Three possible conformers (ct-DMS, tt-DMS, and gg-DMS) of labile dimethoxysilylene (DMS) were obtained in Ar, Kr, Xe, and N-2 matrices and characterized by their IR spectra. The most stable ct-DMS conformer was predominantly stabilized in the matrices after pyrolytic generation of DMS. Irradiation of matrices with lambda = 230 - 1000 nm led to the establishment of a photochemical equilibrium between the conformers, which was strongly shifted to the less stable ones, but changed noticeably after subsequent irradiation with lambda = 280 - 1000 nm. The equilibrium was also affected by the nature of the matrix gas: the fraction of the gg-DMS conformer decreased in the series Ar > Kr similar to N-2 >> Xe, which was attributed to the ability of the host gases to act as relaxants. The tt-DMS conformer existed in the matrices in two forms: the unrelaxed form produced by UV irradiation of ct-DMS and the relaxed form produced upon deposition and after annealing of the matrices containing the unrelaxed form. Annealing of the matrices led to interconversion of the DMS conformers: the rearrangement of gg-DMS into ct-DMS occurred already at 36 K in Ar matrices, whereas the rearrangement of tt-DMS into ct-DMS occurred only at 60 K in Xe matrices. This made it possible to estimate the Gibbs energies of activation of these rotamerization processes as ca. 3 and 4 kcal mol(-1), which agrees with the results of quantum chemical calculations performed. Prolonged photolysis of DMS resulted in the formation of already known isomeric labile methoxymethylsilanone.

A key goal of organic chemistry is to develop new principles for the control of reactions, which can be used to create promising materials demanded in all fields of scientific research and industry. This review is an overview of the scientific advances, which have been made by the N. D. Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences in the past decade within the framework of current trends in organic chemistry. The review covers the results, which are significant for fundamental research and hold great promise for the application in different areas, from the production of materials, petrochemistry, and chemical ecology to medicine, agriculture, and food industry.

A matrix-isolation FTIR study of the reaction between SiCl2 and acetylene showed that at low temperatures, it stops at the stage of complexation between the reactants. This made it possible to examine a photochemical version of this reaction, which led to the simultaneous formation of 1,1-dichloro-1-silacycloprop-2-ene and 3,3-dichloro-3-silaprop-1-yne in comparable yields in contrast to the thermal reaction, which, according to quantum chemical calculations carried out here, results in the formation of 1,1-dichloro-1-silacycloprop-2-ene as the only primary product.

A complex between SiCl2 and CO of the 1:1 composition with coordination of the silylene to the C atom of carbon monoxide is detected in Ar matrices using FTIR spectroscopy. A positive shift of the ν(CO) band of the complex relative to the corresponding band of free CO and a theoretical analysis of the nature of the complex indicate that it is a nonclassical carbonyl complex. It is the first experimentally detected nonclassical carbonyl complex of any silylenes. The main direction of photoinduced transformations of the complex is its decomposition into the starting reactants. The potential energy surface of the SiCl2 + CO system was explored theoretically. It is shown that further thermal transformations of the primary complex are energetically unfavorable.

The review summarizes some of the most prominent results obtained in the laboratory headed by Academician Oleg M. Nefedov at the N. D. Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences in the field of chemistry of carbenes, their heavy analogs, and related intermediates, as well as small-sized cycles. Those include elaboration of safe methodology of cyclopropanation using diazomethane, development and extension of synthetic applications of diazoesters and other diazo compounds in the preparation of valuable chemical products, design of functionalized alkynylcyclopropanes on the basis of alkynylcarbene reactions, creation of versatile synthetic approaches to preparation of various practically useful fluoroorganic compounds on the basis of reactions of fluorocarbenes, development of synthetic applications of heavy carbene analogs and synthesis of small-sized heterocycles containing silicon and germanium atoms, analysis of mechanisms of some important reactions of carbenes, their analogs and related intermediates on the basis of physicochemical studies, direct spectroscopic studies of various labile intermediates of chemical reactions.

A 1 : 1 photostable complex between dichlorosilylene and CO2 was detected using matrix-isolation FTIR spectroscopy. The photostability can be attributed to the existence of photochemical equilibria between this complex and products of its isomerization, first of all, a complex of dichlorosilanone with CO, which are strongly shifted to the complex. A detailed computational study of the potential energy surface of the SiCl2 + CO2 system was carried out.

A 1: 1 donor–acceptor complex between SiCl2 and HCl was detected by matrix IR spectroscopy. The existence of the complex was previously predicted theoretically in the course of analysis of mechanisms of chemical vapor deposition (CVD) processes involving chlorosilanes. The quantum chemical calculations at the G4(MP2) level confirmed the possibility of formation of only one stable complex upon the reaction of SiCl2 with HCl. In addition to the complex of the simplest composition, complexes of SiCl2 with HCl associates were observed upon matrix annealing. The only product formed upon the photolysis of the complexes of all types was trichlorosilane, a product of silylene insertion into the H–Cl bond.

