Metal-containing initiator systems. V. Radical and cationic polymerizations with initiator systems of reduced nickel and chlorosilanes

The polymerizations of methyl methacrylate, styrene, and isobutyl vinyl ether with the binary systems of reduced nickel and chlorosilanes [(CH₃)_nSiCl_4?n, n = 0–3] have been investigated. It was found that these systems could act as both radical and cationic initiators, depending on the nature of vinyl monomers used. The kinetic investigations indicated that methyl methacrylate polymerized via a radical mechanism, and the initiating activity of chlorosilanes decreased in the following order: SiCl₄ > CH₃SiCl₃ > (CH₃)₂SiCl₂ > (CH₃)₃SiCl ? 0. Cationic initiations were observed in the polymerizations of styrene and isobutyl vinyl ether. In the latter case, the activity of chlorosilanes was in the following order: (CH₃)₃SiCl > (CH₃)₂SiCl₂ > CH₃SiCl₃ ? SiCl₄. From the results obtained, a possible mechanism of selective initiation with these systems is proposed and discussed.     

Reactions of organyltrifluoro- and diorganyldifluoro- silanes with 1,1,3,3-tetramethyldisiloxane

1,1,3,3-Tetramethyldisiloxane is decomposed by organyltrifluoro- and diorganyl- difluorosilanes already at 20C in the presence of catalysts with formation of hitherto unknown 1,1-dimethyl-3,3-diorganyl-3-fluoro- and 1,1-dimethyl-3-organyl-3,3-difluorodisiloxanes of general formula R_4–nSiF_n–1OSiH(CH₃)₂ (n=2–3) in yields of 50–70%. On storing these decomposition products easily disproportionate in various directions, in which their propensity to disproportionation is determined by the nature of the substituents at the silicon atoms and the number of fluorine atoms in the molecule.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1162–1166, May, 1991.     

Reaction of α,α,ω-trichloro- and α,α,α,ω-tetrachloroalkanes with alkali metal acylates in aprotic solvents

Polychloroalkanes and -alkenes R(CH₂)_nCl (R=CHCl₂CH₂, CCl₂=CH, n=1, 3) in dipolar aprotic solvents — dimethylformamide (DMF), dimethylsulfoxide (DMSO), hexamethylphosphoramide (HMPA) — at 130–150°C react selectively at the CH₂Cl group with salts of carboxylic acids RCO₂K(Na) to form dichloroesters RCO₂· (CH₂)_nR (R=CHCl₂CH₂, CCl₂=CH). In tetrachloroalkanes CCl₃CH₂(CH₂)_nCl (n = 1, 3, 5) under the same conditions the selectivity of the CCl₃ and CH₂Cl groups relative to the nucleophile RCO₂K(Na) is altered — unsaturated esters RCO₂(CH₂)_nCH=CCl₂ are formed in one stage with yields of 75–90%. Under the selected conditions, high conversion of polychloroalkanes to esters is attained 3 to 5 times more rapidly than in acid media. The structure of the ester obtained has been confirmed by their PMR spectra.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2790–2793, December, 1990.     

Co-hydrolysis products of tetraethoxysilane and methyltriethoxysilane in the presence of tetramethylammonium ions: Part 2 [1]

²⁹Si NMR peaks due to species with the double four-membered ring siloxane backbone composed of both Si(O^–)_4/2 and CH₃Si(O^–)_3/2 units, (CH₃) _n Si₈O _{20 – n} ^{/(8 – n) –} (n=1–3), formed by co-hydrolysis of tetraethoxysilane and methyltriethoxysilane in the presence of tetramethylammonium ions in methanol have been assigned. It has been found that ²⁹Si NMR peaks due to Si(OSi)₃(O^–) units shift to lower frequencies by replacement of the adjacent Si(O^–)_4/2 units by CH₃Si(O^–)_3/2 units, in other words, with increasing m value in Si[OSi(O^–)₃]_{3 – m} [OSi(CH₃) (O^–)₂] _m (O^–) (m=0–2). Peaks from CH₃ Si(OSi)₃ units in the species have also appeared as separated due to the kind of neighbor structural units. On the basis of the assignments, positions of CH₃Si(O^–)_3/2 units in the cubic octameric siloxane framework of (CH₃) _n Si₈O _{20 – n} ^{/(8 – n) –} (n=2, 3), for both of which three isomers are present, have been estimated.     

