Silicon-hydrogen bond oxidation

On the basis of the fact that (R)-BMPP coordinated to the metal center can induce asymmetric addition of methyldichlorosilane across the carbon-carbon double bond of 2-substituted propenes to afford an enantiomeric excess of (R)-2-substituted propylmethyldichlorosilanes, the following processes should be involved in these reactions (a) insertion of the metal center into the silicon-hydrogen bond (oxidative addition of the hydrosilane) (b) addition of the resulting hydridometal moiety to the coordinated olefin preferentially from its re face (in a cis manner) to convert the olefin into an alkyl-metal species and (c) transfer of the silyl group from the metal center to the alkyl carbon to form the product. Since process (b) most likely involves diastereomeric transition states or intermediates, the overall asymmetric bias onto the R configuration at the chiral carbon would have already been determined prior to process (c). A schematic view of such a process is given in Scheme 1. 

The oxidative addition of silanes (with silicon-hydrogen bonds) to coordinatively unsaturated metal complexes is one of the most elegant methods for the formation of metal-silicon bonds. Under this heading normally reactions are considered which yield stable silyl metal hydrides. However, in some cases the oxidative addition is accompanied by a subsequent reductive elimination of, e.g., hydrogen, and only the products of the elimination step can be isolated. Such reactions are considered in this section as well. 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

The oxidation of silanes also can be accomplished by solutions of potassium permanganate, mercuric salts, ferric compounds, and cupric salts. Plain water also will oxidize the silicon-hydrogen bond in the presence of hydroxyl ions as catalysts 6... 

The mechanism of catalytic hydrosilylation involves oxidative addition of a silicon-hydrogen bond to a metal complex as an essential step since it is here the activation of hydrosilane by the catalyst takes place. Thus, many transition metal ions and complexes, especially group VIII metals in low oxidation state containing ir-acid ligands such as CO, tertiary phosphines or olefins display catalytic activity. The sequence of unit reactions in a typical d -metal complex-catalyzed hydrosilylation is summarized as ... 

Silicon-hydrogen bonds also add to dinuclear complexes in reactions that are formally one-electron oxidative additions, or simply metal-metal bond cleavage reactions. The reaction of dicobalt octacarbonyl with an excess of silane leads to silylcobalt carbonyls (equation 13)2 3. Similar reactions involving Mn-Mn29, Re-Re30, Fe-Fe31, Ru-Ru32,... 

A variety of organosilyl-hydridopalladium complexes have been assumed to be generated in situ from hydrosilanes through oxidative addition of the silicon-hydrogen bonds onto palladium complexes. However, few studies have been reported on the isolation and characterization of silyl-hydrido complexes because of their instability. 

The loss of ethylene may be synchronous with the oxidative addition of the hydrosilane to give the platinum(II) species, in which the metal atom inserts into the silicon-hydrogen bond in a three center process. 

Therefore, surface modification strategies for the formation of direct silicon-carbon bonds require, first, a special pre-treatment of the silicon surface to prevent oxidation and, second, an activation of the silicon surface for subsequent reaction with organic moieties. This has been achieved by treatment of the silicon surface with hydrofluoric acid to generate a hydrogen-terminated Si(lll) surface, which can further react with unsaturated co-functionahzed alkenes in the presence of UV irradiation or by thermal activation [27,44,45]. Using this method, carboxylic acid modified silicon substrates have been successfully generated and coupled to thiol modified ONDs via a polylysine/sulfosuccinimidyl 4-(M-maleimidomethyl)-cyclohexane-l-carboxylate couphng (Fig. 12). 

Deposition profile was then followed by an infrared study. Deposition of silicon alkoxlde removed Isolated sllanol at 3745 cm selectively, while that of hydrogen-bonded at 3600 cm was kept unaltered, as shown in Fig. 2a, b. Methyl group of surface residue of silicon compound was seen simultaneously with the disappearance of isolated sllanol. Upon reaction of the surface deposited species with water, the stretch bands of the methyl group disappeared, and the absorption of Isolated sllanol was recovered completely or incompletely, depending upon the kind of zeolite and metal oxide. 

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