State in solution, structure, and regioselectivity of reactions of the lithium 1-(2-methoxyphenyl)-3,3-diphenylpropyne derivative

According to the spectrophotometric data, the lithium 1-(2-methoxyphenyl)-3,3-diphenylpropyne derivative in diethyl ether exists as contact ion pairs, while in THF, according to the spectrophotometric and¹³C NMR data, solvent-separated ion pairs are predominantly formed. According to the¹³C NMR data, the carbanion in the solventseparated ion pairs has a structure close to the propargylic type. The regioselectivity of reactions of the lithium derivative with ethyl halides in diethyl ether, THF, and hexamethyphosphoramide, with benzyl chloride in the first two solvents, and with methanol in THF were studied. The protonation with methanol proceeds exclusively at the allenylic center (C-1) while the ethylation and especially benzylation proceed predominantly at the propargylic center (C-3). The selectivity of ethylation of the propargylic center of both solvent-separated ion pairs in THF and contact ion pairs in diethyl ether increases as the hardness of the ethylating agent increases, and in the case of the same ethyl halide, the selectivity increases from the solvent-separated ion pairs to the contact ion pairs. The spectral data obtained and the data on changes in the regioselectivity do not allow one to believe that the contact ion pairs of the lithium derivative in ether exhibit the intramolecular coordination of the lithium cation to the methoxy group, which might lead to the allenylic structure of contact ion pairs of this derivative. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2043–2051, November, 1997.     

Interpretation paradoxes in unimolecular heterolysis kinetics

The rate constant of the first-order rate equation w = k[RX] that is derived from the variation of the reaction product concentration or determined by the verdazyl method characterizes the lifetime of the transition state or that of the solvent-separated ion pair rather than the heterolysis rate. The diffusion rate constant is equal to the dissociation rate constant of the contact ion pair and to the reverse of the lifetime of the solvent-separated ion pair: k _D ≈ k = 1/τ ≈ 10¹⁰ s⁻¹.     

The stereoregularity of polystyrenes obtained by different ion pairs of polystyryl alkali salts

The stereoregularity of polystyrenes obtained with sodium, potassium, rubidium, and cesium naphthalenes in various solvents was determined by ¹³C-NMR spectroscopy. Polystyrenes produced by contact ion pairs of polystyryl cesium in dioxane and tetrahydrofuran (THF) had the proportions of a 57–58% racemic dyad (P_r), whereas the P_r values increased to 65 and 69% by solvating Cs⁺ counterions in dimethoxyethane and by agent-separating them with crown ether, respectively. Polystyrene obtained by contact ion pairs of polystyryl sodium in dioxane showed a P_r of 66%; polymers produced by solvent-separated ion pairs of polystyryl sodium in THF at ?78°C had a P_r of 71%. A polymerization system which contained alkali counterions with large ionic radii and solvents with low dielectric constants in which only contact ion pairs existed produced polystyrenes with isotactic-rich configurations. The stereoregularity of polystyrene produced by contact ion pairs of polystyryl potassium and rubidium in tetrahydropyran (THP) occurred intermediately between that of polymers obtained in diethyl ether and THF. It was concluded that the stereoregulation of contact ion pairs may be closely related to the interionic distance of the ion pair.     

The influence of polar solvents on ions and ion pairs in the anionic polymerization of styrene

In the rather polar organic solvents dimethoxyethane, tetrahydrofuran and 3-methyltetrahydrofuran, the behaviour of contact ion pairs, solvent-separated ion pairs and free carbanions of polystyryl sodium has been investigated by kinetic and conductivity measurements. Both the equilibrium between the two kinds of ion pairs and the dissociation of solvent-separated ion pairs to free anions are followed over a wide range of temperature. Thereby, conditions can be found under which the polymerization takes place almost exclusively via one of the two types of ion pairs.The thermodynamic parameters of the equilibria and the Arrhenius parameters of the propagation rate constants of the different kinds of propagating chain ends are reported. The equilibria between these species are strongly influenced by the solvent whereas the individual propagation rate constants are scarcely affected by the solvent. The “effective charge distance” in the solvent-separated ion pair can be estimated from the corresponding dissociation constant. The mobility of the polymer carbanion is discussed.     