Formation of donor-acceptor complexes between dichlorosilylene, SiCl2, and allyl halides, AllHal(Hal= Cl, Br) was detected in Ar matrixes using matrix IR spectroscopy. In agreement with the predictions of the performed quantum chemical calculations, only broad unstructured absorption bands contributed by different conformers of the 1 : 1 complexes between SiCl2 and AllHal were observed in IR spectra of matrixes after deposition in the regions of characteristic vibrations of starting reactants. Annealing of matrixes resulted in strong narrowing the bands due to conversions of different conformers into the most stable structures. The predominantly formed conformers in both the reaction systems were those of complexes with SiCl2 coordinated to the Hal atoms of AllHal in the gauche conformations. At the same time, according to the calculations, the complexes with SiCl2 coordination to the double bonds of AllHal can be only slightly less stable than the complexes with coordination to the Hal atoms, and all these basic centers can be considered as comparable in their activity in the complexation. The only products revealed upon photolysis of complexes were the products of silylene insertion into the C–Hal bonds, viz., AllSiCl3 and AllSiCl2Br. Theoretical study of thermal transformations in the SiCl2 + AllHal systems showed that formal insertion of SiCl2 in the C–Hal bonds and its addition to the double bonds of AllHal have low activation barriers of 3–8 kcal mol–1. However, these barriers are too high for these reactions to occur under the matrix isolation conditions.

A matrix FTIR study of interaction between SiCl2 and 1,3-butadiene revealed that at low temperatures, it stops at the step of complexation between the reactants. This allowed us to investigate a photochemical version of this interaction resulting in the formation of both 1,1-dichloro-1-silacyclopent-3-ene and 1,1-dichloro-2-vinylsilirane in contrast to the thermal reaction giving only the first of these products.

It is experimentally found that allyltrichlorosilane dissociates under vacuum pyrolysis (~10–2 Torr) at temperatures above 1100 K to form three labile intermediates: allyl radical, dichlorosilylene, and monoatomic chlorine. On the basis of experimental and theoretical data obtained, it is shown that the decomposition reaction proceeds in two steps. The first step is a typical reaction of homolytic decomposition to two radicals (C3H5 and SiCl3) at the weakest Si—C bond. Due to weakness of the Si—Cl bond in the SiCl3 radical, the energy of which is even somewhat lower than the dissociation energy of the Si—C bond in starting AllSiCl3, this radical undergoes further dissociation to SiCl2 and Cl, thus resulting in three intermediates of different classes of highly reactive species formed from AllSiCl3.

Matrix FTIR study of products of benzene transformations in a pulsed glow discharge at low pressure in highly diluted mixtures of benzene with argon in the presence and absence of small oxygen additions has been carried out. Formation of the following hydrocarbon species has been established: acetylene, butadiyne, fulvene, benzvalene, methane, ethylene, phenyl, ethynyl and butadiynyl radicals. It has been shown that oxygen additions mainly result in deep oxidation of benzene to CO2, CO and H2O, although some products of intermediate oxidation have been detected. Those are formaldehyde, formyl radical, ketene, ketenyl radical, propadiene-1,3-dione, propadien-3-on-1-ilyden and hydroperoxyl radical. At the same time, it has unexpectedly been found that oxygen additions strongly increase the yield of butadiyne. Possible pathways, leading to formation of the listed species have been discussed on the basis of the obtained data and results reported in the literature.

The structure and thermochemistry of typical reactions of cyclic silylenes, 2,5,8-trimethyl-1-sila-2,5,8-triazacycloocta-3,6-dien-1-ylidene and 2,5,8-trimethyl-1-sila-2,5,8-triazacyclo- octa-1-ylidene, was studied by the B3LYP-D/6-311++G(3df,3pd)//B3LYP-D/6-31+G(d) method. The main forms of the silylenes are intramolecular donor-acceptor complexes in which the silylene center is coordinated to the valence-unbonded nitrogen atom. This coordination increases silylene stability and retains its reactivity almost unchanged. The data obtained indicate that the approach to the synthesis of stable and simultaneously reactive silylenes based on the intramolecular coordination of the silylene center is promising.

The PBE/TZ2P method was used to study a concerted σ-dimerization reaction of 1-silacycloprop-2-enes having substituents with different electron effects. The corresponding reaction channels were founds in all the cases, that indicated a general character of this reaction. The reaction barriers varied from moderately high to extremely low. The suggestions made earlier on a possibility for this process to take place in the course of the reaction of silylenes with alkynes at elevated temperature were quantitatively confirmed for the first time. The influence of substituents on the barrier heights and exothermicity of σ-dimerization of 1-silacycloprop-2-enes was studied. The σ-dimerization reaction of 1-silacycloprop-2-enes is one of a few examples of metathesis of s-bonds in the absence of transition metal complexes.

