Bromosilane

Anhydrous diethyl ether, freshly obtained from a commercial supplier, is preferred for the reactions involving lithium hydro-aluminaie. The ether must be peroxide-free. Carbon dioxide must be rigorously excluded from these reaction systems. Explosions have been observed during evaporation of solutions of aluminum hydride and related compounds when carbon dioxide is present as an impurity.If it is necessary to concentrate ether solutions of lithium hydroaluminate by distillation, the following precautions must be observed. A large fluid volume must be maintained in the distillation flask such solutions should never be concentrated so far that very little ether remains in the reaction flask. Distillations must be effected in a protective atmosphere. Cyclic ethers, and especially tetrahydropyran solutions of lithium hydroaluminate, present a far greater explosion risk than diethyl ether solutions. 

A solution of fresh lump lithium hydroaluminate in diethyl ether is prepared by stirring 75 g. (1.98 moles, 25% excess) of 

Into a Pyrex tube, 44 X 275 mm., equipped with a standard-taper 29/42 joint, is condensed 100 g. (1.25 moles) of anhydrous hydrogen bromide. To this, under an atmosphere of nitrogen, is added 60 g. (0.555 mole) phenylsilane, b.p. 119°/754 mm. The reaction tube (with its contents at —196°) is fitted with a vacuum-stopcock adapter (which is wnred firmly into position) 

I Obtainable from Air Products and Chemicals, Inc., Allentown, Pa., or the Matheson Company, Inc., East Rutherford, N.J. It also may be synthesized.  

Bromosilane is purified in 10- to 15-g. portions by slow distillation through a simple low-temperature column held at —78° through a —126° trap and into a —196° trap (no pumping). Bromosilane condenses in the —126° trap. The bromosilane has a vapor pressure of 706.0 mm. at 0° (literature, 710 mm.). The infrared spectrum of the vapor is identical with those spectra published for bromosilane. 

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Only little information is available on carbene formation by rearrangement of bridgehead olefins, generated in thermal reactions. A prominent example is the rearrangement of bridgehead olefin 2, obtained as short-lived intermediate from bromosilane 1 by reaction with potassium fluoride in DMSO at 110°C. Carbene... 

During the preparation of the complexes of chromium, molybdenum and tungsten from K[M(CO)3C5H5] and bromosilane, the residues from sublimation of the products are all pyrophoric. 

Silyl radicals are also involved as the reactive intermediates during one-electron reduction of bromosilanes. As an example. Reaction (1.7) shows the reduction by sodium of a silyl bromide to produce a persistent radical, which has been characterized by EPR spectroscopy [12]. 

Bromosilanes, hydrolysis, 42 162-163 Bromouranate salts, heats of formation, 34 99 Bromyl fluoride... 

The checkers have prepared disilathiane and hexamethyldisilathiane in high yield by reacting the appropriate tertiary and secondary amine with H jS for 2 hr and then treating the solid residue with an excess of the appropriate chlorosilane or bromosilane. 

Moreover, in the divided cell the exo.endo ratio of bromosilanes was 91 9 in the anode compartment bnt only 52 48 in the cathode compartment. Thus, the nature of the ultrasonic effect was explained assuming that beside the electrochemical silylation at the cathode, a parallel silylation process occurs at a magnesium anode, namely the silylation by 70 of an intermediate Grignard reagent produced from dibromide 69. It appears as a rare example of the anodic reduction However, the increase in the current density dnring electrolysis cansed a decrease in the apparent current efficiency. This observation indicates a chemical natnre of the anodic process. Of course, the ultrasonic irradiation facihtates the formation of the organomagnesium intermediate at the sacrificial anode and the anthors reported a similar ultrasonic effect for the nonelectrochemical but purely sonochemical... 

The bromosilane obtained by reaction of the phenylsiloxane 1 with bromine in the presence of triethylamine or sodium siloxide led Kakimoto et al. to the siloxane core building block 4. It contains three phenylsilane-terminated disilox-ane branching units, which should minimise steric hindrance on construction of subsequent generations. A sequence of bromination, amination, and alcoholysis ultimately leads to the third-generation polysiloxane 5 (Fig. 4.50) [99]. 

Tris(trimethylsilyl)chlorosilane is obtained in 50% and 80-90% yield when the corresponding hydride is treated with phosphorus pentachloride and carbon tetrachloride, respectively, while tris(trimethylsiIyl)bromosilane is produced in 79% yield when the hydride is allowed to react with 1-bromo-butane (59). 

The action of liquid sodium amalgam on trimethylchlorosilane (34a, 34b) or -bromosilane (207) at room temperature results in the exclusive formation of bis(trimethylsilyl)mercury, [(CH3)3Si]2Hg, which is relatively stable to heat. This compound, however, undergoes decomposition on heating at 100°-160° C for a day to give hexamethyldisilane in quantitative yield (9, 34a, 34b, 207). 

Fraser-Reid and co-workers have examined serial radical cyclization of pyranose-derivatives [95AJC333] in the stereocontrolled synthesis of Woodward s reserpine precursor [95JOC3859]. Treatment of the bromosilane 188 under reductive conditions resulted in a 5-exo followed by a 6-exo cyclization. The intermediate radical eliminates phenylsulfinyl radical to provide the alkene 189 as the product. The intermediate has been converted to the reserpine precursor 190. The temporary silicon method has been utilized for the synthesis of brassinolide side chain [95SL850]. 

Methylbromosilanes can be obtained by the direct synthesis from MeBr and silicon, but are normally obtained on a small scale by exchange reactions. Siloxanes can be converted into bromosilanes by reaction with phosphorus tribromide91 (equation 55). 

Similarly, 144 has been obtained from the reaction of 1-trimethylsilylcyclopropyl methyl selenide with n-BuLi The a-bromosilane 147 underwent lithiation with n-BuLi in THF at —78 °C to provide 144 with superior efficiency to any other method, Eq. (46))81). 147 was prepared in large quantities by the Hunsdiecker degradation of the 1-trimethylsilylcyclopropanecarboxylic acid 146, obtained by successively reacting the commercially available cyclopropanecarboxylic acid with -BuLi (2 equivalents) and ClSiMe3 82). Uneventfully, 144 added to carbonyl compounds, except for cyclopentanone where enolate anion formation competed the 1-trimethylsilylcyclo-propylcarbinols 148 underwent acid-induced dehydration to the expected 1-trimethyl-silylvinylcyclopropanes 149 79-81) while base induced elimination (KH, diglyme, 90 °C) led to cyclopropylidenecycloalkanes 150 77), Eq. (47). 

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