Silicon compounds thermolysis

All known isolable two-coordinated silylenes are cyclic monomers stabilized by bulky substituted amino groups at silicon and in most cases supplemented by a Huckel-type delocalized electron system. They were obtained by reduction of the corresponding dihalogenated derivates [1]. Attempts to synthesize analogous pyrido- ornaphtho-l,3,2A, -diaminosiloles by the same route gave small amounts of the desired pure product or they failed [Id, 2]. This prompted us to investigate (i) the photolysis of bis(trimethylsilyl)diaminosilanes and (ii) the thermolysis of recently reported disilanes [3] and of the trisilanes as alternative methods to prepare stabilized low-coordinated silicon compounds. 

The former silene is not stable at room temperature, owing to its tendency to dimerize in the absence of other reactants, but it can be stored as its [2+2] cycloadduct with A/-trimethylsilylbenzophenonimine, from which it is cleanly and easily liberated on mild thermolysis.99 The latter silene arose from the loss of LiX from the compound (r-Bu)2SiX-CLi (SiMe3)2, which subsequently underwent a spontaneous 1,3-silicon-to-silicon methyl shift. 

Complex 12 was found to be a good reactant in the double-silylation reaction. Thus, thermolysis of a toluene solution of 12 and diphenylacetylene at 120°C for 12 h afforded 5,6-carboranylene-1,1,4,4-tetramethyl-2,3-diphenyl-1,4-disilacyclohex-2-ene 13. When 1-hexyne was employed in the reaction with 12 under the same reaction conditions, the five-membered disila ring compound 18 was isolated. A key feature in the h NMR spectrum of 18 includes a singlet at 6.24 ppm assigned to the vinyl proton. A characteristic high-frequency 13C NMR resonance at 138.50ppm provides evidence for a tethered sp2 carbon atom between the two silicon atoms. 

The earliest work on silicon carbide fibers was done by Yajima and co-workers [3]. Yajima applied the Kumada [4] rearrangement to Burkhard s [5] dimethylpolysilane - an insoluble and infusible compound - (Eq. 1) and obtained by thermolysis at 400 - 450°C or by catalysis with polyborodiphenyl-siloxane at 350°C a melt spinnable and soluble polycarbosilane (Eq. 2). 

A related scrambling of groups in a silene has also been reported by Eaborn (143) to explain the structure of compounds isolated from the thermolysis of tris(trimethylsilyl)fluorodiphenylsilylmethane at 450°C, where Me and Ph groups freely interchange between silicon atoms [Eq. (19)]. A related rearrangement is probably also involved in the photochemical silene-to-silene isomerizations derived from acylpolysilanes described earlier. 

Orlov (8a) has compared the dissociative ionization and thermal decomposition of a series of Group IV compounds and discusses the use of the mass spectrometer to study the unstable or nonexistent compounds that are presumably thermolysis intermediates. These include bivalent compounds of Group IV such as silicon(II) derivatives and compounds with multiple bonds. Not only are ions observed important in such correlations, but the mass of the neutral fragment, as confirmed by the decomposition of metastable ions, is of equal importance. 

Silylated 1,2,3-trisilacyclopentanes (73) under photolysis split off their central Si atom as a silylene (trapped by a scavenger) <85JOM(292)l67>. l,2,3-Trisilacyclopent-4-enes (74) split off only their central silicon atom as silylene by flash vacuum thermolysis (650-720 °C), whereas by photolysis both central and terminal Si atoms may be extruded depending on the nature of the substituents (Ar better than Alk-stabilized silylene) <84CL393>. Pyrolysis of hexamethyl-1,2,4-trisilacyclopentane (75) is unique for Si-compounds since different products arise at normal <76ZAAC(419)249> and low <820M1453> pressure (Scheme 2). 

Silicon-carbon and silicon-nitrogen multiply bonded compounds 929 A ring-opened isomer 387 is obtained through thermolysis of 386b (equation 110). 

