Methyltrimethoxysilane Sol-Gel Polymerization in Acidic Ethanol Solutions Studied by 29Si NMR Spectroscopy

²⁹Si solution NMR has been used to study the acid-catalyzed polymerization of methyltrimethoxysilane (MTMS) in ethanol. The complex reaction pathways have been analyzed, and the general trends described. New interpretations are given to the rate constants of condensation reactions. The steric effects and inductive effects are found to work in coordination to reduce the reactivity of the silicon sites with the increasing connectivity. The extensive cyclization is attributed to the low steric effects of a tetramer ring closure compared to other end/end species condensations.     

Kinetics of the acid-catalyzed hydrolysis of tetraethoxysilane (TEOS) by <Superscript>29</Superscript>Si NMR spectroscopy and mathematical modeling

Tetraethoxysilane (TEOS) is widely used to synthesize siliceous material by the sol–gel process. However, there is still some disagreement about the nature of the limiting step in the hydrolysis and condensation reactions. The goal of this research was to measure the variation in the concentration of intermediates formed in the acid-catalyzed hydrolysis by ²⁹Si NMR spectroscopy, to model the reactions, and to obtain the rate constants and the activation energy for the hydrolysis and early condensation steps. We studied the kinetics of TEOS between pH 3.8 and 4.4, and four temperature values in the range of 277.2–313.2?K, with a TEOS:ethanol:water molar ratio of 1:30:20. Both hydrolysis and the condensation rate speeded up with the temperature and the concentration of oxonium ions. The kinetic constants for hydrolysis reactions increased in each step k_h1?<?k_h2?<?k_h3?<?k_h4, but the condensation rate was lower for dimer formation than for the formation of the fully hydrolyzed Si(OH)₄. The system was described according to 13 parameters: six of them for the kinetic constants estimated at 298.2?K, six to the activation energies, and one to the equilibrium constant for the fourth hydrolysis. The mathematical model shows a steady increase in the activation energy from 34.5?kJ?mol^?1 for the first hydrolysis to 39.2?kJ?mol^?1 in the last step. The activation energy for the condensation reaction from Si(OH)₄ was ca. 10?kJ?mol^?1 higher than the largest activation energy in the hydrolytic reactions. The decrease in the net positive charge on the Si atom contributes to the protonation of the ethoxy group and makes it a better leaving group.

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Synthesis of regioisomeric naphtho-furans <Emphasis Type="Italic">via</Emphasis> naphthyloxyalkanals

A new route to the regioisomeric 2-alkylnaphtho[2,1-b]- and 2-alkylnaphtho[1,2-b]furans via acid-catalyzed cyclization of the corresponding 2-naphthyloxyalkanals under mild conditions over Amberlyst 15 resin has been described. The 2-naphthyloxyalkanals were obtained by palladium-catalyzed reduction of 2-naphthyloxyalkanoyl chlorides.     

Metal‐Triflate‐Catalyzed Synthesis of Polycyclic Tertiary Alcohols by Cyclization of γ‐Allenic Ketones

It has been established that bismuth(III) triflate catalyzes the cyclization of γ‐allenic ketones under mild reaction conditions. This reaction allows the selective formation of polycyclic tertiary alcohols from cyclic ketone derivatives. The resulting dienols can engage in stereoselective cycloadditions to efficiently afford complex polycyclic systems.     

Synthesis of Y-type zeolites with high degree of Na<Superscript>+</Superscript> cation exchange for La<Superscript>3+</Superscript>

Effect of parameters of the ion exchange of Na⁺ cations for La³⁺ in Y zeolites with SiO₂/Al₂O₃ molar ratios of 5.1 and 6.0 on the degree of exchange and physicochemical properties of the resulting LaNaY samples was studied. The conditions in which deeply substituted La-forms of a Y zeolite with high degree of crystallinity can be obtained were determined.     

Extraction chromatography for the separation of <Superscript>239</Superscript>Np from <Superscript>243</Superscript>Am

Summary An extraction chromatography method was developed for the separation of 239Np from ²⁴³Am in nitric acid solution. A sorbent based on aliphatic quaternary amine Aliquat-336 and hydrophobized silica gel was prepared. ²³⁹Np reduced to the oxidation state(IV) with ferrous sulfamate in 2M or 6M HNO₃ sorbs on the prepared silica gel column. After washing with 0.1M ferrous sulfamate in 2.5M HNO₃, ²³⁹Np is eluted with 0.1M HNO₃ containing 0.02M HF. The separation of ²⁴³Am from ²³⁹Np is very effective. The purity of ²³⁹Np was found to be better than 99.5%. The proposed ²³⁹Np milking procedure is suitable for the preparation of ²³⁹Np tracer that can be used for the determination of ²³⁷Np radiochemical yield.     

