Silanes, ozone

Silanone produced by the silane-ozone photochemical reaction in Ar matrices gives rise to a band at 1202 cm in the IR spectrum that is assigned to the v(Si=0) [766] ... 

Silica is also deposited by the decomposition of diacetoxy-ditertiarybutoxy silane (DADBS) at 450-550°C and low pressure (< 1 Torr) and by the decomposition of octamethyl-cyclotetrasiloxane in ozone at 400°C at low pressure.P ]P 1... 

Recently, Genzer and coworkers [85] presented an interesting new approach for the preparation of stable silane-based SAM systems. As a substrate, cross-linked polydimethylsiloxane (PDMS) was oxidized by UV/ozone treatment to yield a thin sihcon dioxide surface. The surface was then treated with fluorinated alkyltrichlo-rosilanes from the gas phase while being mechanically stretched by a certain length Ax. After modification, the elastomer was allowed to relax resulting in a mecdianically assembled monolayer (MAM) at the surface (Fig. 9.8). 

Silane hydrotrioxides 69 were proposed as transient intermediates in the reaction of ozone with various silanols. Hydrotrioxides can lose oxygen to afford silanol (Scheme 16). 

From silanes 52 are obtained, in high yield, the corresponding silanols 53, which react further to produce disiloxanes 56 and 58-60. Silanes 54 alkoxysilanes 55 and disilanes 57 give high yields of disiloxanes 56. Ozonolysis of tetraethylsilane yields initially acetaldehyde and trimethylsilyl hydroperoxide 61. The latter is partially converted to bis(triethylsilyl) peroxide 62, which is hydrolyzed to silanol 63 and hydrogen peroxide. The ozonolysis is of first order, both in regard to the silanes, and to ozone. The ozonolysis starts with formation of 64 followed by formation of the trioxide 65, which decomposes to acetaldehyde and hydroperoxide 61 (Scheme 14)79 80. 

When a mixture of methylsilane (or dimethylsilane) deposited with ozone in an argon matrix at 17 K was photolyzed, the corresponding silanone, MeHSi=0 (or Me2Si=0), was generated and characterized by their infrared spectra. In the case of the parent silane, H3SiH, a similar photolytic reaction with ozone in an Ar matrix led to the identification of SiO, H2Si=0 (2), (HO)HSi=0 (silanoic acid) and (HO)2Si=0 (silicic acid)18. 

At last, some workers focused their studies on organosilicon compounds [73-82]. Schemes 19 and 20 summarize the action of ozone on silanes, tetra-... 

Only little work has been focused on the action of ozone onto non-hydrocarbon polymers. This area of research has been investigated by Karandinos et al. [148] who cross-linked polydimethylsiloxanes (PDMS) bearing silane groups Si-H after ozonization. These workers studied the reaction shown in Scheme 41. 

The phenyl silane reacts with ozone to give a tri silanol 0Si(OH)3 which... 

In contrast, we took advantage of the addition of ozone to a vinyl silane to produce the desired a-hydroperoxycarbonyl moiety as described by Biichi and Wtiest.54 Thus, the next retron in our analysis was the 2p, 6P-disubstituted cyclohexenyl silane 4, which, according to Biichi, would afford 2 or the synthetically equivalent dioxetane 3 upon exposure to ozone. 

The trimethylsilylated ylides (1), easily generated from trimethyl chlorosilane and ylides, react with aldehydes 2 to form vi-nylsilanes 3 (2,3). The vinylphosphonium silanolates 4 are also formed. Compounds 3 are versatile reagents for further reactions (4). The ylide J (with R1 =H) reacts with aldehydes 2 to give the dienes j). The oxidation of with the adduct 6, from triphenyl-phosphite and ozone, gives access to a generaT synthesis of acyl-silanes (trimethylsilylketones) (2). The silylated ylides react to form phosphonium salts 7 with halogen compounds. The salts 7.can be desilylated by fluorine ions. The disubstituted ylides JO Tormed can be converted in statu nascendi with aldehydes V[ into the tris-substituted olefin J2 (2,3). In the case of R3-I, vinyl... 