The chemistry of some ring systems having two heteroatoms, i.e. dioxetanes, dithietanes, oxathietanes and thiazetidines are described. Next, the review considers compounds having either silicon or boron in a four-membered ring. Some thermolysis processes are interesting in the silicon series and the first thermally stable 1,2-dihydro-1,2-diborete is described. 

Carbodiimides are a unique class of reactive organic compounds having the heterocumu-lene structure R—N=C=N—R. They can be formally considered to be the diimides of carbon dioxide or the anhydrides of 1,3-substituted ureas, and they are closely related to the monoimides of carbon dioxide, the isocyanates. The substituent R can be alkyl, aryl, acyl, aroyl, imidoyl or sulfonyl, but nitrogen, silicon, phosphorous and metal substituted carbodiimides are also known. The unsubstituted carbodiimide HN=C=NH is isomeric with cyanamide, H2NCN. Mono substituted carbodiimides, generated in the thermolysis of 1-substituted tetrazoles, can be isolated at liquid nitrogen temperature but isomerize to the cyanamides at higher temperatures. ... 

Because of the close relationship between silicon and carbon, many attempts have been made to try to synthesize species containing multiple bonds to silicon (Si=C, Si=0, Si=Si, etc.). However, it was not until 1967 that compelling evidence was presented that Si=C might exist in the thermal reaction of 1,1-dimethyl-1-silacyclobutane (equation 90). The first evidence for the existence of Si=Si as transient intermediate was provided in the thermolysis of bridged disilane derivatives (equation 91). Since then, many studies have been published on these unsaturated species, but it was in 1981 that synthesis and characterization of relatively stable crystalline compounds containing Si=C (silene) (equation 92) and Si=Si (disilene) (equation 85) were reported (equations 90-92). 

There are several structurally different types or polymers that are suitable precursors for ternary Si-C-N ceramics. By far the most investigated precursors are polysilazanes of the general type [Si(R )(R°)N(R°)] (R, R°, R° = H, alkyl, aryl, alkenyl, etc.). In contrast to the limited number of starting compounds, H SiCl(4 ) (x = 0-3) as the silicon source and NH3 or H2N-NH2 as the nitrogen source for synthesis of polysilazanes as precursors for binary Si-N ceramics, the chemistry of polycarbosilazanes, that is, carbon-containing or modified polysilazanes, is very multifaceted. The attachment of various organic groups to the silicon atoms allows adjustment of their physicochemical properties, to control their thermolysis chemistry, and also to influence materials properties. The first... 

Amino, alkoxy, and aryloxy polyphosphazenes are typically prepared by nucleophilic displacement reactions of poly(dihalophosphazenes). Analogous reactions with organometallic reagents, however, result in chain degradation and cross linking rather than in linear, alkyl, or aryl substituted poly(phosphazenes). The thermolysis of appropriate silicon-nitrogen-phosphorus compounds can be used to prepare fully P—C bonded poly(organophosphazenes). The synthesis of two of these materials and their Si—N—P precursors is described here. 

Some sulphinyl chlorides have been shown95 to react with silicon-nitrogen compounds or amines to give the sulphinamides (16) and (17). The compounds are reactive intermediates, reacting with trichloromethanesulphenyl chloride and PC15 to form (18) and (19). Compound (19) is cleaved by thermolysis to form sulphinyl chlorides and the trimeric phosphonitrile... 

The first compelling evidence that a silene (a compound containing a silicon-carbon double bond) could exist was provided by Gusel nikov and Flowers in 1967 from results of the thermolysis of dimethylsilacyclobutane26 (equation 16). Thus, it is not surprising that... 

Novel photochemical (and thermal) reactions of macrocyclic oxa-sila-acetylenic ring systems (expected to show unusual optical properties because of electronic effects arising from orbital overlap of the acetylenic n system with the silicon a bonds and the oxygen lone-pair electrons) were described. While thermolysis in the presence of a transition metal carbonyl compound gave cyclization to both benzenoid and fulvene species, photolysis in the presence of the transition metal carbonyl compound (which catalyzes 1,2-silyl shifts across a carbon-carbon triple bond) gave fulvene and vinylidene products, the latter being readily photolyzed to the fulvene 159 (equation 101). 

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