Polycondensation and disproportionation of an oligosiloxanol in the presence of a superbase

Kinetics of reactions of model oligosiloxanols, 1,1,3,3,3-pentamethyldisiloxane-1-ol (MDH) and 1,1,3,3,5,5,5-heptamethyltrisiloxane-1-ol (MD₂H), which occur in the presence of phosphazenium superbase, hexapyrrolidine-diphosphazenium hydroxide, in an acid-base inert solvent, toluene, was studied using sampling and gas chromatographic analysis method. In addition, kinetics of reactions of MDH and MD₂H with trimethylsilanol (MH) was studied. In the MDH and MD₂H systems the oligosiloxanol condensation competes with the oligosiloxanol disproportionation, the latter being the dominating process. The disproportionation products, i.e. MD_n+1H and MD_n−1H, n=1, 2, … undergo analogous consecutive disproportionation and condensation reactions. The kinetic law was derived and rate parameters determined from initial rates and by computer simulation to the best agreement with experimental data. Both competing reactions, the disproportionation and the condensation, conform to the same general kinetic law being first internal order in substrate and first order in catalyst. Activation parameters of these reactions were determined. The results were interpreted in terms of a bimolecular mechanism in which nucleophilic attack of the silanolate anion directed to silicon of the silanol group causes the cleavage of one of its geminal bonds to oxygen, either the one to hydroxyl leading to condensation or the one to siloxane which leads to disproportionation. The latter is faster as the silanolate is a better leaving group compared with OH⁻. Moreover, in the pentacoordinate silicon transition state (or intermediate) the siloxane substituent preferentially enters the apical position, thus driving the OH substituent into the unreactive equatorial position.     

Preparation of substituted network polysilanes by a disproportionation reaction of alkoxydisilanes in the presence of alkoxysilanes

Network methylethoxypolysilanes containing various substituents such as hexyl, phenyl, ethylene, hexamethylene, phenylene, and thiophene groups were prepared by a disproportionation reaction of 1,1,2,2-tetraethoxy-1,2-dimethyldisilane in the presence of the corresponding substituted alkoxysilanes. The reaction was considered to proceed via a silyl anion mechanism. The silyl anion produced from a disilane or a higher homologue attacked the alkoxysilane, and that caused the alkoxysilane moiety to be introduced into the polysilane. The amount of incorporated substituents varied from 3 to 29% of all organic substituents, depending on which alkoxysilane was reacted. The electrical conductivities of some polysilanes were measured, and the polymer with thiophene groups was found to show relatively higher conductivity of 10⁻⁵ Scm⁻¹ after iodine doping. © 1996 John Wiley & Sons, Inc.     

Regioselective electrophilic trifluoromethylation of indolines, oxindoles and indoles in superacid

Treatment of indolines and N-acylindoles with HF/SbF₅/CCl₄ yields 6-trifluoro derivatives (indole numbering) whereas indoles and oxindoles give the 5-trifluoro derivatives in good yield.     

Studies on the disproportionation of trichloromethyltin

The rate of disproportionation of trichloromethyltin in various solvents was studied by two methods, derivatization with sodium tetraethylborate and ¹³C NMR measurements, which gave similar results. The reaction followed second‐order kinetics and was more rapid in coordinating solvents than in uncoordinating ones. Moreover, it was catalyzed by nucleophiles, such as amines or alcohols, which is in favor of a nucleophile‐assisted electrophilic reaction. Copyright © 2003 John Wiley & Sons, Ltd.     

Solid-state structure and condensation reaction of (triphenylmethyl)silanetriol

(Triphenylmethyl)silanetriol [1, (Ph₃C)Si(OH)₃] was obtained in 98% yield by the hydrolysis of (triphenylmethyl)trichlorosilane with ice-water in diethyl ether. Single crystal of [1 · acetone] for X-ray crystallographic determination was grown from a saturated acetone solution. In the single crystal X-ray structure analysis, the silanetriol 1 crystallizes in the centrosymmetric triclinic space group with two independent molecules in the asymmetric unit. Two independent molecules of the silanetriols 1 interact with each other and with acetones by intermolecular hydrogen bondings. Such hydrogen-bonding interactions lead to a one-dimensional columnar polymeric tube. Finally, this tube interacts with others to make sheets alternating hydrophobic organic part and hydrophilic hydroxy groups of the molecules 1 and the oxygen of acetones arranged regularly. The silanetriol 1 is very stable compound in solution and in solid states at room temperature, but decomposed in the presence of KOH, and undergoes a condensation reaction with dicyclohexylcarbodiimide (DCC) as strong dehydrating agent to give polysiloxane. The silanetriol 1 reacts with trimethylchlorosilane to give three type siloxane products (Ph₃C)Si(OH)_3−n(OSiMe₃)_n (n = 1, 2, 3). The number (n) of silylation of hydroxy groups on the silanetriol 1 increase with increasing the mol ratio of trimethylchlorosilane used.     

