Silanones and silanethiones from the reactions of transient silylenes with oxiranes and thiiranes in solution. The direct detection of diphenylsilanethione

The transient silylenes SiMe(2) and SiPh(2) react with cyclohexene oxide (CHO), propylene oxide (PrO), and propylene sulfide (PrS) in hydrocarbon solvents to form products consistent with the formation of the corresponding transient silanones and silanethiones, respectively. Laser flash photolysis studies show that these reactions proceed via multistep sequences involving the intermediacy of the corresponding silylene-oxirane or -thiirane complexes, which are formed with rate constants close to the diffusion limit in all cases and exhibit UV absorption spectra similar to those of the corresponding complexes with the nonreactive O- and S-donors, tetrahydrofuran and tetrahydrothiophene. The SiMe(2)-PrO and SiPh(2)-PrO complexes both exhibit lifetimes of ca. 300 ns, and are longer-lived than the corresponding complexes with CHO, which are both in the range of 230-240 ns. On the other hand, the silylene-PrS complexes are considerably shorter-lived and vary with silyl substituent; the SiMe(2)-PrS complex decays with the excitation laser pulse (i.e., τ ≤ 25 ns), while the SiPh(2)-PrS complex exhibits τ = 48 ± 3 ns. The decay of the SiPh(2)-PrS complex affords a long-lived transient product exhibiting λ(max) ≈ 275 nm, which has been assigned to diphenylsilanethione (Ph(2)Si═S) on the basis of its second order decay kinetics and absolute rate constants for reaction with methanol, tert-butanol, acetic acid, and n-butyl amine, for which values in the range of 1.4 × 10(8) to 3.2 × 10(9) M(-1) s(-1) are reported. The experimental rate constants for decay of the SiMe(2)-epoxide and -PrS complexes indicate free energy barriers (ΔG(?)) of ca. 8.5 and ≤7.1 kcal mol(-1) for the rate-determining steps leading to dimethylsilanone and -silanethione, respectively, which are compared to the results of DFT (B3LYP/6-311+G(d,p)) calculations of the reactions of SiH(2) and SiMe(2) with oxirane and thiirane. The calculations predict a stepwise C-O cleavage mechanism involving singlet biradical intermediates for the silylene-oxirane complexes, and a concerted mechanism for silanethione formation from the silylene-thiirane complexes, in agreement with earlier ab initio studies of the SiH(2)-oxirane and -thiirane systems.     

Theoretical study of halophilic reactions of stable silylenes with chloro- and bromocarbons

A theoretical study of the mechanism of the reaction of stable silylenes with halocarbons has been carried out using the B3LYP density functional method. The main findings are as follows: (1) Lewis acid-base complexes formed between silylenes and halocarbons do not play a role in silylene insertion chemistry into halocarbons; therefore, the acid-base complex mechanism proposed by West et al. (J. Am. Chem. Soc. 2002, 124, 4186) is not appropriate to describe the disilane formation reaction. (2) The disilane formation reactions follow the energetically favorable general reaction pathway (X = halogen): (i) Y2Si: + HCX3 --> TS1 --> Y2XSi-CHX2. (ii) Y2Si: + Y2XSi-CHX2 --> TS2 --> Y2XSi-SiY2CHX2. (3) The observed preference of stable silylenes to undergo C-X bond insertion rather than C-H bond has been investigated. The theoretical findings suggest that this preference is a result of the thermodynamic factor. (4) Stable silylenes prefer to insert into a C-Br rather than a C-Cl bond because the energy barrier to insertion is lower, and the reaction is more exothermic.     

