Silicon direct reaction

The remarkable silicon-directing reaction of electrophiles at the a-carbon of alkynylsilanes is further evident when comparing the intramolecular cyclizations of methyl- vs. silicon-substituted triple bonds, shown below. 

The 1,2-silyl migrative [3+2] cycloadditions of allylsilanes are applicable to a variety of electron-deficient multiple bonds. High diastereoselectivity is usually observed. These silicon-directed reactions are, therefore, valuable for stereocontrolled syntheses of highly functionalized five-membered carbocycles and heterocycles. 

Denmark and co-workers reported a good example of torquoselection in the silicon-directed Nazarov cyclization (see Section 3.4.5.1). They demonstrated that cyclohexenyl-derived divinyl ketones 26 cyclize to give the relative stereoisomer 27 as the major product (see Section 3.4.5.1 for the mechanism of the silicon-directed reaction). The use of bulky alkyl groups (such as /-butyl) and/or bulky silicon substituents gave the best selectivity, at the expense of the chemical yield. It is interesting that the corresponding cyclopentenyl-derived systems gave only poor torque-selectivity. 

An alternative strategy to improve the stereoregularity of electrogenerated PTs consists in the activation of the a-positions of the thiophene ring to increase the selectivity of a-a coupling during polymerization. A first attempt in this direction involved the oxidative polymerization of 2,5-dilithiothiophene in the presence of CuCb [54]. More recently, the use of silicon-directed reactions was proposed as a means to improve the selectivity of a-a coupling. Whereas the polymer synthesized from... 

Organosilanes, especially dimethyldichlorosilane (M2), are important chemicals used in the silicone industries. The direct reaction of silicon with an organic halide to produce the corresponding organosilanes as a gas-solid-solid catalytic reaction was first disclosed by Rochow [1]. In the reaction, a copper-containing precursor first reacts with silicon particles to form the catalytically active component, which is a copper-silicon alloy, the exact state of which is still under discussion. As the reaction proceeds. Si in the alloy is consumed, which is followed by the release of copper. This copper diffuses into the Si lattice to form new reaction centers until deactivation occurs. The main reaction of the direct process is ... 

More than 60 years after its simultaneous discovery by Rochow and Muller, the direct reaction of copper-activated silicon with alkyl chlorides is arguably still the most important industrial process for the preparation of basic organosilanes. An inspiring historic account highlighting the significance of this seminal work has been given by Seyferth.12 A comprehensive review on the subject has been written by Jung and Yoo.13 The most recent work associated with the direct process is concerned with the role of metallic promoters, such as Zn and Cd, as well as mechanistic aspects.14... 

Catalyzed Direct Reactions of Silicon edited by K.M. Lewis and D.G. Rethwlsch... 

It is also possible to produce covalently bonded alkyl MLs on Si(l 11) surfaces using a variety of chemical reactions with passivated H-terminated Si(l 11), but the preparation methods are more complex than the immersion strategy in part due to the higher reactivity of silicon. This is a major achievement because it allows direct coupling between organic and bio-organic materials and silicon-based semiconductors. Both pyrolysis of diacyl peroxides (Linford Chidsey, 1993) and Lewis acid-catalyzed hydrosilylation of alkenes and direct reaction of alkylmagnesium bromide (Boukherroub et al, 1999) on freshly prepared Si(lll)-H produce surfaces with similar characterishcs. These surfaces are chemically stable and can be stored for several weeks without measurable deterioration. Thienyl MLs covalently bonded to Si(l 11) surfaces have also been obtained, in which a Si(l 11)-H surface becomes brominated, Si(lll)-Br, and is further reacted with lithiated thiophenes (He etal, 1998). 

Direct reaction of iron pentacarbonyl with trimethylsilyl isocyanide ( C=N—SiMe3) at 65°-75° yields an air-sensitive substitution product Me3Si—N=C Fe(CO)4 in 93% yield, with elimination of carbon monoxide (152). It was shown by infrared spectroscopy (38) that complex formation lowers the N=C bond order for Me3Si—N=C , whereas it raises the N=C bond order for Me3C—N=C , presumably as a result of interaction between dv orbitals of silicon with the metal d orbitals. 