Time-resolved studies of chlorosilylene, ClSiH, generated by the 193 nm laser flash photolysis of 1-chloro-1-silacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reaction with trimethylsilane-1-d, Me(3)SiD, in the gas phase. The reaction was studied at total pressures up to 100 Torr (with and without added SF(6)) over the temperature range of 295-407 K. The rate constants were found to be pressure independent and gave the following Arrhenius equation: log[(k/(cm(3) molecule(-1) s(-1))] = (-13.22 ± 0.15) + [(13.20 ± 1.00) kJ mol(-1)]/(RT ln 10). When compared with previously published kinetic data for the reaction of ClSiH with Me(3)SiH, kinetic isotope effects, k(D)/k(H), in the range from 7.4 (297 K) to 6.4 (407 K) were obtained. These far exceed values of 0.4-0.5 estimated for a single-step insertion process. Quantum chemical calculations (G3MP2B3 level) confirm not only the involvement of an intermediate complex, but also the existence of a low-energy internal isomerization pathway which can scramble the D and H atom labels. By means of Rice-Ramsperger-Kassel-Marcus modeling and a necessary (but small) refinement of the energy surface, we have shown that this mechanism can reproduce closely the experimental isotope effects. These findings provide the first experimental evidence for the isomerization pathway and thereby offer the most concrete evidence to date for the existence of intermediate complexes in the insertion reactions of silylenes.

A kinetic isotope effect (k(D)/k(H)) of 7.4 has been found for the reaction of chlorosilylene with trimethylsilane (Me(3)SiD vs Me(3)SiH). Such a value can be accounted for by theoretical modeling, but only if an internal rearrangement of the initially formed complex is included in the mechanism. This provides the first concrete evidence for such complexes.

The transformation of a benzene-argon mixture in dielectric-barrier discharge (DBD) was studied. Benzene-soluble polymeric compounds (76.5 wt %), biphenyl (7.6 wt %), and phenylcyclohexadienes (9 wt %) are the major products. Monoalkylbenzenes, cyclohexadienes, and ethynylbenzene are in trace amounts in the reaction mixture. The benzene conversion per pass through the discharge zone was 5.5 wt %. The possible mechanism of the benzene transformation in DBD was suggested on the basis of experimental data and theoretical calculations.

The ESR spectrum of the first representative of highly conjugated triplet ethynylvinylcarbenes, 5-methylhexa-1,2,4-triene-1,3-diyl (1), was recorded in solid argon matrix. The zero-field splitting (ZFS) parameters of carbene 1 (D = 0.5054±0.0006 cm−1 and E = 0.0045±0.0002 cm−1) determined from the experimental ESR spectrum are in between the corresponding parameters of ethynylcarbene C3H2 (2) and vinylcarbene C3H4 (3): D(3) < D(1) < D(2) and E(2) < E(1) < E(3). Quantum chemical calculations of the ZFS parameters of 1, 2, and 3 have been carried out for the first time using two DFT-based approaches, RODFT and UDFT. An analysis of the experimental and theoretical ZFS parameters shows that carbene 1 is characterized by a greater extent of delocalization of the spin density of unpaired electrons than carbenes 2 and 3. The characteristic structural fragments of carbene 1 possess the principal features of the electronic structure of both ethynylcarbene (2) and vinylcarbene (3), respectively. Magnetic spin-spin interactions are identical in carbenes 1 and 2. The dominant contribution to D in 1 and 2 results from the one-center spin-spin interactions on carbon atoms in the propynylidene group, which are subjected to strong spin polarization.

Interactions of carbenes and carbene analogs EH2 and EHX with HX and H2 (E = C, Si, Ge, Sn; X = F, Cl, Br), respectively, were studied by quantum chemical methods. Theoretical analysis of the carbene and silylene systems was carried out at the G3 level of theory using the MP2(full)/6–31G(d) calculated geometries and vibrational frequencies. The stannylene systems were examined at the MP2 level using a modified LANL2DZ basis set for the Sn atoms and the 6–31+G(d,p) basis sets for other atoms. Transformations in the germylene systems were studied within the framework of both approaches, which gave similar results. This allowed one to compare the reaction pathways and their energy profiles for the whole series of systems. In addition to the insertions into the H-X and H-H bonds, the exchange reactions resulting in interconversions of EH2 and EHX can proceed in the systems under consideration. The effects of the nature of the E and X atoms on the reaction barriers and exothermicity of both the insertion and exchange reactions are analyzed. Possible role of radical processes in these systems is assessed.

The cycloaddition reactions of dichlorogermylene GeCl2 to ethylene, buta-1,3-diene, and hexa-1,3,5-triene were studied within the framework of the density functional theory (PBE and B3LYP density functionals) and by the ab initio CBS-QB3 method. The energy characteristics of the reaction of GeCl2 with ethylene were refined and non-empirical quantum chemical calculations of reaction pathways in the GeCl2 + buta-1,3-diene and GeCl2 + hexa-1,3,5-triene systems were carried out for the first time. It was shown that the [2+1] cycloaddition reactions are kinetically hindered and thermodynamically unfavorable, while the [4+1] and [6+1] cycloaddition reactions are characterized by low barriers and result in thermodynamically favorable products. For the [4+1] cycloaddition to buta-1,3-diene and [6+1] cycloaddition to hexa-1,3,5-triene, the most energetically favorable reaction pathways involve a suprafacial and antarafacial approach of reactants, respectively.