Leonid E. Gusel'nikov

Hetero-π-systems from 2+2 cycloreversions. Part 1: Gusel'nikov-Flowers route to silenes and origination of the chemistry of doubly bonded silicon

Abstract The review concerns the production of the unsaturated compounds of a doubly bonded silicon via the four-membered silacycles’ 2+2 cycloreversion, a reaction which allowed 35 years ago the evidence of 1,1-dimethylsilene (DMSE), (CH 3 ) 2 SiCH 2 , the first compound containing the silicon–carbon double bond, and inspired a renewed interest to the chemistry of the multiply bonded silicon compounds at that time believed to be non-existent. Since then a great many of the doubly bonded silicon compounds have been obtained by various methods. Of them sterically non-hindered compounds are kinetically unstable because of their unprecedented reactivity in the bimolecular reactions, whereas those ones kinetically stabilized by the bulky groups, the ‘crowded’ compounds of the multiply bonded silicon, are relatively stable. Thermal and photochemical 2+2 cycloreversions occurring both in gas and liquid phases are reviewed. A special section is devoted to the gas-phase 2+2 thermocycloreversion of 1,1-dimethyl-1-silacyclobutane as a clean reaction of the transient DMSE production for the study of its chemistry as well as for its characterization by physicochemical and spectroscopic methods. Scope and limitations of 2+2 cycloreversion affected by the numbers and the position of silicon atoms in the four-membered ring, by the presence of Groups 15 and 16 elements, by the substituents and by the reactions conditions are evaluated.

Very low pressure pyrolysis (VLPP) of monosilacyclobutanes. Infrared absorptions of 1,1-dimethyl-1-silaethylene, (CH3)2SiCH2, and 1,1-dideuteriomethyl-1-silaethylene, (CD3)2SiCH2 isolated in argon matrices

Abstract The pyrolysis products of 1,1-dimethyl-1-silacyclobutane (DMSCB) and 1,1,3-trimethyl-1-silacyclobutane (TMSCB), and also of the Si-deuteriomethyl analogs (DMSCB-d6 and TMSCB-d6) isolated in argon matrices at 10 K have been studied by IR spectroscopy. Pyrolysis of DMSCB and TMSCB gives rise to an identical set of bands: 644, 696, 817, 824, 932, 992, 1001 and 1253 cm-1 which permanently vanish with the increase in temperature. Correspondingly, the bands at 543, 580, 683, 718, 722, 768, 891, 929, 985 and 1012 cm-1 are observed in the spectra of pyrolysis products of DMSCB-d6 and TMSCB-d6. The appearance of an identical set of bands is attributed to 1,1-dimethyl-1-silaethylene (DMSE) isolated in the argon matrix, but in the case of deuterio analogs, to DMSE-d6. Based on normal coordinate treatment, the above mentioned bands have been preliminary assigned to certain modes of vibrations of DMSE and DMSE-d6.

Production of poly(silaisoprene) by laser-induced decomposition of 1-methyl-1-vinyl-1-silacyclobutane

Abstract Continuous CO 2 laser-induced decomposition of 1-methyl-1-vinyl-1-silacyclobutane (MVS) in the presence of energy conveying sulfur hexafluoride provides a highly selective method for production of poly(2-methyl-2-silabuta-1,3-diene) (polysilaisoprene). Trapping with hexafluoroacetone and tetrafluoroethene confirms the intermediacy of 2-methyl-2-silabuta-1,3-diene (silaisoprene), polymerization of which depends on the ratio MVS/SF 6 . The IR spectrum of the polymer deposited from the gas-phase on to the reactor surface reveals that silaisoprene polymerizes by participation of both double bonds.

Efficient gas phase polymer deposition by infrared laser-photosensitized decomposition of 4-silaspiro[3.4]octane

Abstract Laser-induced thermolysis of 4-silaspiro[3.4]octane is an efficient method for the gas phase deposition of organosilicon polymer, which is assumed to arise from 1-methylene-1-silacyclopentane generated upon the elimination of ethene from the parent molecule.

Infrared spectroscopic study of very low pressure pyrolysis products of cyclosilthianes and 3,3-dimethyl-3-silathietane isolated in argon matrices. A new source of 1,1-dimethyl-1-silaethylene (Me2siCH2), thioformaldehyde (H2CS) and dimethylsilanthione (Me2SiS)

The products of very low pressure pyrolysis (VLPP) of hexamethylcyclotri-silthiane (I), tetramethylcyclodisilthiane (II) and 3,3-dimethyl-3-silathietane (III) were isolated in Ar matrices and were studied by IR spectroscopy. The only pyrolysis product of I was cyclosilthiane II, a dimer of transient dimethylsilanthione (Me2SiS) (IV). The starting material was recovered on pyrolysis of II. Thermal decomposition of III involves three intermediate unsaturated compounds: dimethylsilaethylene (Me2 SiCH2) (V) and thioformaldehyde (H2CS) (VI), both isolated in Ar matrix at 10 K, as well as silanthione IV fixed in the matrix in a form of the cyclic dimer II. The latter was also observed in the study of copyrolysis of 1,1-dimethyl-1-silacyclobutane and thietane, being authentic sources of intermediates V and VI. IR spectra of starting compounds I, II and III isolated in Ar matrices were obtained. The theoretical structure of IV and force constant F(SiS) were determined by the CNDO/2 method. With regard to CNDO/2 errors, SiS bond distance and F(SiS) are equal to 1.993 A and 4.72 mdyn/A, respectively. Calculation of normal vibrations resulted in the following values of vibrational frequencies of dimethylsilanthione (cm−1): 884 νs(SiC2) (A1), 735 νas(SiC2) (B2), 626 νs(SiS) (A1), 200 ⪯ CSiC (A1).