Gem-difluorination in superacid: The dramatic role of halonium ions

In HF/SbF₅, in the presence of CCl₄ or NBS, unsaturated or hydroxylated piperidines yield difluoroanalogues in good yields. The ability to perform hydride abstraction on chloroderivatives, and the ability to form bridged bromonium ions from unsaturated substrates seem to completely influence the reaction course. Some aspects of the mechanism were clarified by DFT calculations.     

Activity and selectivity of zeolite MCM-22 catalysts in the disproportionation of toluene

Zeolite precursor MCM-22 samples were prepared by hydrothermal synthesis. The H-zeolites, obtained from the precursors, were characterized by scanning electron microscopy (SEM), Fourier-transform infrared (FT-IR) spectroscopy, and by their activity in the title reaction. Despite of the lower Br?nsted acid site concentration of the zeolite having smaller crystallites, showed higher activity, while the zeolite, having larger crystals were more selective for p-xylene formation.     

New synthesis of trifluorinated amines from 1-bromopropargylic amines in superacid

Treatment of 1-bromopropargylic amines affords the corresponding 1,1,1-trifluoro in one step in HF-SbF₅ medium. 1-Bromo-1,1-difluoro or 2-bromo-1,1,1-trifluoro derivatives could also be prepared, depending on the reaction conditions.     

Reaction of quinidine acetate, epiquinidine and its acetate in superacid: formation of gem-difluoro derivatives with or without rearrangement

In HF-SbF₅, quinidine 1a or its dihydrochloride cyclises previously obtained with usual acids. A similar reaction is observed in the presence of CCl₄. Under similar conditions quinidine acetate 1b and epiquinidine acetate 2b dihydrochlorides both yield 10,10-difluoro derivatives epimeric at C-3, 6 and 7, and 9c and 10b, and a rearranged difluoro derivative 8b and 11b, respectively. Epiquinidine 2a leads to the expected analogues 10a and 11a and to a ketone 9a. Formation of gem-difluoro compounds implies chloro intermediates at C-10, precursors of α-chlorocarbenium ions, which are trapped by a fluoride ion and which lead by halogen exchange to the products.     

Influence of the solid-state modification of MCM-22 with Cs on toluene disproportionation

The catalytic performance of MCM-22 modified with Cs to different extents by Solid-State Ion Exchange has been investigated in toluene disproportionation. It was suggested that protons on the external surface were preferentially replaced by Cs cations. It was observed that the Cs modifications did not provide improved p-xylene selectivity at conversions close to that of the H-form. The remaining unaffected proton sites in the 10 MR channels are considered as responsible for secondary xylene isomerization proceeding upon toluene disproportionation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Rearrangement or gem-difluorination of quinine and 9-epiquinine and their acetates in superacid

In HF-SbF₅, quinine 1a or its dihydrochloride rearranges into compound 3 (89%), the preferred conformation of the substrate favouring the observed cyclization. Under similar conditions epiquinine 2a dihydrochloride yields in equal amounts two 10,10-difluoro derivatives, epimeric at C-3. In this case, the more stable conformation of the substrate in which the benzylic hydroxyl group is ‘exo’ to the quinuclidyl moiety, prevents the cyclisation. Similarly acetates 1b and 2b give the corresponding 10,10-difluoro derivatives epimeric at C-3. Formation of gem-difluoro compounds implies the formation of chloro intermediates at C-10 followed by an hydride abstraction, yielding an α-chloronium ion. This one is trapped by a fluoride ion and leads to the product by halogen exchange.     

Mechanistic model of disproportionation of nitrogen monoxide on CaHY-type zeolite

Infrared spectra produced by adsorbing, by evacuating and by readsorbing NO on CaHY-type zeolite were examined. The absorption peaks were identified by comparing the spectra produced by adsorbing and by evacuating N₂O and NO₂. Based upon the experimental evidence, an eight-step mechanism was proposed for the behavior of NO on CaHY-type zeolite. A mechanistic model of the disproportionation reaction of NO was deduced from the variation of the spectra with time.     

Peculiarities of disproportionation of toluene on acidic pentasil zeolites

Two samples of pentasil hydrogen forms were obtained: an ordinary sample and a sample lacking surface acid sites. The samples were tested in the disproportionation of toluene over short and very short periods of contact of the reaction mixture with the catalyst layer. The primary conversion products were found to be para-xylene and ethylbenzene in addition to benzene. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 45, No. 2, pp. 108-111, March-April, 2009.     

Catalytic properties of zeolites MCM-22 and NU-87 in disproportionation of toluene

The catalytic properties of MCM-22 and NU-87 were investigated for the disproportionation of toluene to produce benzene and xylene, and the results were compared with those obtained over mordenite, beta and ZSM-5. It turns out that dealumination of MCM-22 removes selectively the acid sites from the external surface and thus suppresses the secondary isomerization of p-xylene, enhancing the para-selectivity. This indicates that the dealuminated MCM-22 is a promising catalyst for the selective formation of p-xylene from toluene disproportionation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Toluene disproportionation and coking on zeolites Y modified with Lewis-connected InO acid sites

The effect of introduction of Lewis acid sites on the reactions of toluene transformation and catalysts coking has been studied over parent (Si/Al = 2.5) and dealuminated (Si/Al = 3.7) Y zeolites modified with Lewis connected InO⁺ cationic acid sites. The catalysts with prevalent amount of Brönsted acid sites (less than 40% of protons exchanged by InO⁺) possess the typical for the proton directed reaction of methyl transfer upon toluene disproportionation long period of activation, the longer the higher the concentration of bridging protons. In contrast, at predominant Lewis sites concentration, the activation period disappear, an enhanced initial activity is observed, followed by deactivation on the expense of a rapid process of accumulation of strongly held reaction products and intermediates. Their further condensation leads to catalysts aging, the faster the higher the concentration of electron acceptor Lewis acid sites. A relation has been found between the initially accelerated processes of alkyl transfer and the reinforced formation of carbonaceous deposits over the In-modified catalysts.     