Bildung siliciumorganischer Verbindungen. LXII. Teilbromierte Carbosilane

Formation of Organosilicon Compounds. LXII. Partial Brominated Carbosilanes The photobromination of 1 leads to compound 2 as well as to C-chlorinated derivatives if the time of reaction is prolonged. Compound 2 is also formed from (Br₂Si–CH₂)₃; Gl. (1) see ?Inhaltsübersicht”?. In a corresponding reaction (Cl₃Si–CH₂)₂SiCl₂ gives successively Cl₃Si–CHBr–SiCl₂–CH₂–SiCl₃, Cl₃Si–CBr₂–SiCl₂–CH₂–SiCl₃ and Cl₃Si–CCl₂–SiCl₂–CH₂–SiCl₃. (Cl₃Si)₂CBr₂ is accessible through the photobromination of (Cl₃Si)₂CH₂. The reactivity of the CBr₂-group is quite obvious in the reaction of Cl₂Si–CBr₂–SiCl₂–CH₂–SiCl₃ with LiAlH₄ yielding (H₃Si–CH₂)₂SiCl₂ as well as in the reaction of compound 2 with CH₃MgCl yielding [(CH₃)₂Si–CH₂]₃. By treatment of the SiH groups with bromine the preparation of compounds with the general formulas CH₃SiH_nBr_3?n; (H_3?nSiBr_n)₂CH₂; (H_3?nSiBr_n? CH₂)₂SiH_2?nBr_n; (H_2?nBr_nSi? CH₂)₃ and (H_3?nSiBr_n)₂CCl₂ is possible. Analysis of the nmr spectra shows that 1,3-Dibromo-1,3,5-trisilacyclohexane is formed to 67% in the trans and to 33% in the cis configuration; 1,3,5-Tribromo-1,3,5-trisilacyclohexane is formed to 80–90% in teh cis-trans configuration. The results of ¹H and ²⁹Si NMR investigations are reported.     

Structural studies on molecular complexes of polyethers. 4 Deoxycholic acid-polyether complexes

Complexes of deoxycholic acid with polyethers, [–(CH₂) _m–1CHR–O–] _n (R=H,m=2–4, 6 andR=CH₃,m=2), were prepared. The crystals are orthorhombic and similar to those most commonly observed for complexes with low molecular weight molecules. Dimensiona in the poly(propylene oxide) complex is about 1 Å longer than those in other polyether complexes, whileb andc do not differ much in these DCA-polyether complexes.     

Acetoxy derivatives of trans-1,2-disilylethylene and their hydrolysis

Conclusions A method was developed for the synthesis of acetoxy-trans-1,2-disilylethylenes of formula (CH₃COO)_n(CH₃)_2–nRSiCH=CHSi(CH₃)R(OCOCH₃), where R and R=alkyl, and n=0 or 1. The corresponding silanols were obtained by the hydrolysis of the latter.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1395–1397, June, 1976.     

Syntheses and pharmacological characterization of achiral and chiral enantiopure C/Si/Ge-analogous derivatives of the muscarinic antagonist cycrimine: a study on C/Si/Ge bioisosterism

