In phenylsilanes

NOE as n is changed from 0 to 4, The slower moving molecules demonstrate an increased importance of the dipolar contribution. In diphenyldichlorosilane (234) DD and SR relaxation mechanisms contribute about equally to the Si T[ value, providing a probable upper limit for a long range intermolecular contribution of dipolar relaxation from phenyl protons in phenylsilanes. 

The small effect induced by one phenyl group on D(Si-H) raises the question of whether these bond dissociation enthalpies in phenylsilanes will foHow a pattern similar to the one observed in methylsilanes. In order to tackle this problem, photoacoustic calorimetry experiments on several phenyl- and methylsilanes were carried out at NRCC. 

Table V. Silicon-Hydrogen and Carbon-Hydrogen Bond Dissociation Enthalpies in Phenylsilanes and Phenylmethanes Data in kJ/mol...

Form Supplied in phenylsilane is a clear liquid cesium fluoride is a white solid both widely available. 

The titanium(IV) chloride catalyzed addition of allylic silanes to (E)-(2-nitroethenyl)benzene affords y,<5-unsaturated nitronates which, on treatment with low valent titanium species [generated in situ from titanium(IV) and zinc], give y,<5-unsaturated nitriles. For example, [(Zs)-2-butenyl]-(dimethyl)phenylsilane underwent reaction with ( )-(2-nitroethenyl)benzene to give 3-methyl-2-phenyl-4-pentenenitrile in 65 % yield as a 3 1 mixture of diastereomers of unassigned configuration22. 

Diiodosilane l2SiH2 1872, prepared by treatment of phenylsilane PhSiHs with iodine, via PhSiH2l, in the presence of catalytic amounts of ethyl acetate at -20 °C, is much more electrophilic than Me3SiI 17 and therefore converts secondary alcohols such as 2-octanol 1871, at room temperature with Walden inversion, into iodides such as 1873 in 93% yield whereas the diol 1874 is nearly quantitatively converted into the monoiodobutane 1875 and only traces of the diiodobutane 1876 [88, 89] (Scheme 12.26). 

Bis(imino)pyridine iron complex 5 acts as a catalyst not only for hydrogenation (see 2.1) but also for hydrosilylation of multiple bonds [27]. The results are summarized in Table 10. The reaction rate for hydrosilylations is slower than that for the corresponding hydrogenation however, the trend of reaction rates is similar in each reaction. In case of tra s-2-hexene, the terminal addition product hexyl (phenyl)silane was obtained predominantly. This result clearly shows that an isomerization reaction takes place and the subsequent hydrosilylation reaction dehvers the corresponding product. Reaction of 1-hexene with H2SiPh2 also produced the hydrosilylated product in this system (eq. 1 in Scheme 18). However, the reaction rate for H2SiPh2 was slower than that for H3SiPh. In addition, reaction of diphenylacetylene as an atkyne with phenylsilane afforded the monoaddition product due to steric repulsion (eq. 2 in Scheme 18). 

In the course of studying the reactions of Si-H compounds with dialkyltitanocenes, with a view to the synthesis of new hydridosilyltitanocene complexes, we adventitiously discovered that phenylsilane undergoes facile, quantitative dehydrogenative coupling to a linear poly(phenylsilylene) under the catalytic influence of dimethyltitanocene. The ease with which this reaction proceeds initially induced us to underestimate the significance of the observation. 

Normally, no small cyclopolysilanes are observed in these reactions. Two exceptions we have noted are the very slow reaction of phenylsilane under the influence of Cp2TiMe2 and the reaction of benzylsilane under the influence of dimethyltitanocene at very long reaction times. From both of these reactions we isolate a single isomer of the cyclohexasilane, in ca. 10 per cent yield in the case of the phenylsilane and ca. 60 per cent yield in the case of the benzylsilane. These isomers are believed to be the all-trans isomers. The phenyl derivative is identical to that... 

The compounds 2 and 3, have all been isolated from reactions of phenylsilane with either dimethyltitanocene (1 2) or dimethyl-zirconocene (r ). All of the evidence points to the fact that these compounds are probably resting species and are not involved in the catalytic cycle. They do nevertheless give some indication of the complex series of reactions that transform the dimethyl-metallocene to active catalyst. 

Unlike pentamethylphenyldisilane, the photolysis of compound 5 in the presence of methanol in hexane gave no products arising from the reaction of a silene with methanol, but polymeric substances were obtained as main products, in addition to small amounts of bis-(trimethylsiloxy)phenylsilane (6) (4%) and bis(trimethylsiloxy)-phenylmethoxysilane (7) (5%). 

