Disilane pyrolysis

SiHMe insertion into Si-H bonds during the vacuum uv photolysis of CH3SiH3. From the study of disilane pyrolysis, Tebben and Ring established the insertion of SiHj into SijHg to give Si3H which was immediately confirmed by Bowrey and Purnell in 1970 ... 

Pyrolysis of polysilanes played an important role in the discovery of silylene reactions. Through the pyrolysis of alkoxydisilanes in the presence of diphenylacety-lene, Atwell and Weynberg obtained a product regarded as a dimer of dimethylsilylene adduct. 1,2-Shift of a methoxy group in disilanes takes place under relatively mild conditions (Scheme 14.1). ... 

Rearrangements of disilanes to a-silylsilenes are well established and are involved in the exchange of substituents between a silylene center and the adjacent silicon.Pulsed flash pyrolysis of acetylenic disilane (41) gave rise to the acetylenic silene (42), which subsequently rearranged to the cyclic silylene, 1-silacyclopropenylidene (43). Irradiation of the cyclic silylene resulted in the isomerization to the isomeric 42, which itself could be photochemically converted into the allenic silylene (44). Both 42 and 43 also were reported to isomerize on photolysis to the unusual (45), which was characterized spectroscopically (Scheme 14.24). 

Disilene and its isomer silylsilylene were neither available by standard vacuum flash pyrolysis of precursors 59-63, nor by the more elaborate method of pulsed flash pyrolysis of 60-63, a pulsed discharge in mixtures of argon and mono- and disilane74 or by the matrix photolysis of educts 59-66 using various light sources (Hg lamps, excimer laser)69,70,72, the microwave discharge in disilane 66 or the cocondensation of silicon atoms with SiFLt. 

Maier and coworkers have found that pulsed flash pyrolysis of an acetylenic disilane 323 gave rise to the acetylenic silylene 324, which subsequently rearranged to the cyclic silylene 1-silacyclopropenylidene 325162. Irradiation of this cyclic silylene resulted in its isomerization to the isomeric acetylenic silylene 324, which itself could be photo-chemically converted to the allenic silylene 326. As well, both 324 and 325 were said to isomerize on photolysis to the unusual silacycloalkyne 327, which was characterized... 

The possibilities of synthesis are limited by the general properties of the Si-Si bond to a small number of reaction types. Disilane derivatives were usually synthesized by a Wurtz-like reaction of monosilane halide derivatives with alkali or alkaline-earth elements. Some pyrolysis reactions that have yielded disilane derivatives are also known. Often, new disilane derivatives were prepared from common derivatives by an exchange of substituents. Some rearrangements have been observed in recent years. 

The pyrolysis of disilane forms mono-, tri-, and n- and i-tetrasilane. An insertion reaction takes place51 in which silylene reacts with an Si—H bond ... 

The most studied reaction of silylene is the insertion reaction into silicon-hydrogen bonds. It was first postulated on the basis that higher silanes were generally formed in pyrolysis of silane, disilane, and trisilane (74, 81, 98). If silylene is the major intermediate, the higher silanes are formed by insertion reactions of the type... 

Gaseous mixtures of disilane 1 and argon (1 1000-2000) were subjected to flash pyrolysis at various temperatures and pressures. After leaving the hot zone the reaction products were directly condensed onto a Csl or BaF2 window at 10 K. The matrix-isolated products were studied by IR and UVA IS spectroscopy. Under the conditions of high-vacuum flash pyrolysis only trimethylsilane (2) and small amounts of acetylene were detected. Any C2H2Si isomer that might have formed was too unstable to be detected under these pyrolysis conditions. 

Brus et al. prepared isolated silicon particles by high temperature pyrolysis of disilane with a subsequent passivation of the surface by oxidation [33]. The particles of various size are then processed by high-pressure, liquid-phase, size exclusion chromatography to separate sizes and obtain various fractions of monosize particles. Such particles represent an almost ideal model of silicon quantum dots. 

Previously, it had been shown that pyrolysis of (86) at 620°C in a vertical nitrogen-flow system gave a complex mixture of products, including (88) (3.5%), (64 R, R = H) (0.8%), 1,1,3,3-tetramethyl-1,3-disila-2-methylidenecyclopentane (2.1 %) and 3,3,4,5,5-pentamethyl-3,5-disilacyclo-pentene (2%) <79JOM(168)23>. Formation of disilacyclopentanes by platinum-catalyzed rearrangement of vinyl disilanes also has been reported <86JAP61207389). 

The gas-phase pyrolysis of disilane and silane is also likely to involve this rearrangement process. The original proposal60, that the thermal elimination of H2 from disilane is a three-center process leading to silylsilylene, was subsequently modified67 in response to the results of chemically activated disilane decomposition. It was concluded that both the... 

Thermal silylcarbene-to-silene rearrangements have been known for a long time1. The pyrolytic product from trimethylsilyldiazomethane, 1,1,2-trimethylsilene, was trapped in an argon matrix230, and the pyrolysis of bis(trimethylsilyl)diazomethane126 was reported to produce fair amounts of 2,4-bis(trimethylsilyl)hexamethyl-l,3-disilacyclobutane, the expected dimerization product of 2-(trimethylsilyl)-1,1,2-trimethylsilene. A second product was the disilane expected from an ene addition of one... 

When subjecting the disilane 48 to a newly developed pyrolysis technique, trimethylsi-lane is split off and 1-silacyclopropylidene 49 is formed as the most stable C2H2S1 isomer. Photolysis of this silylene with monochromatic light yields 50 which was identified by comparing its IR spectrum with a calculated spectrum. Whether 50 is a true cycloalkyne or a diradical, a singlet or a triplet must await further investigation as must the study of its chemical behavior. 

On the other hand, it is unlikely that methylene is formed from methane or other hydrocarbons in the pyrolysis of coke. In contrast to methylene, its analog silylene is a product of the pyrolysis of silane and disilane (8-17 Purnell and Walsh, 1966 Bowrey and Purnell, 1970). Laser-induced fluorescence was used to study the formation of silylene and its reactivity (see, e.g., Baggott et al., 1988 Jasinski and Chu, 1988), but silylene is not within the scope of this book as there are no diazo compounds involved in its chemistry. Literature in which the reactivity of silylene is compared with that of methylene is reviewed briefly in a publication of Skancke (1993, p. 640). 

Polycarbosilanes are usually synthesized by the thermal decomposition of monosilanes or disilanes and by ring-opening polymerization of disilacyclobu-tanes. Fritz et al. sythesized polycarbosilanes (PC-TMS) by heat condensation of tetramethylsilane at 700°C, circulating unreacted silane repeatedly in a continuous pyrolysis furnace [12]. 

Polysilanes have been the first class of precursors used to prepare silicon carbide ceramics. In all cases, on pyrolysis, polysilanes are converted into polycarbosilanes by a Kumada rearrangement prior to the formation of an amorphous silicon carbide network. Several synthetic routes including dehydro-polymerization, ring-opening polymerization of strained cyclosilanes, polymerization of masked disilenes, or base catalyzed disproportionation reactions of disilanes have been described to prepare linear or branched polysilanes but despite its drawbacks the Wurtz-coupling route remains the method applied most, especially to prepare linear polysilanes. 

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