Classical Dynamics of H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> in an Intense Laser Field by Using the Symplectic Method

The classical dynamics of 1D H₂⁺ in an intense field are discussed. The initial conditions are chosen at random in the field-free case, and then the Hamiltonian canonical equations of H₂⁺ system in the intense laser field are solved numerically by mean of the symplectic method under these initial conditions. The probabilities of survival, dissociation, ionization, and Coulomb explosion of H₂⁺ system in the intense laser field are obtained for different laser intensity based on the classical theory.     

Synthesis of substituted 3,4-dihydropyrimidin-2(1<Emphasis Type="Italic">H</Emphasis>)-ones and pyrimidin-2(1<Emphasis Type="Italic">H</Emphasis>)-ones by the Biginelli reaction with 3,5-Di-<Emphasis Type="Italic">tert</Emphasis>-butyl-4-hydroxybenzaldehyde

Three-component acid-catalyzed cyclocondensation of 3,5-di-tret-butyl-4-hydroxybenzaldehyde with urea and ethyl acetoacetate or α-nitroacetophenone (Biginelli reaction) under homogeneous conditions gave the corresponding 5-substituted 3,4-dihydropyrimidin-2(1H)-ones having in position 4 of the heteroring an aryl substituent with sterically shielded hydroxy group. The condensation catalyzed by inorganic salts (Fe³⁺, Co²⁺, Zn²⁺, Li⁺) was successful only with ethyl acetoacetate as initial methylene-active component. Under analogous conditions, acetophenone and 4-fluoroacetophenone gave rise to 4,6-diarylpyrimidin-2(1H)-ones which are capable of undergoing phenol-quinonemethide tautomerism.     

<Superscript>29</Superscript>Si NMR Spectroscopy Investigation of Alcoholic Phenyltrimethyl Ammonium Silicate Solutions

²⁹Si NMR spectroscopy is a powerful tool for studies of the silicate species existing in both aqueous and non-aqueous solutions. In this report ²⁹Si NMR spectroscopy was used to characterize species present in alkaline alcoholic silicate solutions. Phenyltrimethylammonium (PTMA) hydroxide was used as a base. The effects of polymerization/depolymerization of silicate anions in alcoholic alkaline solutions were investigated with different alcohols by ²⁹Si NMR spectroscopy. The esterification of monomeric silicate, Si(OH)₄, in the presence of different alcohols was also studied. Esterification depends on the alkyl chain as well as number of hydroxyl groups in the alcohol.     

Novel heterotricyclic systems: 2,6-dioxa- and 2-oxa-6-thia-10-azatricyclo-[5.2.1.04,10] deeanes; 2,6-dioxa-11-azatricyclo[5.3.1.04,11] undecane; and 9,13-dioxa-14-azatetracyclo[6.5.1.02,7.011,14]tetradeca-2,4,6-triene

The acid-catalyzed condensation of 2-amino-1,3-propanediols and 1,4-diketones under water-azeotroping conditions has led to the first synthesis of the new tent-like, essentially strain-free heterotricyclic system, 2,6-dioxa-10-azatricyclo[5.2.1.0^4,10]decane. Over thirty new compounds containing this system are reported. In cases where the aminodiol is unsubstituted or bears a phenyl substituent a mixture of the tricycle and the isomeric pyrrole resulted. These observations indicate alternative pathways in the Knorr-Paal condensation which lead to one, or the other, or both of these products. A scheme postulating common intermediates is presented. Examples of three additional, related, novel heteropolycyclic systems synthesized in analogous fashion are also reported.     