Cycloaddition reactions of N-heterocyclic stable silylenes with ethylene and formaldehyde

The cycloaddition reactions of N-heterocyclic silylenes 1-4 with ethylene (C₂H₄) and formaldehyde (CH₂O) molecules were performed at the MP2/6-31G** level of theory, respectively. Full optimizations and frequency analyses were done for the stationary points on the potential energy surface. The intrinsic reaction coordinate (IRC) was also calculated for all the transition states at the same level of theory. The possible cycloaddition mechanisms were investigated and results from various reactions were compared in detail. The theoretical results indicated that the cycloaddition reactions of N-heterocyclic silylenes with C₂H₄ and CH₂O proceeded through a concerted mechanism to form a three-membered ring containing C1, C2 (O), and Si atoms in the products, which was similar to those of simple silylene H₂Si. Silylenes 1-4 exhibit some electrophilicity toward C₂H₄ whereas nucleophilicity toward CH₂O leading to the reaction process. Based on the reaction energy barrier and the exothermic energy, the reaction activities of saturated silylene (2) were stronger than those of unsaturated silylenes (1, 3, and 4).     

Mass-spectrometric detection of an unstable product in silver-methanol interaction

The low pressure interaction of methanol vapors with a silver catalyst was studied by mass spectrometry. An unstable product contributing to m/e-15 was detected in this reaction. While yield of formaldehyde increased with addition of oxygen, the unstable product showed towards oxygen a reversed, decreasing trend.

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- , m/e=15. , .

     

Primary reactions in the pyrolysis of allyltrimethylsilane

The pyrolytic conversion of allyltrimethylsilane to vinyltrimethylsilane is rate determined by a bimolecular reaction; the main initial unimolecular step is siliconallyl bond rupture.     

Synthesis and structures of heteroleptic silylenes

The reaction of benzamidinato silicon trichloride [{PhC(NR)2}SiCl3] [R = Bu(t) (1), SiMe3 (2)] with 2 equiv of potassium in THF afforded mononuclear chlorosilylene [{PhC(NBu(t))2}SiCl] (3) and [{PhC(NSiMe3)2}2SiCl2] (4), respectively. Compound 4 was formed by the disproportionation of unstable [{PhC(NSiMe3)2}SiCl]. The reaction of [{PhC(NBu(t))2}SiCl3] (1) with 1 equiv of LiR (R = NMe2, OBu(t), OPr(i), PPr(i)2) in THF yielded [{PhC(NBu(t))2}SiCl2R] [R = NMe2 (5), OBu(t) (6), OPr(i) (7), PPr(i)2 (8)]. Treatment of 5-8 with 2 equiv of potassium in THF resulted in the novel heteroleptic silylene [{PhC(NBu(t))2}SiR] [R = NMe2 (9), OBu(t) (10), OPr(i) (11), PPr(i)2 (12)]. Compounds 4, 9, and 12 have been analyzed by X-ray crystallography.     

Mineral reactions and pyrolysis of Israeli oil shale

The mineral reactions of Israeli oil shale fed to the GENESIS ($\text{G}$eneration of $\text{Ene}$rgy from $\text{S}$hale of $\text{Is}$rael) split-stage fluidized-bed reactor have been studied by infrared. X-ray and chemical means. Organically bonded sulphur released during pyrolysis reacts with oxygen from the fluidizing air and with CaO resulting from decomposition of calcite to form CaSO₄, mostly in a surface layer. At pyrolysis and coke-oxidation temperatures near 600°C, kaolinite reacts with calcite to form amorphous modified metakaolinite. At organic gas combustion temperatures near 1000°C, the amorphous phase in overhead fines reacts with additional calcite and quartz to yield gehlenite and larnite.     

Mass-spectrometric multielement analysis of copper and copper-base alloys, with electrical detection

Relative sensitivity coefficients have been determined for 21 elements in copper and its alloys, with iron as an internal standard, by spark-source mass-spectrometry with electrical detection and magnetic peak-switching. Twenty calibration standards ranging from pure copper to 60%-copper alloys were used. The sensitivity coefficients measured appear to be independent of the elemental concentration and are obtained with a mean precision of 15%.     