Ozone Competition Reaction Procedures. In the relative rate studies, the solvent-saturated ozone—oxygen stream was passed into a glass bubbler reactor vessel charged with 4 ml of about 4 X 10"2M concentration of each of the two silanes to be competitively ozonized as well as of an inert saturated aliphatic hydrocarbon to function as internal standard. (For example, n-undecane was used for the tributylsilane/trihexylsilane study.) The effluent from the reactor passed through a solvent-filled bubble counter to visualize the flow. The inlet stream splitter mentioned earlier was adjusted to allow 2—4 hours for each runs completion, as determined by experience. The temperature was controlled at 0°C in both the saturator and the reactor by ah ice bath. 

A second possibility is that the ozone forms some kind of complex with the silane before attack on the hydrogen. From this complex, all hydrogens are equally accessible, and the decomposition is first order in complex. In the hope of observing such a complexation, the ultraviolet spectra of ozone/silane mixtures in carbon tetrachloride were examined (33). Although no spectral bands attributable to a silicon-ozone complex were found, it was observed that any silicon-containing species catalyzed the decomposition of ozone. That is, not only triethylsilane, but tri-ethylsilanol and tetramethylsilane as well, destroy ozone in carbon tetrachloride. This result indicates an association of the ozone with the silicon atom, regardless of the functionality of the silicon species (within the types examined) and completely independent of the silicon substrate s... 

If the reaction is four-centered (5 - 7), a silicon hydroperoxide should be formed. The reaction of triethylsilane with ozone was monitored by NMR at —57°C. The only species observed under these conditions were the silane and the silanol no evidence for a hydroperoxide intermediate which might have been stable at that temperature (35) was detected, and chemical tests for peroxides proved negative (5, 6). As the concentration of ozone was increased from zero to saturation, the spectrum of the silane completely disappeared with the concurrent appearance of the silanol NMR spectrum. 

When all the mechanistic evidence is taken into consideration, the following reaction sequence appears to best satisfy the data. The silane undergoes reversible complexation (A) with the ozone, the complex being present in only small concentrations. The rate step then involves electrophilic attack on the hydridic hydrogen, passing through a five-center transition state. This may decompose to either a silyl hydrotrioxide (Bi) or directly to the radical pair (B2). The silyl hydrotrioxide, if present, must decompose rapidly to the radical pair (C). This radical pair then recombines with retention of configuration to afford the ultimate product, the silanol (D). 

Vin silanes follow a similar course on oxidation with peroxy acid or with ozone. - Depending upon the conditions of oxidation they can be converted either to a carbonyl compound (2) or to the a-hydroxy-caibonyl compound (3), as in Scheme Vinyl silanes are useful synthetic intermediates, and this oxidation rearrangement procedure is an important component in their spectrum of reactivity. 

Finally we mention the interesting observation that vinyl silanes afford dioxe-tanes on ozonization (Eq. 20). In this unusual manner, dioxetane (12) was obtained in the ozonization of 2-methyl-3-trimethylsilyl-2-butene. The general scope of this potentially convenient method merits investigation, especially since the conditions are mild and convenient. The 1,2-dioxetanes that have been prepared via these miscellaneous methods are shown in Table 1. 

Cyclic silanes, reactions with ozone 85UK923. 

Scheme 2.126 Preparation of trifluoromethyltrimethyl silane (MejSiCFj, Ruppert s Reagent). Ruppert s original method [2c] above) leads, after aqueous work-up, to the formation of stoichiometric amounts of the carcinogenic HMPA (OP(NMe2)3). In addition, ozone-depleting CFjBr is used as the starting material. A recent method beloiv), with potential for technical upscale, utilizes the inexpensive CHF, and depends on a catalytic cycle initiated by diphenyldisulfide [69].

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