Synthesis,Structure, and Keto-Enol Tautomerism of 3-R<Superscript>1</Superscript>-5,5-R<Superscript>2</Superscript>,R<Superscript>2</Superscript>-6-R<Superscript>3</Superscript>-2,3,5,6-Tetrahydropyran-2,4-diones

Ethyl esters of 2,4-dibromo-2-R¹-4-R²-3-oxopentanoic and -hexanoic acids react with zinc and aliphatic or aromatic aldehydes under the conditions of the Reformatskii reaction to give 3-R¹-5,5-R², R²-6-R³-2,3,5,6-tetrahydropyran-2,4-diones, which are obtained in three forms: keto, enol with enolization of the keto group, and enol with enolization of the ester group. The keto form is isolated by crystallization from a mixture of CCl₄ and petroleum ether; the first enol form, from MeOH, EtOH, and polar aprotic solvents; and the second enol form, from CHCl₃. The second enol form is oxidized in DMSO to form a keto compound containing a hydroxy group at the 3-position of the heteroring.     

POCl3‐Mediated Reaction: A New Entry to Polysubstituted 2‐Pyridones via Self‐condensation of β‐Keto Amides

Synthesis of polysubstituted 2‐pyridones and their analogues from β‐keto amides via self‐condensation cyclization is described. Notable advantages include mild reaction conditions, reduced synthetic steps, high yields, and a readily available starting materials. Further mechanistic studies suggest that the transformation proceeds through self‐condensation, intramolecular nucleophilic cyclization, and elimination.     

Synthesis of new <Emphasis Type="Italic">N</Emphasis>-polysubstituted oxaazaisowurtzitanes by acid-catalyzed condensation of sulfonamides with glyoxal

By application of acid-catalyzed condensation of propane-2-, benzene- and methanesulfonamides with glyoxal a series of new derivatives of oxaazaisowurtzitanes were obtained, in particular, new heterocyclic systems: 2,4,6,8,12-pentaoxa-10-aza- and 2,4,8,12-tetraoxa-6,10-diazatetracyclo[5.5.0.0^3,11.0^5,9]-dodecanes.     

Solid-state <Superscript>29</Superscript>Si MAS NMR studies of diatoms: structural characterization of biosilica deposits

Four different diatom species (Chaetoceros debilis, Chaetoceros didymum, Cylindrotheca fusiformis, Nitzschia angularis) were studied by solid-state (29)Si MAS NMR spectroscopy. To determine the Q(2):Q(3):Q(4) ratios in the biosilica deposits of the diatoms, quantitative (29)Si MAS NMR experiments were performed. This analysis did not reveal any differences regarding the molecular architecture of the silica (i.e. the degree of condensation of the SiOH units (2 identical with SiOH --> identical with Si-O-Si identical with + H(2)O)) from the different diatom species. However, complete cells showed significantly smaller Q(4):Q(3) ratios (1.8-1.9) than extracted cell walls (2.5-2.8), indicating the existence of intracellular pools of less condensed silica.     

<Superscript>18</Superscript>F- and <Superscript>11</Superscript>C-labelling of quantum dots with n.c.a. [<Superscript>18</Superscript>F]fluoroethyltosylate and [<Superscript>11</Superscript>C]methyliodide: a feasibility study

Quantum dots functionalized on the outer surface with either amino- or carboxyl functions were labelled with [¹⁸F]fluoroethyltosylate and [¹¹C]methyliodide in order to use the positron emitter-labelled fluorescence agents for multimodality imaging techniques, i.e. fluorescence imaging and positron emission tomography. ¹⁸F-Labelling of both compounds was realized with yields up to 5% as determined by size exclusion chromatography, which is twice as much as reported in literature before [1]. ¹¹C-Labelling of amino- and carboxyl-QDs proceeded with good yields (up to 45 and 35%, respectively) under optimized reaction conditions. In general for both QD-types and both labelling agents the labelling yield increased with the amount of QDs used in the reaction as well as with reaction time and reaction temperature.     

A combined multicomponent-acid catalyzed cyclization reaction as an efficient route to novel tricyclic pyrrolo[2,1-a]isoquinoline derivatives

A simple, new two-step procedure for the synthesis of novel tricyclic pyrrolo[2,1-a]isoquinoline derivatives is described. The initially prepared polysubstituted pyrroles obtained via the four-component condensation of arylglyoxals, cyclic 1,3-dicarbonyls, aminoacetaldehyde dimethyl acetal, and β-keto esters subsequently underwent intramolecular acid-catalyzed cyclization to the desired products in moderate to good yields.     

