Silicon reaction scheme

The structurally simplest silicon reagent that has been used to reduce sulphoxides is the carbene analog, dimethylsilylene (Me2Si )29. This molecule was used as a mechanistic probe and did not appear to be useful synthetically. Other silanes that have been used to reduce sulphoxides include iodotrimethylsilane, which is selective but unstable, and chlorotrimethylsilane in the presence of sodium iodide, which is easy to use, but is unselective since it cleaves esters, lactones and ethers it also converts alcohols into iodides. To circumvent these complications, Olah30 has developed the use of methyltrichlorosilane, again in the presence of sodium iodide, in dry acetonitrile (equation 8). A standard range of sulphoxides was reduced under mild conditions, with yields between 80 and 95% and with a simple workup process. The mechanism for the reaction is probably very similar to that given in equation (6), if the tricoordinate boron atoms in this reaction scheme are replaced... 

The two acetylenic functions in the silicon or tin derivative make possible a competition between 1 1 and 2 1 reactions (Scheme 39, pathway b), and the product ratio depends significantly on the reaction conditions. For silicon derivatives an excess of 1-boraadamantane in the reaction mixture (acetylenic component added to 1-boraadamantane) leads to the octacyclic compounds 88 (up to 60%), while when the much more reactive 1-alkynyltin analogues are used, a second intramolecular 1,1-organoboration often takes place <2001CEJ775>. The stmcture of 88c (M = Sn, R = Me, R1 = SiMe3) was confirmed by X-ray analysis <2001CEJ775>. 

The reaction of triallylborane with silicon triyne 123 is interesting. A113B attacks both internal and external triple bonds giving rise to silole 124 and two heterocycles with bridgehead boron 125 and 126 in a 1 3 3 ratio as a result of competitive sequential reactions (Scheme 52). When 1,1-allylboration of the internal C C bond followed by intramolecular 1,1-vinyIboration takes place, the silole 124 is formed, while in another case 1,1-allylboration followed by a series of intramolecular 1,2-allylboration reactions leads to boron derivatives 125 and 126 <2002JOM(657)146>. 

The intermediate formation of betaines and their subsequent irreversible isomerization to silylated phosphorus ylides have previously been postulated for the reactions of phosphorus ylides with hexamethylsilirane,107 sila-108 and disilacyclobutanes109 with different substituents at the silicon atom (Schemes 34 and 35). 

In the second, complementary, approach the polycondensation of silica polymer is followed by the formation of an organic network made by cross-linking reaction of monomers covalently bound to silicon compounds (Scheme 4.1) resulting in polymeric materials with outstanding protective abilities, including thermal, mechanical and corrosion resistance. 

The sensing of fluoride with a reaction-based indicator can be based on its unique reactivity with silicon. As shown in Fig. 20, the O-Si bond cleavage with fluoride can be used to produce the coumarin dye 53. For this system, a 100-fold sensitivity increase was seen after incorporation of the reaction scheme into a conjugated polymer [143]. 

The final step in this sequence is the coupling of a suitable chiral group to the silicone. In reaction scheme 3 the attachment of an amino acid amide residue is given as an example. The carboxy functional copolymer is dissolved in dichlormethane/dimethylformamide and an amino aid -amide is coupled with dicyclohexyl... 

Fig. 4.1 Reaction scheme proposed for the chemical dissolution of (100) oriented silicon surfaces in alkaline solutions.

Fig. 4.3 Reaction scheme proposed for the anodic, divalent dissolution of silicon electrodes in HF.

Fig. 4.4 Reaction scheme proposed for the anodic, tetravalent dissolution of silicon electrodes in aqueous HF. The reaction can be separated into two parts first an oxide is...

In addition to silicon, sulfur groups have been used as auxiliaries for amide oxidation reactions (Scheme 31) [63], However, in these cases the mechanism of the reaction is different. The sulfur is oxidized to form a sulfur radical cation that is then eliminated from the molecule in order to... 

Nakajima has shown that a-cyclopropyl acyl silane (23) results from reaction of 1-trimethylsilyl cyclopropyl lithium with dichloromethyl methyl ether at low temperature in THF solution, in a reaction said to involve a carbene intermediate and a 1,2-silicon shift (Scheme 56)147. 