Hydrogen bonding mediated ion pairs of some aprotic ionic liquids and their structural transition in aqueous solution

Ion pair speciation of ionic liquids(ILs) has an important effect on the physical and chemical properties of ILs and recognition of the structure of ion pairs in solution is essential. It has been reported that ion pairs of some ILs can be formed by hydrogen bonding interactions between cations and anions of them. Considering the fact that far-IR(FIR) spectroscopy is a powerful tool in indicating the intermolecular and intramolecular hydrogen bonding, in this work, this spectroscopic technique has been combined with molecular dynamic(MD) simulation and nuclear magnetic resonance hydrogen spectroscopy(~1H NMR) to investigate ion pairs of aprotic ILs [Bmim][NO_3], [BuPy][NO_3], [Pyr_(14)][NO_3], [PP_(14)][NO_3] and [Bu-choline][NO_3] in aqueous IL mixtures. The FIR spectra have been assigned with the aid of density functional theory(DFT) calculations, and the results are used to understand the effect of cationic nature on the structure of ion pairs. It is found that contact ion pairs formed in the neat aprotic ILs by hydrogen bonding interactions between cation and anion, were still maintained in aqueous solutions up to high water mole fraction(say 0.80 for [BuPy][NO3]). When water content was increased to a critical mole fraction of water(say 0.83 for [BuPy][NO3]), the contact ion pairs could be transformed into solvent-separated ion pairs due to the formation of the hydrogen bonding between ions and water. With the further dilution of the aqueous ILs solution, the solvent-separated ion pairs was finally turned into free cations and free anions(fully hydrated cations or anions). The concentrations of the ILs at which the contact ion pairs were transformed into solvent-separated ion pairs and solvent-separated ion pairs were transformed into free ions(fully hydrated ion) were dependent on the cationic structures. These information provides direct spectral evidence for ion pair structures of the aprotic ILs in aqueous solution. MD simulation and ~1H NMR results support the conclusion drawn from FIR spectra investigations.     

Synthesis and Characterization of the Dilithium Salt of the Octasilyl[4]radialene Dianion: Evidence for a Lithium Walk on the Eight-Center,Ten-Electron π-Electron System

A highly symmetric structure is displayed in toluene by 1 , which was obtained by reduction of octasilyl[4]radialene with lithium in THF. The two Li⁺ ions of 1 are not fixed to the π-electron system in toluene, but are fluxional, giving rise to a bis-contact ion pair. In a solvating medium such as THF, one of the Li⁺ ions dissociates in such a way that 1 exists as a contact ion pair and a solvent-separated ion pair.     

Radical heterolysis reactions. Dynamics of formation, collapse, and solvation of ion pairs determined by a competition kinetic method

The kinetics of radical heterolysis reactions, including rate constants for radical cation-anion contact ion pair formation, collapse of the contact pair back to the parent radical, and separation of the contact pair to a solvent-separated ion pair or free ions were obtained in several solvents for a beta-mesyloxy radical. Rate constants were determined from indirect kinetic studies using thiophenol as both a radical trapping agent via H-atom transfer and an alkene radical cation trapping agent via electron transfer. [reaction: see text].     

Transition metal carbonylates in nucleophilic aromatic substitution: II. Medium effects for substitution in 2-(pentacarbonylmanganese)-4,6-difluoro-1,3,5-triazine

Effects of counterion, solvent and solvating agents were studied for the reactions of transition metal carbonylates CpFe(CO)₂⁻, (Cp = C₅H₅), Re(CO)₅⁻¹, Mn(CO)₅⁻, CpW(CO)₃⁻¹, CpMo(CO)₃⁻ with 2-(pentacarbonylmagnanese)-4,6-difluoro-1,3,5-triazine. The observed trends substantiate the hypothesis of enhanced reactivity of contact ion pairs as compared to solvent-separated ion pairs and free ions.     