An Isotope (<Superscript>18</Superscript>O, <Superscript>15</Superscript>N,and <Superscript>2</Superscript>H) Technique to Investigate the Metal Ion Interactions Between the Phosphoryl Group and Amino Acid Side Chains by Electrospray Ionization Mass Spectrometry

Cationic metal ion-coordinated N-diisopropyloxyphosphoryl dipeptides (DIPP-dipeptides) were analyzed by electrospray ionization multistage tandem mass spectrometry (ESI-MS ⁿ ). Two novel rearrangement reactions with hydroxyl oxygen or carbonyl oxygen migrations were observed in ESI-MS/MS of the metallic adducts of DIPP-dipeptides, but not for the corresponding protonated DIPP-dipeptides. The possible oxygen migration mechanisms were elucidated through a combination of MS/MS experiments, isotope (¹⁸O, ¹⁵N, and ²H) labeling, accurate mass measurements, and density functional theory (DFT) calculations at the B3LYP/6-31 G(d) level. It was found that lithium and sodium cations catalyze the carbonyl oxygen migration more efficiently than does potassium and participation through a cyclic phosphoryl intermediate. In addition, dipeptides having a C-terminal hydroxyl or aromatic amino acid residue show a more favorable rearrangement through carbonyl oxygen migration, which may be due to metal cation stabilization by the donation of lone pair of the hydroxyl oxygen or aromatic π-electrons of the C-terminal amino acid residue, respectively. It was further shown that the metal ions, namely lithium, sodium, and potassium cations, could play a novel directing role for the migration of hydroxyl or carbonyl oxygen in the gas phase. This discovery suggests that interactions between phosphorylated biomolecules and proteins might involve the assistance of metal ions to coordinate the phosphoryl oxygen and protein side chains to achieve molecular recognition.     

Acyl chloride‐facilitated condensation polymerization for the synthesis of heat‐sensitive poly(anhydride‐ester)s

We succeeded in developing the acyl chloride‐facilitated condensation polymerization method for the synthesis of new poly(anhydride‐ester)s with aromatic side groups, which cannot be polymerized by the classic melt condensation polymerization method. Using chlorinated and acylated carboxylic acids as the intermediates, the polymerization was carried out at low temperatures of 120 or 135 °C to yield pure poly(anhydride‐ester)s of molecular weights as high as 1.55 × 10⁵ with minimal side‐reactions. A homogeneous route of preparation was developed and optimized, using butyric anhydride as the acylating reagent and oxalyl chloride as the chlorinating reagent. A comparison of the mechanisms of the classic method and the new method indicates that the effects of transacylation—cyclization and oligomer formation—were greatly reduced due to the high reactivity of carboxylic acid chloride and the steric effect of bulky acyl groups. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5899–5915, 2007     

Base‐Promoted/Gold‐Catalyzed Intramolecular Highly Selective and Controllable Detosylative Cyclization

A highly selective, controllable and synthetically useful base‐promoted intramolecular detosylative cyclization of bis‐N‐tosylhydrazones has been achieved, affording N‐containing heterocycles and cyclic olefins under transition‐metal‐free or gold‐catalyzed procedures, respectively. Moreover, an effective and practical metal‐free or gold‐catalyzed approach to synthesize polycyclic aromatic compounds is also reported.     

Electroactive Polymerization Behaviors of Fused-Pentagon Chlorofullerenes: <Superscript>#1809</Superscript>C<Subscript>60</Subscript>Cl<Subscript>8</Subscript> and <Superscript>#271</Superscript>C<Subscript>50</Subscript>Cl<Subscript>10</Subscript>

The fullerenes that violate isolated pentagon rule (IPR) have unusual electronic properties resulting from their fused-pentagon structures. Numerous non-IPR fullerenes have now been captured by chlorination, affording opportunity to go insight into the properties involved in non-IPR fullerenes in the forms of chlorofullerenes (CFs). Here cyclic voltammetry (CV) is employed to probe the electrochemical properties of non-IPR ^#1809C₆₀Cl₈ in comparison with those of ^#271C₅₀Cl₁₀. Differing from IPR-satisfying CFs such as C₆₀Cl₈ and C₆₀Cl₁₀ (referring to I _h-symmetric C₆₀), the two non-IPR CFs exhibit divergent electroactive polymerization characters. In addition, the electrocatalytic effect of ferrocene that is otherwise employed as internal reference has been shown in the CV process of CFs.