Other examples of Si—C(Cp) bond cleavage reactions deal with pentamethylcyclopen-tadienyl (Cp ) substituted silicon compounds. Scheme 11 shows a collection of different kinds of reactions with (pentamethylcyclopentadienyl)trimethylsilane as the substrate. Nucleophilic as well as electrophilic attack is the basis for Si—C(Cp) bond fission97. Reactions with various element halides involved in pentamethylcyclopentadienyl transfer are very useful in synthetic chemistry88-91 (see also Sections II.B and HE). 

In 1982, Sakurai [7] described a catalytic version of this reaction (Scheme 13.4). The addition of small quantities of fluoride anions to the allylsilane 1 generates the pentacoordinated silicon species 10, probably in equilibrium with the starting materials 1 and 11. This activated species can react with the carbonyl derivative 6 to yield the alkoxide 12 which is trapped by fluorotrimethylsilane. This last step not only furnishes the silylated compounds 13 but also regenerates the fluoride catalyst 11. Acidic work-up then leads to the desired homoallylic alcohol 7. 

Mixing of an aminosilane with silica gel results in a fast adsorption, by hydrogen bonding of the amine to a surface hydroxyl group.2 After adsorption, the amine group can catalyze the condensation of the silicon side of the molecule with a surface silanol. Thus siloxane bonds with the surface may be formed in the absence of water.3,4 For other silanes the siloxane bond formation requires an initial hydrolysis of the ethoxy groups or the addition of an amine in the reaction mixture.5 This general reaction scheme has been presented in chapter 8. Here we will go into further detail on the types of interaction of the aminosilane with the silica surface and the characterization of the bonded silane species. 

Again the silicon atom of the ylide obtained bears a methyl group, which must originate from the ylide carbon of (CHS) 3PCH2 if the reaction scheme (Scheme 5) is correct. 

Asymmetric cyanosilylation of ketones and aldehydes is important because the cyanohydrin product can be easily converted into optically active aminoalcohols by reduction. Moberg, Haswell and coworkers reported on a microflow version of the catalytic cyanosilylation of aldehydes using Pybox [5]/lanthanoid triflates as the catalyst for chiral induction. A T-shaped borosilicate microreactor with channel dimensions of 100 pm X 50 pm was used in this study [6]. Electroosmotic flow (EOF) was employed to pump an acetonitrile solution of phenyl-Pybox, LnCl3 and benzal-dehyde (reservoir A) and an acetonitrile solution of TMSCN (reservoir B). LuC13-catalyzed microflow reactions gave similar enantioselectivity to that observed in analogous batch reactions. However, lower enantioselectivity was observed for the YbCl3-catalyzed microflow reactions than that observed for the batch reaction (Scheme 4.5). It is possible that the oxophilic Yb binds to the silicon oxide surface of the channels. 

Fig. 8.16. Reaction scheme for photoanodic dissolution of silicon in low intensity limit illustrating the competition between hole capture steps (rate constants k to k ) and electron injection steps (rate constants k to k,). The nominal valence states of the silicon intermediates are indicated. The final product Si(IV) is the soluble hexafluorosilicate species.

The second major class of non-umpolung nucleophilic carbene catalysis comprises reactions by initial NHC-activation of various silicon compounds. Their proposed common pathway is thought to lead to a hypervalent silicon complex4 and thus provide carbene-catalyzed activation of the corresponding nucleophiles such as TMSCN, TMSCF3 etc. (Kano et al. 2006 Song et al. 2005 2006). It is not only certain carbon-silicon bonds that can be effectively activated, but a comparable activation of Si-O bonds, e.g. of trimethylsily enol ethers etc., allows for mild, NHC-promoted Mukaiyama aldol reactions (Scheme 6 Song et al. 2007). 

The wide range of silicon compounds that are readily hydrolyzed means that the siloxane H3SiOSiH3 is formed (probably via H3 SiOH) in numerous reactions (Scheme 35). Similarly, hydrolysis of halodisilanes tends to give siloxane rather than silanol products (Scheme 36). ... 

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