Classical planar doubly bonded Si-substituted silenes—a boundary system between olefins and heavier Group 14 analogs: Geometric and electronic structures, rotational barriers about the SiC double bond, and comparison with the analogs and isoelectronic phosphenes, further evidence against C-ylides of silicon

Abstract An ab initio study of a number of isostructural ethenes, silenes, and germenes at the MP4/6-311G(d)//MP2/6-31G(d)+ZPE level of theory showed that R 2 SiCH 2 silenes are the last classical planar doubly bonded system because unlike the heavier Group 14 analogs electronegative substituents do not disturb a planar geometry, shorten and weaken the SiC double bond. The calculations of the potential energy profiles and the rotational barriers of isoelectronic silene and phosphene as well as phosphorane are in favor of silenes to be more like phosphenes rather than phosphoranes. The rotational barriers decrease as more electronegative substituents are attached to the Group 14 atom. For ethenes, silenes, and germenes the maximal effect is observed for fluorine substitution. Fluorine does not affect the rotational barrier in phosphenes. A thermochemical approach based on the strain energies and 2+2 cycloreversion enthalpies was used to estimate the difference between the EC (E=C, Si, Ge, P) σ- and π-bond energies in elementaalkenes. The Bader analysis of the electron density distribution results in a covalent and highly polar double bonds whose polarity decreases in the order: silenes>phosphenes>germenes.

IR laser photosensitized decomposition of 1-methyl-1-silacyclobutane. Efficient gas-phase polymer deposition

Abstract Laser-induced thermolysis of 1-methyl-1-silacyclobutane is a highly selective method for the gas-phase deposition of organosilicon polymer, which is formed by the elimination of ethene and the major participation of methylsilaethene and dimethylsilylene in the polymerization.

Novel 2,4-disilathietane ring system from [2 + 2]cycloaddition of 1,1-dimethyl-1-silaethylene,Me2SiCH2, to dimethylsilanthione, Me2SiS. Perturbation molecular orbital (PMO) study on reactivity of intermediates with a double-bonded silicon atom

Copyrolysis of 1,1-dimethyl-1-silacyclobutane (I) with both hexamethylcyclotrisilthiane (II) and tetramethylcyclodisilthiane (III) at 560°C involves 1,1-dimethyl-1-silaethylene, Me2SiCH2 (IV), and dimethylsilanthione, Me2SiS (V), intermediates and yields the following cycloaddition products: the new 2,2,4,4-tetramethyl-2,4-disilathietane (VI), 1,1,3,3-tetramethyl-1,3-disilacyclobutane (VII), and III. Six-membered cyclocarbosilthianes, 1,1,3,3,5,5-hexamethyl-2-thia-1,3,5-trisilacyclohexane (VIII) and 1,1,3,3,5,5-hexamethyl-2,4-dithia-1,3,5-trisilacyclohexane (IX) have also been derived by inserting IV and V into the SiS bond of VI. Copyrolysis of I with thietane (X) also results in four- and six-membered cyclocarbosilthianes, the major product being VI. This is discussed in terms of dimethylsilanthione formation via [2 + 2]cycloaddition of IV to thioformaldehyde (XI) followed by [4 → 2 + 2]cyclodecomposition of the 2-silathietane intermediate. A perturbation molecular orbital study of [2 + 2]cycloaddition involving intermediates IV, V, and XI has shown that IV reacts more readily with V and XI than it cyclodimerizes. Dimerization of V is the most prominent reaction.

Laser-induced chemical vapour deposition of polymethanimine

Continuous-wave CO2 laser photosensitized (SF6) decomposition of azetidine, dominated by expulsion of ethene and formation of polymethanimine, represents a convenient process for chemical vapour deposition of thin polymeric films.

Direct infrared spectroscopic evidence of allyl radical and definition of the initiation step in thermal decomposition of cycloalkanes.: A comparison with very low pressure pyrolysis of alkenes

The very low pressure pyrolysis (VLPP) of both C4–C8 cycloalkanes and their corresponding isomeric 1-alkenes as well as 2-heptene (750–950°C, 10−3 s, 10−2–10−3 Torr) was studied using the low temperature matrix infrared spectroscopy method (LTM/IR) to identify radicals and to evaluate reactions responsible for their generation. Apart from the pyrolysate of cyclobutane the reaction products of all other cycloalkanes exhibit absorption bands in the spectra due to the allyl radical (518, 808 and 985 cm1) besides those of ethene, propene and the isomeric 1-alkenes. Moreover, in the case of cyclohexane the methyl radical and in the case of cyclopentane and of cycloheptane the ethyl radical were identified amongst cyclopropane and cyclopentane, respectively. The VLPP/LTM/IR spectra of 1-alkenes on their part show absorption bands due to both allyl and methyl (from C4H8, C6H12, C8H16) or ethyl (from C5H10, C7,H14) radicals. These results are considered to be direct evidence of the initiation step of the conventional radical chain process of cycloalkanes pyrolysis consisting in the generation of radicals from isomeric 1-alkenes which are considered primary products of the unimolecular thermal 1-alkenes conversion.