The C/Si/Ge-analogous compounds rac-Ph(c-C₅H₉)El(CH₂OH)CH₂CH₂NR₂ (NR₂=piperidino; El=C, rac-3a; El=Si, rac-3b; El=Ge, rac-3c) and (c-C₅H₉)₂El(CH₂OH)CH₂CH₂NR₂ (NR₂=piperidino; El=C, 5a; El=Si, 5b; El=Ge, 5c) were prepared in multi-step syntheses. The (R)- and (S)-enantiomers of 3a–c were obtained by resolution of the respective racemates using the antipodes of O,O′-dibenzoyltartaric acid (resolution of rac-3a), O,O′-di-p-toluoyltartaric acid (resolution of rac-3b), or 1,1′-binaphthyl-2,2′-diyl hydrogen phosphate (resolution of rac-3c). The enantiomeric purities of (R)-3a–c and (S)-3a–c were ≥98% ee (determined by ¹H-NMR spectroscopy using a chiral solvating agent). Reaction of rac-3a–c, (R)-3a–c, (S)-3a–c, and 5a–c with methyl iodide gave the corresponding methylammonium iodides rac-4a–c, (R)-4a–c, (S)-4a–c, and 6a–c (3a–c→4a–c; 5a–c→6a–c). The absolute configuration of (S)-3a was determined by a single-crystal X-ray diffraction analysis of its (R,R)-O,O′-dibenzoyltartrate. The absolute configurations of the silicon analog (R)-4b and germanium analog (R)-4c were also determined by single-crystal X-ray diffraction. The chiroptical properties of the (R)- and (S)-enantiomers of 3a–c, 3a–c·HCl, and 4a–c were studied by ORD measurements. In addition, the C/Si/Ge analogs (R)-3a–c, (S)-3a–c, (R)-4a–c, (S)-4a–c, 5a–c, and 6a–c were studied for their affinities at recombinant human muscarinic M₁, M₂, M₃, M₄, and M₅ receptors stably expressed in CHO-K1 cells (radioligand binding experiments with [³H]N-methylscopolamine as the radioligand). For reasons of comparison, the known C/Si/Ge analogs Ph₂El(CH₂OH)CH₂CH₂NR₂ (NR₂=piperidino; El=C, 7a; El=Si, 7b; El=Ge, 7c) and the corresponding methylammonium iodides 8a–c were included in these studies. According to these experiments, all the C/Si/Ge analogs behaved as simple competitive antagonists at M₁–M₅ receptors. The receptor subtype affinities of the individual carbon, silicon, and germanium analogs 3a–8a, 3b–8b, and 3c–8c were similar, indicating a strongly pronounced C/Si/Ge bioisosterism. The (R)-enantiomers (eutomers) of 3a–c and 4a–c exhibited higher affinities (up to 22.4 fold) for M₁–M₅ receptors than their corresponding (S)-antipodes (distomers), the stereoselectivity ratios being higher at M₁, M₃, M₄, and M₅ than at M₂ receptors, and higher for the methylammonium compounds (4a–c) than for the amines (3a–c). With a few exceptions, compounds 5a–c, 6a–c, 7a–c, and 8a–c displayed lower affinities for M₁–M₅ receptors than the related (R)-enantiomers of 3a–c and 4a–c. The stereoselective interaction of the enantiomers of 3a–c and 4a–c with M₁–M₅ receptors is best explained in terms of opposite binding of the phenyl and cyclopentyl ring of the (R)- and (S)-enantiomers. The highest receptor subtype selectivity was observed for the germanium compound (R)-4c at M₁/M₂ receptors (12.9-fold).     

Lithiated γ-O-functionalized propyl phenyl sulfides and sulfones of the type Li[CH(SOxPh)CH2CH2OR] (x = 0, 2). [Li{CH(SPh)CH2CH2Ot-Bu}(tmeda)] – A structurally characterized organolithium inner complex

Lithiation of O-functionalized alkyl phenyl sulfides PhSCH₂CH₂CH₂OR (R = Me, 1a; i-Pr, 1b; t-Bu, 1c; CPh₃, 1d) with n-BuLi/tmeda in n-pentane resulted in the formation of α- and ortho-lithiated compounds [Li{CH(SPh)CH₂CH₂OR}(tmeda)] (α-2a–d) and [Li{o-C₆H₄SCH₂CH₂CH₂OR)(tmeda)] (o-2a–d), respectively, which has been proved by subsequent reaction with n-Bu₃SnCl yielding the requisite stannylated γ-OR-functionalized propyl phenyl sulfides n-Bu₃SnCH(SPh)CH₂CH₂OR (α-3a–d) and n-Bu₃Sn(o-C₆H₄SCH₂CH₂CH₂OR) (o-3a–d). The α/ortho ratios were found to be dependent on the sterical demand of the substituent R. Stannylated alkyl phenyl sulfides α-3a–c were found to react with n-BuLi/tmeda and n-BuLi yielding the pure α-lithiated compounds α-2a–c and [Li{CH(SPh)CH₂CH₂OR}] (α-4a–b), respectively, as white to yellowish powders. Single-crystal X-ray diffraction analysis of [Li{CH(SPh)CH₂CH₂Ot-Bu}(tmeda)] (α-2c) exhibited a distorted tetrahedral coordination of lithium having a chelating tmeda ligand and a C,O coordinated organyl ligand. Thus, α-2c is a typical organolithium inner complex.Lithiation of O-functionalized alkyl phenyl sulfones PhSO₂CH₂CH₂CH₂OR (R = Me, 5a; i-Pr, 5b; CPh₃, 5c) with n-BuLi resulted in the exclusive formation of the α-lithiated products Li[CH(SO₂Ph)CH₂CH₂OR] (6a–c) that were found to react with n-Bu₃SnCl yielding the requisite α-stannylated compounds n-Bu₃SnCH(SO₂Ph)CH₂CH₂OR (7a–c). The identities of all lithium and tin compounds have been unambiguously proved by NMR spectroscopy (¹H, ¹³C, ¹¹⁹Sn).     