Preparation of substituted network polysilanes by a disproportionation reaction of ethoxydisilane initiated by a small amount of organolithium reagents

Network methylethoxypolysilanes containing various substituents such as phenyl, butyl, phenylene, thiophene, and anthracene groups were prepared by a disproportionation reaction of 1,1,2,2-tetraethoxy-1,2-dimethyldisilane initiated by addition of a small amount of organolithium reagents that have the corresponding substituents. The reaction was considered to be catalyzed by lithium ethoxide, which was formed by the substitution reaction of the ethoxydisilane with the lithium reagents. Both the substituted and pristine disilanes participated in the disproportionation reaction to yield the network polysilanes. The amount of the substituents and the molecular weight of the polysilanes varied, depending on what and how much of the lithium reagent were used. The electrical conductivity of some polysilanes was measured, and polymers with thiophene or anthracene groups were found to show relatively higher conductivity of 10⁻⁴ Scm⁻¹ after iodine doping. © 1997 John Wiley & Sons, Inc.     

Effect of ion-pair formation between 1,4-benzosemiquinone and alkaline-earth metal cations on the semiquinone disproportionation equilibrium at physiological pH

The decrease in the 1,4-benzosemiquinone (BQ^−.) disproportionation constant upon addition of either MgCl₂ or BaCl₂ was used to determine the corresponding semiquinone-M⁺² ion pair formation constant in 10 mM cacodylic-cacodylate buffer, at pH 7.4, utilizing electron paramagnetic resonance (EPR) spectroscopy. These constants are 5.6±2.8 and 2.5±1.3 for magnesium and barium, respectively, at an ionic strength of 1.0. A decrease in the BQ^−. g-value was also detected upon addition of either MgCl₂ or BaCl₂ due to ion pair formation, being more pronounced for MgCl₂ than for BaCl₂. No changes in proton coupling constants were onserved by addition of these salts to the BQ^−. containing solutions. No ion pair formation was detected if KCl or Nacl were added, up to a concentration of 3.0M.     

Chain-growth polycondensation for well-defined condensation polymers and polymer architecture

The historical development of our research on polycondensation that proceeds in a chain-growth polymerization manner ("chain-growth polycondensation") for well-defined condensation polymers is described. We first studied polycondensation in which change of the substituent effect induced by bond formation drove the reactivity of the polymer end group higher than that of the monomer. In this approach, well-defined aromatic polyamides, polyesters, polyethers, and poly(ether sulfone)s were obtained. The second approach was the study of the phase-transfer polymerization of a solid monomer dispersed in an organic solvent. In this type of polymerization, the solid monomer was physically unable to react with another monomer and was carried with the phase transfer catalyst into the solution phase where it reacted with an initiator and the polymer end group in the solvent in a chain polymerization manner. We also found catalyst-transfer polycondensation as a third approach to chain-growth polycondensation. In the Ni-catalyzed polycondensation of 2-bromo-5-chloromagnesiothiophenes, the Ni catalyst transferred to the polymer end group, and a coupling reaction occurred there to yield a well-defined polythiophene. This chain-growth polycondensation was applied to the synthesis of condensation polymer architectures such as block copolymers, star polymers, graft copolymers, and so on.     

Understanding the reactivity and basicity of zeolites: a periodic DFT study of the disproportionation of N(2)O(4) on alkali-cation-exchanged zeolite Y

The disproportionation of N(2)O(4) into NO(3)(-) and NO(+) on Y zeolites has been studied through periodic DFT calculations to unravel 1) the role of metal cations and the framework oxygen atoms and 2) the relationship between the NO(+) stretching frequency and the basicity of zeolites. We have considered three situations: adsorption on site II cations with and without a cation at site III and adsorption on a site III cation. We observed that cations at sites II and III cooperate to stabilize N(2)O(4) and that the presence of a cation at site III is necessary to allow the disproportionation reaction. The strength of the stabilization is due to the number of stabilizing interactions increasing with the size of the cation and to the Lewis acidity of the alkali cations, which increases as the size of the cations decreases. In the product, NO(3)(-) interacts mainly with the cations and NO(+) with the basic oxygen atoms of the tetrahedral aluminium through its nitrogen atom. As the cation size increases, the NO(3)(-)...cation interaction increases. As a result, the negative charge of the framework is less well screened by the larger cations and the interaction between NO(+) and the basic oxygen atoms becomes stronger. NO(+) appears to be a good probe of zeolite basicity, in agreement with experimental observations.