Effects of mercaptides on anionic polymerization. III. Polymerization of methyl methacrylate by thiophenoxides in polar aprotic solvents

Sodium thiophenoxide initiated the polymerization of methyl methacrylate in polar aprotic solvents (DMF, DMSO, HMPA). The active species that initiated the polymerization of the monomer was found by spectrophotometric measurements and by the sodium fusion method to be sodium thiophenoxide itself. The activation energy for the polymerization of the monomer in DMF solvent obtained was E = 3.4 kcal/mole below 30°C, and E = ?3.3 kcal/mole above the temperature. The phenomena were reasoned as the result of the formation of two active species: a solvent-separated ion pair and a contact ion pair. The effects of counterions on the reactivity of thiophenoxide increased with increasing electropositivity of the metals: Li < Na < K. Sodium phenoxide, the oxygen analog of thiophenoxide, was also found to initiate the polymerization of the monomer in the solvents. The relative reactivity of thiophenoxide to phenoxide for the monomer in HMPA at 30°C was thus determined: phenyl-SNa > phenyl-ONa. The relative effect of the polar aprotic solvents on the reactivity of thiophenoxide was also as follows: HMPA > DMF > DMSO. The kinetic studies were made by the graphical evaluation of rate constants. The following results were obtained for the monomer at 20°C in DMF solvent: K_p = 3.5 × 10² 1./mole-hr and K_t = 9.8 × 10^?2/hr.     

离子对内电子转移型染料阴离子-翁盐阳离子光敏体系中的溶剂效应和结构效应

本文研究了溶剂效应和结构效应对染料碘翁盐光物理, 光化学性质的影响。观察到在溶剂中离子对可以各种形式存在, 如紧密离子对、溶剂分隔离子对或溶剂化的自由离子, 溶剂的极性不仅影响各种存在形式的光谱性质, 而且影响它们之间的平衡关系, 进而影响离子对体系的物理化学性质。染料母核和碘翁阳离子的结构均对离子对体系的性质有影响。光诱导电子转移反应的热力学驱动力越大, 反应速度越快。用分子模拟技术(Molecular Modeling)对离子对体系的立体结构进行了研究, 为理解离子对体系的各种物理化学行为提供了重要的参考。     

Inversion of stereoselectivity in a metallocene catalyst

Inversion of stereoselectivity of a particular metallocene (Me₂Si(Flu) (N-t-Bu)ZrCl₂) (Me: methyl, Flu: fluorenyl, t-Bu: tert-butyl) from syndiospecific into isospecific was observed by changing the cocatalyst from methylaluminoxane (MAO) to [Ph₃C]⁺ [B(C₆F₅)₄]⁻/AliBu₃ (iBu:isobutyl). The change of the solvent-separated ion pair (in case of MAO as cocatalyst) into the contact ion pair (in case of the more polar and less bulky borate anion) was proposed as the plausible explanation of this phenomenon.     

Solvent-shared radical ion pairs [pyrene·⊖Na⊕(C2H5)2]∞: ESR evidence for two different aggregates in solution,room temperature crystallization,and structural proof of another polymorphic modification

The reduction of pyrene with sodium in aprotic diethyl ether allows to crystallize the extremely air-sensitive radical ion pair pyrene-sodium-diethylether. The single-crystal structure determination at 130 K shows that each sodium counter cation, solvated by one diethyl-ether molecule, is η³- and η⁶-coordinated to one of the short-axis six-membered rings of two pyrene radical anions. The resulting dibenzene-sodium sandwiches form a string, in which the hydrocarbon planes are canted to each other by 62°. In the pyrene radical-anion skeleton, no distortion due to its negative charge can be detected relative to that of the neutral molecule. From the temperature-dependent signal multiplets of preceding ESR investigations, the solvent-separated pyrene radical anion as well as two different contact radical-ion pairs had been identified and their structures in solution approximated by potential-energy estimates. Referring to the recently discovered long-axis Na⊕ contact ion pair polymorph, crystallized at lower temperatures, the structure reported here represents the second and probably thermodynamically more stable one. Both the ESR and the structural results provide some insight into the multidimensional networks of equilibria in aprotic solution, which are activated by alkali-metal reduction of unsaturated organic compounds.     

