Si-O bonds, formation

These percentages were determined using 29Si and 13C NMR spectroscopies, the highest rates of Si-O bond formation being observed with X = OTf, whereas chlorosilanes rather led to pentacoordinated compounds. 

In silicon derivatives, X-ray studies of compound 27 were consistent with a covalently bonded trigonal bipyramidal molecule <2000CC565>. In addition, nucleophilic substitution at silicon for similar compounds was modeled either by NMR or X-ray techniques and both methods correlate in the calculation of % Si-O bond formation <2003JOM66, 2003JOM154>. 

As for heterogeneous olefin polymerization catalysis, the activity of rare-earth metal catalysts may be also enhanced in organic transformations by the use of silica supports or other carriers [7]. Indeed, several catalytic C-C and C-X (with X = H/D, Si, O) bond formation reactions as weU as functional group transformations witness to the potential of SOLn/AnC-based heterogeneous catalysts for fine chemical synthesis. 

The extent of Si—O bond formation is found to increase as X becomes a better leaving group (F2CF3). Additionally, substituents Y on the pyridone ring... 

The properties of these monolayers are similar to the alkyl monolayers discussed above, although no direct evidence of Si-O bond formation has yet been put forward. Aldehydes also react thermally with the H/Si(lll) surface, resulting in monolayers with similar properties to those formed via the alcohol reactions. A noticeable difference between the properties of the monolayers formed from the reaction of alcohols and aldehydes is their stability in boiling water. Aldehyde derived monolayers are stable with respect to this procedure while alcohol derived ones are not [41]. This could suggest differences in the structure of the monolayer or in the nature of the bond to the surface. Reactions of alcohols and amines with chlorinated surfaces, forming Si-OR and Si-NHR monolayers, have also been reported [42,43]. 

Si-O bond formation competes with dimerization for 1, the dimer is preferred by 106.9 kJ/mol, for molecule 2 the exothermicity of dimerization is already reduced to 55 kJ/mol. Noteworthy, for molecule 3 the dimerization becomes endothermic ( rei(dimer) = 79.5 kJ/mol). 

As mentioned in the previous section, the Peterson reaction proceeds by an irreversible addition of the silyl-substituted carbanion to a carbonyl. It has generally been assumed that an intermediate p-oxidosi-lane is formed and then eliminated. In support of this mechanistic hypothesis, if an anion-stabilizing group is not present in the silyl anion, the p-hydroxysilanes can be isolated fixrm the reaction, and elimination to the alkene carried out in a separate step. Recent studies by Hudrlik indicate that, in analogy to the Wittig reaction, an oxasiletane (304) may be formed directly by simultaneous C—C and Si—O bond formation (Scheme 43). The p-hyd xysilanes were synthesized by addition to the silyl epoxide. When the base-induced elimination was carried out, dramatically different ratios of cis- to rranr-alkenes were obtained than from the direct Peterson alkenation. While conclusions of the mechanism in general await further study, the Peterson alkenation may prove to be more closely allied with the Wittig reaction than with -elimination reactions. 

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