Organosilicon compounds selectivity

Selective excitation and selective detection have been recently reviewed by the recognized leaders in this field135 who viewed 29Si NMR applications from a broader perspective. In this review we focus on those selective experiments that have found wider use in studies of organosilicon compounds, selective INEPT and HEED as representatives of the two classes (excitation and detection) of selective experiments. Some other selective experiments are mentioned in other sections (e.g. SPT, selective decoupling, SPINEPTR, V>-BIRD HMQC, DQF COSY). 

The crystal structure investigation of the organosilicon model compounds selected (Fig. 1) aim at an improved understanding of the spatial requirements of bulky substituents such as trimethylsilyl groups [3] and their effects on the molecular properties. 

Exceeding the limitation of molecular dynamics, the steric requirement of trimethylsilyl groups can cause drastic changes both in structure and of molecular properties of organosilicon compounds. For illustration, the so-called "Wurster s-Blue11 radical ions are selected On one-electron oxidation of tetramethyl-p-phenylenediamine, its dark-blue radical cation, detected as early as 1879 [11a], is gene-... 

Chemical reactivity of unfunctionalized organosilicon compounds, the tetraalkylsilanes, are generally very low. There has been virtually no method for the selective transformation of unfunctionalized tetraalkylsilanes into other compounds under mild conditions. The electrochemical reactivity of tetraalkylsilanes is also very low. Kochi et al. have reported the oxidation potentials of tetraalkyl group-14-metal compounds determined by cyclic voltammetry [2]. The oxidation potential (Ep) increases in the order of Pb < Sn < Ge < Si as shown in Table 1. The order of the oxidation potential is the same as that of the ionization potentials and the steric effect of the alkyl group is very small. Therefore, the electron transfer is suggested as proceeding by an outer-sphere process. However, it seems to be difficult to oxidize tetraalkylsilanes electro-chemically in a practical sense because the oxidation potentials are outside the electrochemical windows of the usual supporting electrolyte/solvent systems (>2.5 V). 

It is the purpose of this chapter to review selectively the advances in the synthetic applications of organosilicon compounds, excluding those organosilanes covered in other chapters. Since the publication of The Chemistry of Organic Silicon Compounds, two monographs1,2 and several reviews3-5 have been published, as have three conference proceedings6-8 and a symposium-in-print 9. 

Enantioselective enzymatic amide hydrolyses can also be applied for the preparation of optically active organosilicon compounds. The first example of this is the kinetic resolution of the racemic [l-(phenylacetamido)ethyl] silane rac-84 using immobilized penicillin G acylase (PGA E.C. 3.5.1.11) from Escherichia coli as the biocatalyst (Scheme 18)69. (R)-selective hydrolysis of rac-84 yielded the corresponding (l-aminoethyl)silane (R)-85 which was obtained on a preparative scale in 40% yield (relative to rac-84). The enantiomeric purity of the biotransformation product was 92% ee. This method has not yet been used for the synthesis of optically active silicon compounds with the silicon atom as the center of chirality. 

Of the other methods for preparing organosilicon compounds, the Grignard and direct methods have been selected for further consideration here. This is not to say that the Wurtz synthesis and the meta-thetical reactions of silicon tetrachloride with alkyls of zinc and mer-... 

In spite of the known phases, the advantage of suggested selective immovable phase is the possibility of selective analysis of various classes of organic and organosilicon compounds. Of spe-cial attention should be peculiarity of the phase to separation of cyclic and naphthene hydrocarbons, amines and alcohols with preservation of high thermal stability. 

A large number of different a,(o-bis[(trifluoromethyl)sulfonyloxy]-substituted organosilicon compounds can be obtained by relatively simple methods from the corresponding amino-, allyl-, or phenylsilanes. Moreover, it is remarkable that these silyl triflate derivatives are often easily formed, when the synthesis of the corresponding chloro- or bromosilanes is difficult or does not appear to have been attempted. Eq. 2 and Eq. 3 show selected examples of this synthesis [10-12]. The products were prepared in high purities and yields. The resulting triflates should be used for the polycondensation without further purification, because they often cannot be destilled without decomposition. 

The first observation of penta- and hexaeoordinate silicon compounds was reported at the beginning of the 19th century by Gay-Lussac [87] and Davy [88], Subsequent investigation of hypercoordination in silicon compounds stimulated widespread use of nucleophilic activation and catalysis in the application of organosilicon compounds as reactive species in organic synthesis. Synthetic application for silicon-fluorine bond formation can be found in several reviews over the last two decades, and this section focuses on recent advances in the use of hypervalent organosilicon compounds in selective organic synthesis, in particular, selective carbon-carbon bond formation [89]. 

Apart from the measurements of spin-spin couplings between rare spin nuclei there are, of course, other applications of selective excitation and selective detection in NMR of organosilicon compounds. 

V. Bazant, V. Chvalovskj), and J. Rathousky Chemistry of Organosilicon Compounds. Publishing house of the Czechoslovak Academy of Sciences, Prague, and Academic Press, New York. Volumes 1 and 2 (two parts) (1965) 3 and 4 (three parts) (1973). Volume 1 summarizes the chemistry of organosilicon compounds and documents the literature to September 1961 (616 pp.). Volume 2 is a formula register with the same cut-off date. Volume 3 reviews selected topics (NMR, IR and Raman spectroscopy w-bonding penta- and hexa-co-ordinate silicon stereochemistry) up to December 1969 (761 pp.) and Volume 4 is a formula index for the period October 1961-December 1969 (2349 pp.). 

Substituted vinylsilanes such as ,Z-RCH=CHSiR 3 and R(SiR 3)C=CH2 are a class of organosilicon compounds commonly used in organic synthesis. Many efficient stereo- and regio-selective methodologies for the synthesis of substituted vinylsilanes involving classical stoichiometric routes from organometallic reagents and, more recently, transition metal-catalyzed transformations of alkynes, silylalkynes, alkenes, and simple vinylsilanes have been reported [1-3]. 

When silicon compounds were becoming more and more important, a gioup of chemists met to discuss the nomenclature problems involved. The purpose of the meeting is well stated in the preamble to the rules (13) which resulted from this and subsequent meetings. The purpose of the following rules is to provide one systematic and unique name for each of the simple organosilicon compounds and to guide in the selection of an accurate name for each of the more complex or polyfunctional compoimds of this nature. ... 

The catalyst system for the modem methyl acetate carbonylation process involves rhodium chloride trihydrate [13569-65-8]y methyl iodide [74-88-4], chromium metal powder, and an alumina support or a nickel carbonyl complex with triphenylphosphine, methyl iodide, and chromium hexacarbonyl (34). The use of nitrogen-heterocyclic complexes and rhodium chloride is disclosed in one European patent (35). In another, the alumina catalyst support is treated with an organosilicon compound having either a terminal organophosphine or similar ligands and rhodium or a similar noble metal (36). Such a catalyst enabled methyl acetate carbonylation at 200°C under about 20 MPa (2900 psi) carbon monoxide, with a space-time yield of 140 g anhydride per g rhodium per hour. Conversion was 42.8% with 97.5% selectivity. A homogeneous catalyst system for methyl acetate carbonylation has also been disclosed (37). A description of another synthesis is given where anhydride conversion is about 30%, with 95% selectivity. The reaction occurs at 445 K under 11 MPa partial pressure of carbon monoxide (37). A process based on a montmorillonite support with nickel chloride coordinated with imidazole has been developed (38). Other related processes for carbonylation to yield anhydride are also available (39,40). 

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