Structural parameters of concentrated aqueous solutions of LiCl under extreme conditions,as calculated by the method of integral equations. Effect of temperature

Peculiarities of structure formation of aqueous LiCl solutions at different salt : water molar ratios (LiCl : n H₂O, n = 3.15, 8.05, 14.90) under conditions of isobaric heating (p = 100 bar, T = 298, 323—523 K, T = 50 K) were studied by the method of integral equations. Heating of LiCl : 14.90H₂O solution was found to lead to disappearance of tetrahedral ordering of solvent molecules, appreciable weakening of the coordination abilities of both ions, and to an increase of the number of contact ion pairs and a decrease of the number of solvent-separated ion pairs. For the LiCl : 8.05H₂O system, the tetrahedral structure of the solvent disappears at a lower temperature and heating has a less pronounced effect on the coordination and associative abilities of the ions. In the LiCl : 3.15H₂O solution, tetrahedral ordering of the solvent molecules disappears at 298 K and the number of contact ion pairs decreases as temperature increases. Other structural changes in this system upon heating are similar to those found for the LiCl : 14.90H₂O and LiCl : 8.05H₂O solutions.     

Conductance and ion association in dimethylsulfite

The electrolytic conductances of LiClO₄, LiAsF₆, NaClO₄ and the n-butylammonium salts Bu₄NCl, Bu₄NClO₄, and Bu₄NBPh₄ (tetraphenylborate) have been determined in dimethylsufite at 25°C. Ion association constants calculated from these data are interpreted in terms of solvent-separated ion pairs (for LiClO₄, LiAsF₆, and NaClO₄), and contact ion pairs (for the n-butyl-ammonium salts). Comparisons are made for analogous electrolytes in acetone, and in which all salts form contact ion pairs.     

Molecular dynamics simulations of LiCl association and NaCl association in water by means of ABEEM/MM

Constrained molecular dynamics simulations have been used to investigate the LiCl and NaCl ionic association in water in terms of atom-bond electronegativity equalization method fused into molecular mechanics (ABEEM/MM). The simulations make use of the seven-site fluctuating charge and flexible ABEEM-7P water model, based on which an ion-water interaction potential has been constructed. The mean force and the potential of mean force for LiCl and NaCl in water, the charge distributions, as well as the structural and dynamical properties of contact ion pair dissociation have been investigated. The results are reasonable and informative. For LiCl ion pair in water, the solvent-separated ion pair configurations are more stable than contact ion pair configurations. The calculated PMF for NaCl in water indicates that contact ion pair and solvent-separated ion pair configurations are of comparable stability.     

High Temperature Stability of Hydrated Ion Pairs Na<Superscript>+</Superscript>Cl<Superscript>–</Superscript>(H<Subscript>2</Subscript>O)<Subscript><Emphasis Type="Italic">N</Emphasis></Subscript> under Conditions of a Flat Nanopore

The high-temperature stability of hydrated ion pairs under conditions of a nanoscopic flat pore with hydrophobic structureless walls is studied by computer simulations. The limited space of the nanopore stimulates dissociation of the contact ion pair (CIP) with its transition to the state of the solvent-separated ion pair (SSIP); moreover, the ion pair demonstrates a high degree of stability on heating. The inverse temperature effect where the heating renders a moderate consolidating effect on the state of a hydrated contact ion pair is observed: when heated to the electrolyte boiling point, the free energy barrier that separates the CIP and SSIP states shifts by 2 molecules towards the larger hydration shells. On the pressure scale, the boundary between CIP and SSIP states shifts at the same rate as the saturating pressure with the increase in the temperature.     

