phase. The most effective fluorous moieties are linear or branched perfluoroalkyl chains with high carbon number; they may also contain heteroatoms. The chemical transformation may occur either in the fluorous phase or at the interface of the two phases. The application of FBS has been demonstrated for the extraction of rhodium from toluene and for the hydroformylation of olefins. The ability to separate a catalyst or a reagent from the products completely at mild conditions could lead to industrial application of homogeneous catalysts or reagents and to the development of more environmentally benign processes.

An improved design of fluorophilic molecules: Prediction of the ln P fluorous partition coefficient, fluorophilicity, using 3D QSAR descriptors and neural networks

Abstract A combination of 3D QSAR molecular descriptors and artificial neural networks have been used to predict fluorophilicities, the natural logarithm of the perfluoro(methylcyclohexane)/toluene partition coefficients, for a wide range of partially fluorinated organic compounds. The average error of the predictions was less than twice the 0.2 experimental error. Multiple linear regression proved to be much less efficient. To better characterise the fluorous partition phenomenon, specific fluorophilicity was defined as the product of fluorophilicity and of the ratio of the van der Waals volumes of the expelled fluorous solvent and the entering solute molecules. This dimensionless term correlates well in a compound family with the calculated Hildebrand parameters of the fluorous molecules. The trifluoromethyl group was found highly effective for increasing the fluorous phase affinities of model compounds when used in combination with longer perfluoroalkyl groups.

Fluorous Polymeric Membranes for Ionophore-Based Ion-Selective Potentiometry: How Inert Is Teflon AF?

Fluorous media are the least polar and polarizable condensed phases known. Their use as membrane materials considerably increases the selectivity and robustness of ion-selective electrodes (ISEs). In this research, a fluorous amorphous perfluoropolymer was used for the first time as a matrix for an ISE membrane. Electrodes for pH measurements with membranes composed of poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole]-co-poly(tetrafluoroethylene) (87% dioxole monomer content; known as Teflon AF2400) as polymer matrix, a linear perfluorooligoether as plasticizer, sodium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate providing for ionic sites, and bis[(perfluorooctyl)propyl]-2,2,2-trifluoroethylamine as H+ ionophore were investigated. All electrodes had excellent potentiometric selectivities, showed Nernstian responses to H+ over a wide pH range, exhibited enhanced mechanical stability, and maintained their selectivity over at least 4 weeks. For membranes of low ionophore concentration, the polymer affected the sensor selectivity noticeably at polymer concentrations exceeding 15%. Also, the membrane resistance increased quite strongly at high polymer concentrations, which cannot be explained by the Mackie-Meares obstruction model. The selectivities and resistances depend on the polymer concentration because of a functional group associated with Teflon AF2400, with a concentration of one functional group per 854 monomer units of the polymer. In the fluorous environment of these membranes, this functional group binds to Na+, K+, Ca2+, and the unprotonated ionophore with binding constants of 10(3.5), 10(1.8), 10(6.8), and 10(4.4) M(-1), respectively. Potentiometric and spectroscopic evidence indicates that these functional groups are COOH groups formed by the hydrolysis of carboxylic acid fluoride (COF) groups originally present in Teflon AF2400. The use of higher ionophore concentrations removes the undesirable effect of these COOH groups almost completely. Alternatively, the C(=O)F groups can be eliminated chemically, or they can be used to readily introduce new functionalities.

Fluorophilic Ionophores for Potentiometric pH Determinations with Fluorous Membranes of Exceptional Selectivity

