Silyl bromide

A number of approaches have been tried for modified halo-de-diazoniations using l-aryl-3,3-dialkyltriazenes, which form diazonium ions in an acid-catalyzed hydrolysis (see Sec. 13.4). Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides (Ku and Barrio, 1981) without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60-95%. The authors explain the reaction as shown in (Scheme 10-30). The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X = CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the A-aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations. 

According to spectral evidence the complexes of silyl bromide and silyl iodide with strong donors are also ionic 40h... 

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]. 

Evidence for the existence of trisilylphosphine was found for the first time by Aylett, Emeleus and Maddock in the reactions between silyl iodide, HsSil, and phosphine or between silyltrialkylammonium iodide and phosphine. According to Amberger the reaction of potassium dihydrogenphosphide with silyl bromide is suitable for the preparation of trisilylphosphine. The reaction. 

With ethyl iodide, ethylphosphine and phosphine are formed. The reaction with excess silyl bromide results in the formation of silylphosphine. Silylphosphine, like methylphosphine, forms an adduct with diborane... 

In addition, 20 is assumed to be formed as the initial product, when the silyl bromide 21 is treated with Na/K alloy in n -heptane. The fast coupling of intermediate 20 with bromide 21 results in the formation of dimer 22 in 80% yield (equation 3A)M. 

Group IV element halides, such as silyl bromide, trimelhylstannyl chloride, and dimethylgermyl dichloride, cleave the Te —Si bond in organo trimethylsilyl tellurium with removal of trimethylsilyl halide1. 

The synthesis of small arrays of organic compounds derived from multicomponent condensations was recently reported by Studer et al. (119). A 10-member amino acid amide library L7 (Fig. 8.21) was prepared using the fluorous Ugi ( Flugi ) condensation, and another 10-member dihydropyrimidine library L8 (Fig. 8.21) was prepared using the Biginelli ( Fluginelli ) condensation adapted to the fluorous phase. The key intermediates for library preparation were the silyl bromide 8.36, prepared from a fluorous iodide (120), and the acyl bromide 8.37 and the acid 8.38, prepared from an orthothiobenzoate (121), as shown in Fig 8.21. The structure of the fluorous tag was... 

The TDS ester is prepared from a carboxylic acid and the silyl bromide by reaction with AgOTf in CH2CI2 (84-93% yield). It is stable to -BuLi, LiAlHt, AcOH, aqueous NaOH at 50 C, and 1 N HCl at 40°C, but is cleaved with Pyr-HF, THF, 50°C and f-BuOK/DMSO at 25°C. It is not deprotonated at the a-carbon of the ester with -BuLi and thus this group also serves to protect these hydrogens from enolization. ... 

The unusual amino acid 56 has been synthesised6 by alkylation with the silylated bromide 61 prepared from commercially available 59. You might notice the radical bromination step giving only moderate yields. 

The synthesis of a,j5-dihydroxyphosphonic acids is easily accomplished by the addition of lithiated diisopropyl phosphite to 464. Unfortunately, under these conditions there is no asymmetric induction, and a 1 1 mixture of 563 and 564 is produced, but the two are separable by flash chromatography [177]. The highest ratio of diastereomers (1 3) is obtained using diisopropyl trimethyl phosphite. Treatment of either 563 or 564 with trimethyl-silyl-bromide followed by hydrogenolysis of the benzyl protecting group furnishes the corresponding free dihydroxyphosphonic acid. 

Mono- or difluorophosphonates la and lb were reacted with trimethyl-silyl bromide leading to the phosphonic adds 2a and 2b after solvolysis in methanol (Scheme 1). 5-Substituted 1,2-oxaphospholane formation was accomphshed by intramolecular DCC-mediated couphng. The last step consisted of the formation of the esters 3a and 3b by reaction of the appropriate fatty acyl chloride in the presence of pyridine. 

Under Lewis acidic conditions, the silyl bromide (1) and a, -dimethoxylated amides undergo a [3 + 3] type annulation which is useful for the construction of the piperidine skeleton (eq 13). 7 xfjg silyl bromide (1) is also an excellent reagent for the stereoselective introduction of a functionalized allyl group at the anomeric center of the glucosides (eq 14). ... 

Coupling Reactions to Dienes and Enynes. The silyl bromide (1) participates readily in copper or transition metal-mediated coupling reactions to produce 1,3-butadienes, which are very useful s)mthetic intermediates. For example, 2,3-bis[(trimethylsilyl)methyl]-1,3-butadiene (8), derived from the oxidative dimerization of cuprate (2) (M = Cu) is useful for rapid construction of multicyclic systems via tandem Diels-Alder reactions, as depicted in eq 15 The diene 2-dimethylaminomethyl-3-... 

Palladium-catalyzed coupling of the silyl bromide to the terminal alkyne (10) gives the propargylic allylsilane (11), which is a key intermediate in the synthesis of a 10-membered cyclodiynol analog (12) of the antitumor agent neocarzinostatin (eq 19). ... 

---