Reaktionen mit phosphororganischen Verbindungen, 17. Mitt.: Zur Oxydation von β-Oxophosphoniumrhodaniden mit Bleitetraacetat

Zusammenfassung Die Oxydation von Phosphoniumrhodaniden (C₆H₅)₃P–CH (Alkyl) CO–CHR₁R₂ SCN mitPbTA liefert in Abhängigkeit von den Resten R₁ und R₂ Verbindungen des Typs R₁R₂C=C=C (SCN) (Alkyl) und die daraus durch 1,3-Umlagerung resultierenden Senföle R₁R₂C(NCS)–CC-Alkyl. Für R₁=R₂=H und R₁=Cl, R₂=C₂H₅ entsteht fast nur die Allenverbindung. Für R₁=H und R₂=Alkyl isoliert man ein Gemisch aus Allenrhodanid und Acetylenverbindung. Wenn R₁=R₂=(–CH₂–CH₂–CH₂–) oder CH₃ darstellen, kommt es zur ausschließlichen Bildung des Acetylensenföls.Aus -Methoxyphosphoniumrhodaniden (C₆H₅)₃P–CH (OCH₃)CH–CHR₁R₂ SCN erhält man bei der Umsetzung mitPbTA R₁R₂CH–CO–CH(OCH₃) (NCS) und R₁R₂CH–CO–CH(OCH₃) (S–CO–CH₃).

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The oxidation of phosphoniumrhodanides (C₆H₅)₃P–CH (Alkyl)CO–CHR₁R₂ SCN with lead tetraacetate results in the formation of R₁R₂C=C=C=(SCN) (alkyl) type compounds, dependent on the substituents R₁ and R₂ sometimes followed by a 1,3-rearrangement to mustard oils R₁R₂C(NCS)–CC–alkyl. The case R₁=R₂=H and R₁=Cl, R₂=C₂H₅ yields almost exclusively the allenic compound. With R₁=H and R₂-alkyla mixture of allene rhodanide and the acetylenic compound is isolated. If R₁ and R₂ stand for –CH₂–CH₂CH₂– or CH₃, only the mustard oil with an acetylenic group is produced.Upon the oxidation of (C₆H₅)₃P–CH(OCH₃)CO–CHR₁R₂ SCN^– we observed formation of R₁R₂CH–CO–CH(OCH₃) (NCS) and R₁R₂CH–CO–CH(OCH₃) (S–CO–CH₃).

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Mit 5 Abbildungen

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Herrn Prof. Dr.L. Schmid zum 70. Geburtstag gewidmet.

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16. Mitt.:Elisabeth Werner undE. Zbiral, Angew. Chem.79, 899 (1967).     

2-(Trialkoxysilylalkylthio)ethyl vinyl sulfides

Conclusions Trialkoxysilylalkanethiols (CH₃O)₃Si(CH₂)_nSH (n=1–3) react with divinyl sulfide at 100–110° to give 2-(trialkoxysilylalkylthio)ethyl vinyl sulfides (CH₃O)₃Si(CH₂)_nS (CH₂)₂SCH=CH₂ in high yield. The reactivity of the trialkoxysilylalkanethiols decreases with increase in the number of CH₂ groups between the S and Si atoms. A second molecule of the organosilicon thiol acids adds with difficulty to divinyl sulfide to give the diadduct.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 197–199, January, 1977.     