Ionophore-doped sensor membranes exhibit greater selectivities and wider measuring ranges when they are prepared with noncoordinating matrixes. Since fluorous phases are the least polar and least polarizable liquid phases known, a fluorous phase was used for this work as the membrane matrix for a series of ionophore-based sensors to explore the ultimate limit of selectivity. Fluorous pH electrode membranes, each comprised of perfluoroperhydrophenanthrene, sodium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate, and one of four fluorophilic H(+)-selective ionophores were prepared. All the ionophores are highly fluorinated trialkylamines containing three electron withdrawing perfluoroalkyl groups shielded from the central nitrogen by alkyl spacers of varying lengths: [CF(3)(CF(2))(7)(CH(2))(3)](2)[CF(3)(CF(2))(6)CH(2)]N, [CF(3)(CF(2))(7)(CH(2))(3)](2)(CF(3)CH(2))N, [CF(3)(CF(2))(7)(CH(2))(3)](3)N, and [CF(3)(CF(2))(7)(CH(2))(5)](3)N. Their pKa values in the fluorous matrix are as high as 15.4 +/- 0.3, and the corresponding electrodes exhibit logarithmic selectivity coefficients for H(+) over K(+) as low as <-12.8. The pKa and selectivity follow the trends expected from the degree of shielding and the length of the perfluoroalkyl chains of the ionophores. These electrodes are the first fluorous ionophore-based sensors described in the literature. The selectivities of the sensor containing [CF(3)(CF(2))(7)(CH(2))(5)](3)N are not only greater than those of analogous sensors with nonfluorous membranes but were of the same magnitude as the best ionophore-based pH sensors ever reported.

Practice of fluorous biphase chemistry: convenient synthesis of novel fluorophilic ethers via a Mitsunobu reaction

Abstract The evolution of the term fluorous is addressed first, then a concise terminology is proposed, including fluorous partition coefficient, specific fluorophilicity and fluorousness. Some examples are shown for the design of higher generation fluorophilic molecules, involving Class I to Class III ponytails. Fluorophilic ethers of the structure of ArC(CF3)2O(CH2)m(CF2)nF (m=1, n=1, 7; m=3, n=8) are obtained in high yields, when 2-aryl-1,1,1,3,3,3-hexafluoro-propanols are reacted either with trifluoroethyl- and 1H,1H-perfluorooctyl triflates (NaH/DMF, Williamson ether synthesis) or with 3-perfluorooctyl-propanol (Ph3P/EtO2CNNCO2Et/PhCF3, Mitsunobu reaction), respectively. Fluorophilic phenol- and perfluoro-tert-butyl ethers can also be prepared effectively by the latter method. In case of higher homologues (n=7, 8) product isolation can be facilitated using fluorous extraction (C6F14/CH3OH). Specific fluorophilicity values of target molecules are estimated using a 2D method and compared with experimentally determined ones.

Perfluoroalkylated amphiphiles with a morpholinophosphate or a dimorpholinophosphate polar head group

Some previously synthesized (perfluoroalkyl)alkyldimorpholinophosphates, CnF2n+1CmH2mOP(O)- [N(CH2CH2)2O]2, were found remarkably to stabilize heat sterilizable water-in-fluorocarbon reverse emulsions and to have a strong proclivity to self-aggregate into microtubular assemblies when dispersed in water. This series has now been extended in order to allow structure–property relationships to be established and product optimization to be achieved. A new series of even more fluorophilic compounds consisting in bis[(perfluoroalkyl)alkyl]monomorpholinophosphates, (CnF2n+1CmH2mO)2P(O)N(CH2CH2)2O, was also synthesized. Preliminary surfactant activity and biocompatibility data are presented and compared to data obtained with non-fluorinated analogues.

Diaryldiacyloxyspirosulfuranes—I : Syntheses from sulfides with halogenating agents

Abstract A series of diaryldiacyloxyspirosulfuranes has been prepared from bis(2-carboxyaryl) sulfides with different halogenating agents. In pyridine, sulfides with two electron-withdrawing nitro groups cannot be converted into spirosulfuranes, suggesting the participation of pyridine in Cl+-transfer reactions. UV and IR spectroscopic data for spirosulfuranes are also reported and briefly discussed.