Mechanistic study of samarium diiodide-HMPA initiated 5-exo-trig Ketyl-Olefin coupling: the role of HMPA in post-electron transfer steps

The mechanistic importance of HMPA and proton donors (methanol, 2-methyl-2-propanol, and 2,2,2-trifluoroethanol) on SmI2-initiated 5-exo-trig ketyl-olefin cyclizations has been examined using stopped-flow spectrophotometric studies. In the presence of HMPA, the rate order of proton donors was zero and product studies showed that they had no impact on the diastereoselectivity of the reaction. Conversely, reactions were first-order in HMPA, and the additive displayed saturation kinetics at high concentrations. These results were consistent with HMPA being involved in a rate-limiting step before cyclization, where coordination of the intermediate ketyl to the sterically congested Sm(III)HMPA both stabilizes the intermediate and inhibits cyclization. Liberation of the contact ion pair through displacement by an equivalent of HMPA provides a solvent-separated ion pair releasing the steric constraint to ketyl-olefin cyclization. The mechanism derived from rate studies shows that HMPA is important not only in increasing the reduction potential of Sm(II) but also in enhancing the inherent reactivity of the radical anion intermediate formed after electron transfer through conversion of a sterically congested contact ion pair to a solvent-separated ion pair. The mechanistic complexity of the SmI2-HMPA-initiated ketyl-olefin cyclization is driven by the high affinity of HMPA for Sm(III), and these results suggest that simple empirical models describing the role of HMPA in more complex systems are likely to be fraught with a high degree of uncertainty.     

Studies on the reactive species in fluoride-mediated carbon-carbon bond-forming reactions: carbanion formation by desilylation with fluoride and enolates

The reactive species in fluoride-mediated carbon-carbon bond-forming reactions was investigated. The regio- and diastereoselectivities of silanes reacting with cyclohexenone in the presence of a catalytic amount of fluoride was compared to the reactivity of analogous solvent-separated lithium ion pairs. Closely analogous behavior was observed, showing that carbanions and not siliconate complexes are the reactive species in the fluoride-catalyzed reactions. Spectroscopic investigations unambiguously show that phenylthiobenzyl anion will form by reaction of silane with tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) or crypt[2.1.1]-solvated lithium enolates. The catalytic cycle runs smoothly with the crypt[2.1.1] complex of alpha-(phenylthio)benzyllithium as the initiator and enolate as the carrier of the desilylation reaction.     

Dynamics of alkene radical cation/phosphate anion pair formation from nucleotide C4' radicals. The DNA/RNA paradox revisited

The fragmentation of nucleotide C4' radicals generated by thiyl radical addition to C4'C5' exocyclic glycals has been re-examined and found to be a function of the thiol and, probably, the initiating system employed. It has been demonstrated that C4' radicals of DNA and RNA models fragment even in the very nonpolar benzene solution if the correct (aliphatic) thiol is employed. (17)O-Labeling experiments are used to demonstrate that the fragmentation of nucleotide C4' radicals (2-deoxyribo- and ribo-) to contact ion pairs is either irreversible or so rapidly reversible as to preclude prior reorganization of the contact ion pair. Formation of the solvent-separated ion pair is an irreversible step, with all such ion pairs proceeding to product formation.     

The Li+-Initiated Twofold Dehydrogenation and C−C Bond Formation of Hexaphenylbenzene to the Dilithium Salt of the 9,10-Diphenyltetrabenz[a,c,h,j]anthracene Dianion

Partly solvent-separated and partly solvent-shared , the contact ion triple of the 9,10-diphenyltetrabenz[a,c,h,j]anthracene dianion is formed by the ultrasonically activated reaction of hexaphenylbenzene with lithium metal powder in 1,2-dimethoxyethane (dme) [Eq. (1)]. The proposed microscopic pathway for this reaction—twofold dehydrogenation and C−C bond formation—is supported by quantum-chemical calculations.