Fig.40a-c Silicon-29 and proton spectra of phenylsilane PhSiH3 in C6D6. a INEPT spectrum with complete proton decoupling, b proton-coupled INEPT spectrum l]sm 200 Hz) the fine structure is due to coupling with the aromatic protons, c proton spectrum showing 29Si satellites for the SiH protons)... 

Figure 4. Schematic of torsional energy levels in Si electronic state of phenylsilane and Do electronic state of phenylsilane+. The torsional state symmetry labels arise under the molecular symmetry group G12.

Reduction of either the exo or endo isomer of 2-phenyl-2-norbornanol with trifluoroacetic acid and triethylsilane, triphenylsilane, or phenylsilane in dichloro-methane gives endo-2-phenylnorbomane quantitatively (Eq. 24).164 The stereospecific formation of only the endo-hydrocarbon can be understood on the basis that only exo approach by organosilicon hydride toward the 2-phenylnorbornyl cation intermediate is kinetically competitive for product formation.164... 

The 1,4-reduction of a,/3-unsaturated aldehydes is best carried out with diphenylsilane in the presence of zinc chloride and tetrakis(triphenylphosphine) palladium436 or a combination of triethylsilane and tris(triphenylphosphine) chlororhodium 437 Other practical approaches use phenylsilane with nickel (0) and triphenylphosphine438 and diphenylsilane with cesium fluoride.83 It is possible to isolate the initial silyl enol ether intermediate from the 1,4-hydrosilylation of o, /3-unsaturated aldehydes (Eq. 264).73,411 The silyl enol ethers are produced as a mixture of E and Z isomers. 

Alkylated (R,RHetrahydroindcnyItitanium difluoride and phenylsilane serve to asymmetrically reduce a variety of ketones, especially aryl alkyl ketones, in excellent chemical yields and >96% ee.587 The use of the easier to handle and less expensive PMHS is also highly effective in these transformations. In a related study using the (W, W)-tctrahydroindcny I titanium l,T-binaphth-2,2/-diolate precursor to the active catalyst, similarly impressive results are obtained.588... 

The reaction was carried out by addition of 1.95 equivalents of -BuLi to a THF solution of 22 at 0°C to generate the active catalyst, which was then combined with substrate (S/C about 20 1) under an inert atmosphere using phenylsilane as the stabilizing agent. Trisubstituted unfunctionalized olefins can be hydrogenated in good yield with high ee. Representative results are listed in Table 6-3. 

Ti-F bond and generate a Ti-H species when 99 was treated with phenylsilane. The chirality transfer may take place through imine insertion into the Ti-H bond, similar to that in the catalytic hydrogenation process.1000 The reaction can be carried out by the subsequent addition of imines. The corresponding silylated amines can be obtained and further converted to enantiomerically enriched amines upon acid treatment. For example, in the presence of 99, N-methylimine 100 undergoes complete hydrosilylation within 12 hours at room temperature, with 97% ee and up to 5000 turnovers.103... 

The initial electron transfer to form the anion radical species seems to be reversible. For example, Allred et al. investigated the ac polarography of bis(trimethylsilyl)benzene and its derivatives which showed two waves in di-methylformamide solutions [71] the first one is a reversible one-electron wave, and the second one corresponds to a two-electron reduction. Anion radicals generated by electrochemical reduction of arylsilanes have been detected by ESR. The cathodic reduction of phenylsilane derivatives in THF or DME at — 16° C gives ESR signals due to the corresponding anion radicals [5] (See Sect. 2.2.1). 

Silane redistribution 6 mg of (3-(methylsilyl)propyl) triphenylphosphonium hexafluorophosphate(V) (0.0121 mmol, 1 equivalent), 93 mg of Karstedt s catalyst (Pt 3.91% 0.000954 mmol, 0.8 equivalent), 6 mg of triphenylphosphine (0.0229 mmol, 1.9 equivalent) were dissolved in 20 mL of freshly distilled fluoroben-zene in a 50 mL Schlenk flask in an inert atmosphere glovebox. The flask was equipped with a septum, removed from the glovebox and installed with PEEK tubing leading to the mass spectrometer. The flask was pressurized with N2 and monitored by ESI-MS. Once a steady signal was obtained, 0.15 g of freshly distilled phenylsilane (1.39 mmol, 115 equivalents) was injected into the reaction flask. The reaction was initiated at room temperature and then was heated to reflux after 20 min to drive it to completion. 

The adverse effect of the hydrophilic silica was attributed to the condensation reaction of surface silanol groups on the silica and phenylsilane moieties on the polymer backbone. This results in increased cross-linking via formation of siloxane bonds between the polymer and silica. 

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