Neighbouring group participation in reactions of sulphides with chloramine-t

Abstract Neighbouring group participation in reactions of chloramine-T with ortho -substituted aryl methyl sulphides and diaryl sulphides has been studied. The reaction is markedly hindered by the steric effect of the ortho substituent of the phenyl ring, but groups having a CO moiety show an anchimeric effect in the following order: o -CH 2 CO 2 Me ~ o -CH 2 CO 2 H o -CH 2 CO 2 - o -CO 2 Me ~ o -CO 2 H o -CO 2 - ⪡ 2o-CO 2 in the rate-determining step may be ruled out on the basis of salt and isotope effect. Substituents with neighbouring group participation diminish the yield of sulphilimine in solvents containing water. The electrophilic chlorination of sulphides by TsNHCl may be assumed to be the rate-determining step with the positively charged sulphonium centre stabilized by the negatively polarized or charged carbonyl-oxygen in the transition state. This type of interaction hinders the nucleophilic attack of sulphonamidate ion at the sulphonium centre in the fast product-controlling steps, decreasing the yield of sulphilimine.

Convenient syntheses and characterization of fluorophilic perfluorooctyl-propyl amines and ab initio calculations of proton affinities of related model compounds

Abstract A convenient and effective method for the preparation of perfluorooctyl-propyl amines ([R f8 (CH 2 ) 3 ] n NH 3− n ( 1 – 3 ), n =1,2,3; R f8 (CH 2 ) 3 NHMe ( 4 ); [R f8 (CH 2 ) 3 ] 2 NMe ( 5 ); R f8 (CH 2 ) 3 NMe 2 ( 6 ); R f8 =F(CF 2 ) 8 ) via a step by step alkylation with R f8 (CH 2 ) 3 I is described. The fluorophilicity values of 1 – 6 were determined by GC and range from 0.79±0.07 ( 1 ) to 5.3±0.2 ( 3 ). Systematic ab initio calculations of proton affinities of model compounds ( 7a – j ) using Hartree–Fock and density functional theory imply that the inserted trimethylene spacer unit efficiently reduces the electron-withdrawing effect of the perfluorinated segment. All structures were verified by multinuclear one- and two-dimensional NMR experiments involving both homo- ( 19 F – 19 F ) and hetero-nuclear ( 1 H – 13 C , 1 H – 15 N , 19 F – 13 C ) correlations based on the GMQFCOPS and inverse 1 H and/or 19 F detected GHSQC, GHMQC sequences with broadband adiabatic 13 C -decoupling.

Diaryldiacyloxyspirosulfuranes. Part 3. Sulfuranes with five-, six- and seven-membered spirorings: syntheses and molecular structures

Three novel diaryldiacyloxyspirosulfuranes (2–4) with five-, six- and seven-membered spirorings have been prepared, and their molecular structures determined by X-ray diffraction. The structure of spirosulfurane 1 with two identical five-membered spirorings has been reinvestigated. In all spirosulfuranes 1–4 the arrangement of the ligands about the central sulfur atom show a slightly distorted trigonal bipyramidal (TBP) geometry resulting in chiral molecular structure. The axial (hypervalent) S–O and equatorial (covalent) S–Car bond lengths range from 1.838(1) to 1.872(3) and from 1.771 (3) to 1.794(4)A, respectively. The axial O–S–O and equatorial Car–S–Car bond angles lie in the narrow intervals of 174.9(2)–177.4(4)° and 105.8(2)–106.9(2)°, respectively. The five-membered spirorings are practically planar in 1–3. The six-membered spirorings in 2 and 4 assume distorted sofa conformations. The seven-membered spiroring in 3, having four Car atoms with planar configuration, is irregular. The aromatic rings fused with the spirorings are perpendicular to the equatorial plane only if the spiroring is five-membered. In other cases the stable conformations with TBP geometry about sulfur involve a significant torsion of the aromatic ring about the S–Car axis. Thus, in diaryldiacyloxyspirosulfuranes having acyloxy-O ligands at axial positions the structural parameters about the sulfur are not considerably influenced by the shape and size of the spirorings. The O–S–O moiety with hypervalent bonds may be regarded as a structural unity. The sum of the individual S–O bond lengths, (3.684–3.744 A) which is characteristic of diaryldiacyloxyspirosulfuranes, differs markedly from the values found earlier for dialkoxy- and acyloxy-alkyloxy analogues (3.601 and 3.910 A, respectively). For spirosulfurane 3 a rearrangement into the isomeric ten-membered cyclic anhydride 13 is observed.