Organosilyl ethers

The application of the RCM approach to create a range of cyclic silyl ethers successfully expands the scope of reactions utilizing the temporary sihcon tether approach, and the novel concept of heteroatom activation of the sihcon from the coupling substrate itself Thus, in cases where homoallylic alcohols or mediumsized rings are the desired products, the aforementioned methods offer a facile and direct route for their synthesis. 

Despite the diversity of organosilyl ether cross-coupling systems, all of the aforementioned examples are similar in one aspect they employ alkenylsilyl ethers. Arylsilyl ethers (specifically, aryl orthosiliconates), however, have played a significant role in biaryl synthesis. 

Alkyl-alkyl cross-coupling reactions have historically been the most difficult to realize. Among the many obstacles to the effective development of such a system are the lower reactivity of alkyl groups relative to alkenyl and aryl groups, as well as side processes such as -hydride elimination that are accessible with alkyl substrates. More reports by Fu et al. have disclosed successful alkyl cross-coupling protocols 

The breadth of organosilyl ether cross-coupling now rivals that of organosi-lanols. There are, however, a few limitations most notably that some of these systems - particularly in biaryl synthesis - require a large excess of organosilane relative to the electrophile employed. In spite of this, organosilyl ethers are a useful class of substrates because of the ease of synthesis of many of the precursors, as well as their stability and high reactivity. 

All of the aforementioned variants of silicon cross-coupling have highlighted the activating ability of oxygen substitution on the silicon. The advantages of silanols, silyl ethers, and siloxanes over other classes of organosilicon precursors such as 

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The bonding arrangement Si—O—C— is characteristic of both organosilyl ethers, Ri —Si(OR ) and the two kinds of cyclic compounds, organo-l,3,2-dioxasila heterocycles (Section 3.5.5) and silatranes (Section 3.5.6). 

Organosilyl ethers are versatile reactants in the manufacture of silicones and in other chemical syntheses (Chapter 4). They are liquids which can be distilled. They are more slowly hydrolysed than haloorganosilane, and the reaction is more easily controlled. 

In the presence of a catalytic amount of iodine, tetraalkoxysilanes react directly with magnesium and an alkyl or aryl halide. A mixture of 2 parts magnesium, 1 part tetraethoxysilane and 2 parts bromobenzene in toluene yields, after 6 h at 100 °C, the following organosilyl ethers triethoxyphenylsilane (18-20 % b.p. 234°C), diethoxy-diphenylsilane (40-42% b.p. 302-304°C) and ethoxytripkenylsilane (5-6% m.p. 63-64°C) [275]. At the molar ratio 4 1 4, ethoxytriphenylsilane is the favoured product (60-65%). [276]. 

Triethoxyphenylsilane and diethoxydiphenylsilane (weight ratio 3 1) are the products of the reaction between tetraethoxysilane, magnesium and chlorobenzene at 160°C in an autoclave [276]. Triethoxy(phenylethynyl)silane [277] (75% b.p. 141-142 °C) can be prepared from (phenylethynyl)magnesiumbromide, or triethoxy-ethylsilane [278] (84 % b. p. 158-160 °C) from ethylmagnesium bromide. Trialkylox-yhydrosilanes also react with olefins to form organosilyl ethers, e.g. to (3-benzyloxypropyl)-trimethoxysilane [279] (63% Eq. 3.117) ... 

Diethylmethylsilane is an effective precursor for the preparation of organosilyl ethers. In the presence of rhodium, ruthenium or cobalt catalysts, this hydroorganosilane... 

E)-AUylsilanes and allenylsilanes. The preparations from allylic and propargylic alcohols via the (silyl)silyl ethers involve bis(organosilyl)Pd(II) complexes to insert into the multiple C-C bonds and a subsequent yyn-elimination of the ensuing oxasilacyclobutanes to afford the products and PhjSi=0. 

Like organosilyl cations stabilized by two 2-(methoxymethyl)phenyl ligands [4], 11 reacts with pyridine with transfer of a methyl group and formation of a cyclic silyl ether 17. In contrast to organosilyl cations chelated by two 2-(methylthiomethyl)phenyl ligands, a displacement of the intramolecular donors by pyridine is not possible in 12 (Scheme 3). 

A still more stable organophilic surface on silica particles is obtained by reacting the surface with alkyl chlorosilanes, thus attaching organosilyl groups. For example, the surface of colloidal silica was covered with trimethylsilyl groups by Her (460), who transferred silica from water to triethyl phosphate, dried the sol, added trimethyl-chlorosilane, then heated and removed excess reagent and solvent by vacuum evaporation. The solid product was dispersible to form sols in benzene, ether, and chloroform, but not in water. 

The zinc and cadmium organosilyl compounds are about equally unstable, in sharp contrast to the mercury compounds. The reaction of lithium tetrakis(trimethylsilyl) aluminate with zinc acetate in diethyl ether yields (25 %) bis(trimethylsilyl) zinc [435]. This compound can be kept for about three weeks under an inert gas at — 20 °C. The reaction of lithium tetrakis(trimethylsilyl) aluminate with cadmium acetate forms bis(trimethylsilyl) cadmium [435] (27%, very unstable, sensitive to light). Bis(tri-t-butylsilyl) cadmium [436] (m. p. 140 °C, slightly yellowish crystals which turn greenish black on exposure to air) can be obtained from tri-r-butylsilane and diethylcadmium (Eq. 3.230) ... 

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