Silanes cleavage

A controlled reaction of dichloride (1) with lithium in THF affords dodecamethylsUane, a reagent that has been used for the preparation of sUyl enol ethers through photolytic cleavage. Silanes and polysUanes are readily available from the addition of an organometaUic species to (1) Thus the bisvinylsUane (2) can be prepared from (1) (eq 6). ... 

Trialkyltin substituents are also powerful ipso-directing groups. The overall electronic effects are similar to those in silanes, but the tin substituent is a better electron donor. The electron density at carbon is increased, as is the stabilization of /S-carbocation character. Acidic cleavage of arylstannanes is formulated as an electrophilic aromatic substitution proceeding through an ipso-oriented c-complex. ... 

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

Particularly interesting examples of mechanistic considerations can be found in the work of Graham [124-126], Schubert [127,128] and Crabtree [129]. Cophotolysis of hexamethylbenzenechrom- (or tungsten) tricarbonyl or Cp Mn(CO)3 with diphenylsilane yields, after cleavage of CO, a side-on coordinated silane with a M(H)Si 3c2e bond 30 [130, 131],... 

This excellent method of oxidative cleavage (/) of carbon-silicon bonds requires that the silane carry an electronegative substituent (2), such as alkoxy or fluoro. Either hydrogen peroxide or mcpba may be used as oxidant, and the alcohol is produced with retention of configuration (3). Fluoride ion is normally a mandatory additive in what is believed to be a fluoride ion-assisted rearrangement of a silyl peroxide, as shown below ... 

Most of the kinetic studies have been carried out by Eaborn and by Benkeser and their coworkers, and the first study was that of Eaborn656, who measured, spectrophotometrically, the rates of cleavage of 4-methoxyphenyltrimethylsilane by hydrochloric and perchloric acids in methanol, and by hydrochloric acid in aqueous dioxan, both at 47.9 °C (Table 226). The reaction was first-order in silane... 

The polysulfidic moieties of the silanes are unstable, and cleavage of the sulfur groups results in active sulfur species. A notorious problem with this kind of coupling agents is the balance between its reactivity towards the silica, requiring a temperature of at least 130°C to obtain an acceptable speed, and its eagerness to react with the rubber polymer, which starts to become noticeable at temperamres above 145°C. Furthermore, the primary and secondary reactions are chemical... 

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

Transfer of the hydride from the Cu to the electrophilic carbon and cleavage of the copper alkoxide by the silane regenerates 69. Recent reports point to the influence of the type of the counter ion X" of the homoleptic 66-67 on the activity, the BF being superior to the PF analogue this effect has been attributed to differences in the rate of active catalyst generation from the homoleptic [Cu(NHC)2] X and NaO Bu due to solubility differences of the inorganic salts formed during the displacement of the NHC by BuO" [54] (Scheme 2.10). 

Recent work has shown that there are many assumptions required in order to use the silane-cleavage method. However, an advantage of this approach is that it can be nsed to determine acidities at regiospecific positions in very weak acids, including those for which the conjugate base anion is unstable with respect to electron detachment. 

As with the halogens, the thiols, phosphines, silanes, and germanes undergo photoaddition to olefins via homolytic cleavage. Representative examples follow a24 130) ... 

Silanes with heterocyclic residues were also found to be reactive (Equation (20)), although the most likely reason is the in situ cleavage of the heterocyclic residue in the presence of hydrated TBAF to form silanols.290-292 Similarly, among various arylgermanes, only those containing 2-furyl groups were reactive in cross-coupling reactions.293... 

In contrast to the observed photochemistry of adamantyltris(trimethylsilyl)silane which efficiently yields the appropriate silene compound, similar photolysis of the germanium analogue provided no evidence for the production of germene13. However, photolysis of the germanium compound in CCI4 did result in a Norrish type 1 cleavage of... 

In analogy to the mechanism of the palladium-catalyzed enyne cyclization, it is postulated that exposure of palladium(O) to acetic acid promotes in situ generation of hydridopalladium acetate LnPd"(H)(OAc). Alkyne hydrometallation affords the vinylpalladium complex A-10, which upon r-carbopalladation of the appendant alkyne provides intermediate B-7. Silane-mediated cleavage of carbon-palladium bond liberates the cyclized product along palladium(O), which reacts with acetic acid to regenerate hydridopalladium acetate to close the cycle (Scheme 33). 

As an example of the selective reactivity of borazirconocene alkenes, their hydrolysis was examined [1]. The carbon—zirconium bond is more reactive than the carbon—boron bond towards various electrophiles, and so hydrolysis can be expected to occur with preferential cleavage of the former bond. Since hydrolysis of alkenylzirconocenes is known to proceed with retention of configuration [4,127—129], a direct utility of 45 is the preparation of (Z)-1-alkenylboronates 57 (Scheme 7.17) [12]. Though the gem-dimetalloalkenes can be isolated, in the present case it is not necessary. The desired (Z)-l-alkenylboronates can be obtained in a one-pot procedure by hydrozirconation followed by hydrolysis with excess H20. The reaction sequence is operationally simple and is compatible with various functional groups such as halides, acetals, silanes, and silyloxy protecting groups [12]. 

Cleavage of Si—C bonds (12,243-245). This oxidation can be used to convert vinylsilanes in three steps to syn- or anti-1,2-diols. Thus Grignard reagents cleave epoxides of vinylsilanes selectively to (3-hydroxy silanes, which can be oxidized with retention of configuration to 1,2-diols. When applied to an (E)-vinylsilane, the sequence results in the syn-l,2-diol the an -l,2-diol is obtained from a (Z)-vinylsilane by the same reactions. 

Organosilylphosphines are conveniently prepared by cleavage of alkyldiaryl-phosphines with lithium in THF, followed by treatment with chlorotrimethyl-silane,15 and tris(trimethylsilyl)phosphine has been prepared from the reaction of chlorotrimethylsilane with a mixture of sodium and potassium phosphides.16... 

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