Tin(IV) and organotin(IV) derivatives of novel β-diketones: Part IV. Triorganotin(IV) complexes of fluorinated 4-acyl-5-pyrazolones. Crystal structure of (1-(4-trifluoromethylphenyl)-3-methyl-4-acetylpyrazolon-5-ato)triphenyltin(IV)

The synthesis of three 1-(4-trifluoromethylphenyl)-3-methyl-4-R¹(C=O)-5-pyrazolone proligands LH (L¹H; R¹=C₆H₅: L²H; R¹=CH₃: L³H; R¹=CF₃) and their interaction with R₃Sn(IV) acceptors (R=Me, Buⁿ, Ph) are reported. When R=Me or Buⁿ, aquo (4-acylpyrazolonate)SnR₃(H₂O) derivatives are obtained and the anionic donors 4-acylpyrazolonate (L⁻) act in the O–monodentate form. These triorganotin complexes are not stable in chlorohydrocarbon solvents and decompose to R₄Sn and bis(4-acyl-5-pyrazolonate)₂SnR₂. When R=Ph, stable (4-acyl-5-pyrazolonate)SnPh₃ derivatives, both in solution and in the solid state, are obtained. The crystal structure of (1-(4-trifluoromethylphenyl)-3-methyl-4-acetylpyrazolon-5-ato)triphenyltin(IV) shows a five-coordinate tin atom in a strongly distorted cis-bipyramidal trigonal environment (axial angle=161.2(2)°) with the acylpyrazolonate donor acting as an asymmetric O₂–bidentate species (Sn–O(1)=2.081(6) Å: Sn–O(2)=2.424(5) Å). Electronic effects are responsible for the different behavior shown by these trialkyl and triphenyl derivatives.     

Synthesis of α-(perfluoroalkyl)-ω-(trialkylsilyl)alkanols

The synthesis of alkanols of the general formula R₃Si(CH₂)_nCH(OH)R_F (n = 1–3) is elaborated and their behavior under GC-MS is studied.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1516-1519, June, 1996.     

Alkyliminophosphorsäuretrihalogenide, 1. Mitt.: Halogenaustausch zwischen (CH3NPCl3)2 und (CH3NPF3)2

Zusammenfassung Durch Reaktion von dimerem Methyliminophosphorsäuretrichlorid mit dimerem Methyliminophosphorsäuretrifluorid werden die gemischt halogenierten dimeren Methyliminophosphorsäuretrihalogenide der allgemeinen Zusammensetzung (CH₃NP)₂Cl _n F_6–n mitn=1–5 dargestellt. Die analytischen Daten und die IR-Spektren werden mitgeteilt.

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Alkylimino phosphoric acid trihalogenides, I.: Halogen exchange between (CH₃NPCl₃)₂ and (CH₃NPF₃)₂

The reaction of (CH₃NPCl₃)₂ and (CH₃NPF₃)₂ yields the unknown 1.3-diaza-2.4-fluorochloro-phosphetidines of the general formula (CH₃NP)₂Cl _n F_6–n withn=1–5. The synthesis, the analytical data and the IR-spectra are reported.

     

Reaktion von Alkyl- und Arylchlorsilanen mit Phosph(III) en-imidamiden

The reaction of RSiCl₃ (R=CH₃, C₂H₅, C₆H₅) and R₂SiCl₂ (R=CH₃) with one mole of the phosphenimidous amides R₂N–P=NR [R=R=Si(CH₃)₃; R=Si(CH₃)₃, R=C(CH₃)₃] yieds a four membered PN₂Si-ring system under elimination of (CH₃)₃SiCl.     

