Organosilicon compounds oxidation

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-... 

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

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). 

The electrooxidation of organosilicon compounds containing heteroatoms has been investigated extensively and various synthetic applications have been developed. Cooper and Owen studied the oxidation potentials of a series of silyl-substituted amines, phosphines, and sulfides, and observed that silyl substitution at the carbon adjacent to the heteroatom caused a significant decrease in the oxidation potentials (Table 4) [35]. 

On the basis of these concepts a number of electrochemical reactions of organosilicon compounds have been developed. Although a rich variety of synthetic applications of the anodic oxidation of organosilicon compounds has been made in recent years, the application of the cathodic reduction of such compounds has been less studied and will hopefully be uncovered in the near future. 

TABLE 2. Anodic oxidation of organosilicon compounds in the presence of fluoride ions... 

Anodic oxidation of organosilicon compounds bearing heteroatoms and its synthetic applications have been studied intensively. The oxidation potentials of organosilicon compounds bearing nitrogen, sulfur and phosphorus atoms at the a-, ji- and /-positions were shown in Table 512-21-23. it should be noted that the silyl group a to the heteroatom decreases appreciably the oxidation potential. 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

In recent years remarkable progress has been made in the chemistry of organosilicon compounds containing a double bond to group 14 and group 15 elements. The reaction of a silylene-isocyanide complex with nitrile oxide generated the intermediate silanone, which with phenyl isothiocyanate afforded 1,3,2-oxathiasiletane 63, which is sensitive to hydrolysis (Equation 10) <2000CL244>. 

D. One-Electron Ionization and One-Electron Oxidation of Organosilicon Compounds... 

It has seemed quite natural to think of silicon only in terms of the oxide, for practically all of the earth s silicon is bound up with oxygen. Together these two elements constitute some 76 per cent of the solid crust of the earth, and there is more than enough oxygen to combine with all the silicon. Free silicon therefore does not occur in nature, nor do its organic compounds. The only natural substance which has been demonstrated to have carbon-silicon bonds is the rare mineral moissanite, which is silicon carbide, and this ordinarily is not thought of as an organosilicon compound. 

It is true that straw and feathers contain silicon, for its oxide is found in the ash when these materials are burned, but the mechanism by which this silicon entered into the plant or animal is not understood, and it has not been demon-strated that silicate esters or organosilicon compounds are involved. 

Since the orbital interaction plays a major role, the oxidation potentials of a-het-eroatom-substituted organosilicon compounds depend on the geometry. As a matter of fact, the oxidation potentials of a-alkoxysilanes in which the rotation around the C-O bond is restricted vary dramatically with the torsion angle of Si-C-O-C (Table 4) [115]. 

Table 3 Oxidation Potentials of Nitrogen, Sulfur, and Phosphorus-Substituted Organosilicon Compounds [114]...

Table 5 Oxidation Potentials of Sila-Functional a-Heteroatom-Substituted Organosilicon Compounds [116]...

The oxidation of allyl and benzyl silanes provides a method for umpolung of such organosilicon compounds. 

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