Hard-soft acid-base interactions of silylenes and germylenes

A detailed investigation of the electrophilic and nucleophilic character of singlet silylenes and germylenes, divalent compounds of silicon and germanium, respectively, substituted by first- and second-row elements is presented. In a first part, the Lewis acid properties of these compounds were studied through their complexation reaction with the Lewis bases NH3, PH3, and AsH3. The results indicate that this complexation is most favorable with the hardest base NH3, classifying these compounds as hard Lewis acids. This is confirmed by the linear correlation between the interaction energies and the value of the electrostatic potential, used as an approximation to the local hardness, near the empty p orbital of these compounds, indicating a charge-controlled interaction in the complex. Also the electrophilicity index, proposed by Parr et al., computed both at the global and the local level, correlates linearly with the complexation energies of the compounds with NH3. The Lewis base character of these silylenes has been investigated, through their interaction with the acids BH3 and AlH3. Also in this case, the electrostatic potential can be used to probe the reactivity of the compounds. It will finally be demonstrated that an increasing stability of the silylenes and germylenes is accompanied by an increase in their nucleophilicity and a decrease of the electrophilicity.     

Quantum-chemical calculations of the mechanism of some reactions of the insertion of silylenes and dichlorosilylenes into a single bond

Carbenes and silylenes can participate in some insertion reactions both in singlet and in triplet electronic states. The reactivity of silylenes depends on the nature of their substituents. AM1 and PM3 semiempirical calculations were performed for the reactions of silylene and dichlorosilylene insertion into the H-Cl bond of the hydrogen chloride molecule and the C-O bond of the furane molecule. The data obtained was used to propose probable mechanisms of these reactions.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 810–813, May, 1994.     

Recent developments in the chemistry of stable silylenes

This paper reviews progress in the field of thermally stable silylenes since 2000. Developments in the synthesis and characterization of new stable silylenes are discussed, followed by a summary of recent theoretical calculations upon model silylenes. The main part of the article concerns the reactivity patterns displayed by silylenes 1-4, which are discussed in terms of their behavior toward radical, organic, inorganic, and organometallic species.     

Mechanistic modeling of the wall reactions in the pyrolysis of pentachloroethane

The thermal dehydrochlorination C₂HCl₅ → C₂Cl₄ + HCl has been studied in a static system between 565 and 645 K at pressures ranging from 5 to 21 torr. The course of the reaction was followed by measuring the pressure rise in the conditioned quartz reaction vessel and by analyzing the products by gas chromatography. The observed experimental results and data from the literature for flow systems can be explained quantitatively in terms of a radical reaction model involving heterogeneous chain initiation and termination steps. The rate constants have been deduced for reactions of Cl, Cl₂, and C₂HCl₅ over reactor walls covered with a pyrolytic carbon film and for reactions of adsorbed Cl atoms. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 322–330, 2002     

Diastereoselective addition of alcohol to diastereotopic silylenes

The successful example of diastereoselective addition of alcohol to diastereotopic silylenes is reported.     

The middle-value iteration algorithm and its application in investigating hydrocarbon pyrolysis reactions

A new algortihm, the middle-value iteration algorithm (MVIA), is proposed. The precision obtained is similar to that of the Geaer algorithm although the algorithm is simpler. It can be used for investigating pyrolysis reaction systems and analogus systems involving radical reactions.     

Quantum-chemical calculations of the electronic structure and reactivity of silylenes

The origin of the different reactivities of carbenes and silylenes has been discussed. Molecular orbital calculations have been carried out in order to estimate the effects of substituents on the singlet-triplet splittings E _ST in these molecules. The data ofab initio calculations have been used to establish the linear correlation between E _ST values for carbenes and silylenes. Both qualitative and quantitative appraisals of the applicability of the semiempirical MNDO method to calculations of E _ST values for carbenes and silylenes have been derived. E _ST values for a large number of substituted carbenes and silylenes have been computed by the PM3 method, one of the latest versions of the MNDO approach (Ref. 14).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1006–1008, June, 1994.