Reactivity of certain hydrogen donors and olefins with respect to α,α-dichloroalkyl radicals

The rate constants of hydrogen splitting from hydrogen donors (DH) by the ClCH₂(CH₂)₃CCl₂ radicals (R¹) were determined by the method of competitive kinetics in the temperature range of 413–433 K. As a competing reaction with splitting of hydrogen from the DH, the isomerization of radicals R¹ into ClCH(CH₂)₃CCl₂H radicals with 1,5-migration of hydrogen was chosen, for which log k (sec^–1)=9.343 –10.56/2,303RT (kcal/mole) was calculated. It was found that the relative reactivities are similar of R¹ and (CH₃)₃CO radicals (R⁴) in splitting hydrogen from HD. This makes it possible to predict the values of analogous constants for radicals R¹ from known rate constants of splitting hydrogen from various DH by radicals R⁴. The rate constants of the addition of Cl(CH₂)₂Cl₂ radicals (R²) to amethylstyrene and methyl methacrylate were determined. Taking into account the data in [7], where the same constants were determined for styrene, methyl acrylate, acrylonitrile and vinyl methyl ketone, the existence of a linear dependence was shown of the logarithm of the rate constant of the addition of radicals R² to olefins on the polar properties of the above-enumerated olefins.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1811–1816, August, 1990.The authors wish to express their gratitude to T. T. Vasil'eva for providing the reference samples of 1,5,5-trichloropentane and 1,1,5,5-tetrachloropentane.     

Complexes of apatitic layered compound Ca8(HPO4)2(PO4)4·5H2O with dicarboxylates

Octacalcium phosphate(OCP), Ca₈(HPO₄)₂(PO₄)₄·5H₂O, consists of alternative stackings of layers with an apatitic structure and a brushite-like composition. Here we consider whether or not OCP is able to complex with organic substances. The interplanar spacing (d₁₀₀) of OCP prepared in the presence of dicarboxylates (RC₂O₄ ^2–; R=organic group) expanded from the original value of 18.7 Å to 19.2–26.1 Å depending on the length of R. Examples of R were C_nH_2n(n=1–6), CH(CH₃)CH₂, C(CH₃)=CH, CH=CH, CH₂CH=CHCH₂ and C₆H₄. Structural considerations and experimental data suggested that dicarboxylates were incorporated into the OCP structure through the replacement of HPO₄ ^2– by RC₂O₄ ^2–.     

Reaktion von Alkyl- und Arylchlorsilanen mit Phosph(III) en-imidamiden

The reaction of RSiCl₃ (R=CH₃, C₂H₅, C₆H₅) and R₂SiCl₂ (R=CH₃) with one mole of the phosphenimidous amides R₂N–P=NR [R=R=Si(CH₃)₃; R=Si(CH₃)₃, R=C(CH₃)₃] yieds a four membered PN₂Si-ring system under elimination of (CH₃)₃SiCl.     

Nitrogen-containing organosilicon compounds

Fifteen previously unknown piperidinosilanes of the type R_4–m Si[(CH₂) _n NC₅H₁₀] _m (m=1–3; n=0–3). have been synthesized by the reaction of piperidine with diethylaminotrimethylsilane, bis(diethylamino)dimethylsilane, chlorobis(dimethylamino)methylsilane, trichloro(methyl)silane, trialkyl(chloromethyl)silanes, dialkoxy(alkyl)(chloromethyl)silanes, trialkylvinylsilanes, trialkyl(3-chloropropyl)silanes, and 3-chloropropyl(diethoxy)methylsilane. The piperidinosilanes (n>0) have been converted into the corresponding hydrochlorides and methiodides.For part VIII, see [1].     

Chromato-mass spectral study of the reactions of organylsiloxanes with metal oxides

A study was carried out on the reaction of 1,1,3,3-tetramethyldisiloxane with metal oxides such as CuO, HgO, and Sb₂O₅ in the presence of compounds containing a siloxane bond such as hexamethyldisiloxane, octamethylcyclotetrasiloxane, and trimethylacetoxysilane, which, at 100–110°C over 0.5–10 h, leads to the formation of linear organyl siloxanes (CH₃-[-Si(CH₃)₂O-]_n-Si(CH₃)₃, CH₃-[-Si(CH₃)₂O-]_n-Si (CH₃)₂H, and H-[-Si(CH₃)₂O-]_n-Si(CH₃)₂H, where n=2–6, and cyclic organylsiloxanes. The reaction does not proceed in the presence of aluminum, bismuth, germanium, and lead oxides.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1658–1660, July, 1990.