In a simple strategy to biaryl formation, Han et al.89 showed that silicon-directed ipso-substitution and concomitant cleavage from supports could be used for formation of functionalized biphenyls. For this they used a tethered silyl aryl bromide in a Suzuki cross-coupling reaction, followed by the ipso-substitution/cleavage step (Scheme 39). A variety of boronic acids were coupled in this manner. The only difficulty occurred with electron-deficient nitrophenylboronic acid where the desired product was formed under anhydrous conditions in only 33% yield (the remainder being starting material). Reversion to the more usual conditions of aqueous base-DME (i.e., those used by Frenette and Friesen)70 improved the yield to 82%. 

Han, Y. Walker, S. D. Young, R. N. Silicon Directed ipso-Substitution of Polymer Bound Arylsilanes Preparation of Biaryls via the Suzuki Cross-Coupling Reaction, Tetrahedron Lett. 1996, 37, 2703—2706. 

Catalyzed Direct Reactions of Silicon Lewis, K. M., Rethwisch, D. G., Eds. Elsevier Amsterdam, 1993. 

In the late 1980s, E.A. Pngar and P.E.D. Morgan and a team of researchers at the Rockwell International Science Center, Thousand Oaks. California, conducted a thorough effort to understand Low Temperature Direct Reactions Between Elemental Silicon and Liquid Ammonia or Amines for Ceramics and Chemical Intermediates. Details are given in reference died. 

Gung and coworkers42 have located (at the MP2/6-31 G level of theory) the transition state for the silicon-directed aldol reaction between a silyl enol ether and formaldehyde. They found it to be a boatlike six-membered ring 21 with a pentacoordinate silicon (equation 9). 

V. SILICON DIRECT PROCESS REACTIONS WITH REAGENTS OTHER... 

Lewis and coworkers have also made significant contributions to the understanding of the MCS reaction via the use of surface studies72. XPS and AES analysis of catalytically active surfaces showed that zinc causes a restructuring of the Cu3Si surface. Additionally, zinc enrichment is enhanced by the addition of SnCLt. Lead is a well known poison for the direct reaction and the Lewis group found that lead suppressed enrichment of the Cu3Si surface in zinc and silicon. 

There are many examples of the direct reaction of silicon with ethyl chloride, vinyl chloride and chlorobenzene1. Vinyl and allylchlorosilanes76 were first made via a direct process in 1945 as were phenylchlorosilanes77. Jung s group has recently extended the direct reaction of silicon with a variety of substrates including allyl chloride78. Silicon... 

A key report investigated a variety of substrates in their reaction with silicon in an effort to find evidence for silylene intermediates during the silicon direct process reaction. When silicon, copper and methanol were reacted as described above but in the presence of alkenes, alkyldimethoxysilanes and (MeO SiH were formed95-97. The use of allyl propyl ether instead of alkenes gave allyldimethoxysilane, with 38% selectivity. These results and the reaction of silicon with MeCl in the presence of butadiene to give silacyclopent-3-enes indicates intermediate formation of silylenes. 

Silicon-directed stereoselective syn addition of the hydroxy group to the olefinic double bond occurs intramolecularly in a TiCLt-or TsOH-catalyzed cyclization of vinylsilanes 139 bearing a hydroxy group207. The reaction gives mainly the irans-isomer (trans cis = 80-90 20-10) (equation 116)208. It is noteworthy that no cyclization is observed in the absence of the silyl substituent under the same conditions. 

The direct reaction of methyl chloride with silicon metal is the foundation stone of the worldwide silicone industry352. In corporate laboratories over the years, the reaction has been carefully engineered to provide the maximum amount of the desired product, dimethyldichlorosilane. Despite the industrial importance of the direct reaction, and the great amount of research devoted to it, its mechanism is still obscure353. Recently, however, a model has been suggested in which silicon in silylene form provides the crucial intermediate. 

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