Silanes boron halides

Halosilanes - [SILICONCOMPOUNDS - SILANES] (Vol 22) -boron tnchlonde in prodn of [BORON COMPOUNDS - BORON HALIDES] (Vol 4)... 

Matteson, D. S. and R. Soundararajan. 1990. Hydroboration with boron halides and trialkyl-silanes. J. Org. Chem. 55 2274. 

The simplest composite fibres are the B and SiC monofilaments manufactured by chemical vapour deposition of boron halide or silane gases onto a heated substrate, normally W or C. Their diameter is in the range of 50-150 p.m, and they exhibit a concentric microstructure in which four different regions can be distinguished, the composition and thickness of each one depending on the actual fibre characteristics (Fig. 9). A thin (1-2 i.m) layer of pyrolytic graphite is deposited onto the C core before the deposition of... 

One of the earliest methods for preparing aromatic boronic acids involved the reaction between diaryl mercury compounds and boron trichloride [198]. As organomer-curial compounds are to be avoided for safety and environmental reasons, this old method has remained unpopular. In this respect, trialkylaryl silanes and stannanes are more suitable and both can be transmetallated efficiently with a hard boron halide such as boron tribromide [199]. The apparent thermodynamic drive for this reaction is the higher stability of B-C and Si(Sn)-Br bonds of product compared to the respective B-Br and Si(Sn)-C bonds of substrates. Using this method, relatively simple arylboronic acids can be made following an aqueous acidic workup to hydrolyze the arylboron dibromide product [193]. For example, some boronic acids were synthesized more conveniently from the trimethylsilyl derivative than by a standard ortho-metallation procedure (entry 11, Table 1.3). 

In common with the synthesis of trifluoroisopropenyl-boronic acid, trifluoro-isopropenylsilane can be also prepared in 80% yield by treatment of 2-BrTFP with 1.2 equivalents of magnesium in the presence of 2.0 equivalents of dimethylphenylsilyl chloride in THF at — 10°C to room temperature (Scheme 26.45). This reagent can employ as CF2=C -CH2 synthon, but there have been no reports on the transition metal-catalyzed cross-coupling reaction of tri-fluorinated silane with halides. 

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]. 

Allenyllithium reagents are commonly prepared through lithiation of propargylic halides or by deprotonation of alkynes or certain allenes (Eq. 9.1). Lithiated allenes often serve as precursors to stable allenylmetal compounds such as stannanes or silanes. They can also be employed for the in situ synthesis of allenylzinc, -titanium and -boronate compounds, which can be further transformed to substitution products not accessible from their allenyllithio precursors. 

Li, L.-F. and You, X.-Z. (1993a). A Topological Index and Its Application. Part 3. Estimations of the Enthalpies of Formation of Mixed Halogen-Substituted Methanes, Silanes and Boron Mixed Halides. Thermochim.Acta, 225,85-96. 

Some authors have described the generation of boronates and stannanes bound to a support starting from the corresponding aryl haHdes [372, 6]. Boranes, boronic esters, and stannanes can furthermore be readily obtained from vinyl halides or from alkenes or alkynes by means of hydroboration or hydrostannylation (see Section 4.2). Boronates and silanes or stannanes can act as carbanion equivalents. Thus, support-bound boronates can release aryl alkenyl groups upon transmetala-tion to Rh. The intermediately formed Rh species can act as nucleophiles, and react with aldehydes to give alcohols (618) or can perform Michael additions (621, 622) [437, 438, 439] (Scheme 129) (see also Sections 4.7.15 and 4.2). 

The most common catalysts in order of decreasing reactivity are halides of aluminum, boron, zinc, and iron (76). Alkali metals and their alcoholates, amines, nitriles, and tetraaLkylureas have been used (77—80). The largest commercial processes use a resin—catalyst system (81). Trichlorosilane refluxes in a bed of anion-exchange resin containing tertiary amino or quaternary ammonium groups. Contact time can be used to control disproportionation to dichlorosilane, monochlorosilane, or silane. 

Coupling Reactions. The palladium-catalysed cross-coupling of E—(1-alkyl-l-alkenyl)boronates with alkyl halides has been shown to permit stereo- and regio-specific access to trisubstituted alkenes in which the introduced alkyl group occupies the position formerly occupied by the boron substituent.201 A1lyl-silanes may be obtained... 

Allyl silanes are rather like silyl enol ethers they react with electrophiles, provided they are activated, for example by a Lewis acid. Titanium tetrachloride is widely used but other successful Lewis acids include boron trifluoride, aluminium chloride, and trimethylsilyl triflate. Electrophiles include acylium ions produced from acid chlorides, carbocations from tertiary halides or secondary benzylic halides, activated enones, and epoxides all in the presence of Lewis acid. In each case the new bond is highlighted in black. 

Type I Terminal alkenes, aUyl silanes, primary aUylic alcohols, ethers, esters, aUyl boronate esters, aUyl halides... 

Some alkenes with heteroatoms can be used in cross-coupling reactions. Vinyl boranes are an important group, as the carbon-boron bond can subsequently be converted into so many other functional groups, such as halides (Scheme 8.97). ° An example of cross-metathesis of a vinyl borane, followed by Suzuki coupling, can be found in Scheme 11.40. An example of a vinyl silane metathesis can be found in Scheme 2.110. 

CM reactions can also be used to prepare partners for cross-coupling reactions such as vinyl boronates, vinyl silanes, or vinyl halides, which are type II olefins. 

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