Silicon-hydrogen bond substitution

Fig. 11. Models of neutralized substitutional boron in silicon. Hydrogen bonded to a) Si, b) B, c) bridging bond, and d) OH bonded to B.

Table I includes the relative bond dissociation enthalpies obtained for some group 14 hydrides by photoacoustic calorimetry,7 10 The data demonstrate that, for the trialkyl-substituted series, the bond strengths decrease by 6.5 and 16.5 kcal/mol on going from silane to germane and to stannane, respectively. The silicon-hydrogen bonds can be dramatically weakened by successive substitution of the Me3Si group at the Si-H functionality. A substantial decrease in the bond strength is also observed by replacing alkyl with methylthio groups.

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... 

There is an interesting report of carbonyl substitution in the reaction between phosphorus pentafluoride and MeSiH2Co(CO)4 (114, 7). There is evidence that PF5 is reduced by the silicon-hydrogen bonds yielding HCo(CO)4, fluorosilane, and trifluorophosphine the latter subsequently displaces carbon monoxide from the hydridocarbonyl complex. 

The application of classical and non-classical (photoacoustic) reaction-solution calorimetry to probe the energetics of metal-ligand bonds in organometallic systems is briefly analysed and illustrated by thermochemical results involving two families of compounds. The classical reaction-solution studies enabled the discussion of the systematics of metal-carbon bond enthalpies in several complexes M(t) -C H ) L. The photoacoustic studies addressed the effect of phenyl goups on the energetics of silicon-hydrogen bonds in phenyl-substituted silanes. 

The constancy of silicon-hydrogen bond dissociation enthalpies in methyl substituted silanes is supported by a thermochemical analysis which does not rely on the enthalpies of formation of the corresponding silyl radicals. 

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 reaction of tetramethylsilane with fluorine led to the isolation of several, partially fluorine-substituted tetramethylsilanes (see Tables VII-IX), and preservation of over 80% of the silicon-carbon bonds in the initial, tetramethylsilane reactant. The stability of many of the partially fluorinated germanes and silanes (some are stable to over 100°C) is very surprising, for the possibility of elimination of hydrogen fluoride is obvious. Indeed, before the first reported synthesis (12) of... 

Isomerically pure 1 is obtained when HMes2SiSiPh2SiPh2SiMes2H (Mes = mesityl) 5 is used as a starting material, because bonds between silicon and methyl substituted aryl groups like p-tolyl or mesityl are cleaved much faster by hydrogen halides than Si-Ph bonds [3], Further reaction of I with HCl or HBr leads to the formation of the corresponding halo-derivatives 2 and 3 ... 

On the other hand polysilylalkynes with phenyl or allyl substituents are converted with triflic acid into polymeric alkynylsilyltriflates. These polymers react with many acidic element hydrogen compounds or lithium element compounds with formation of silicon element bonds. Thus we found an easy approach to numerous new functional substituted alkynes [12], Eq.(9) shows selected examples of this reaction type. 

The proposed mechanism is depicted in Scheme 38. The aldehyde is activated due to coordination on the silicon atom. A hydrogen bond between the aldehyde function and the benzyl substituted tertiary nitrogen atom stabilizes the transition state, and the benzyl group ensures that the cyclopentadiene attacks the dienophile only from one side. 

These three metalloles are the lower C-alkyl substituted Si—H bond-containing siloles which were isolated. The possible substitution of hydrogen bonded to silicon by another atom or a functional group appeared very attractive. Scheme 13 shows the functionalization reactions of 1,3,4-trimethylsilole (63) described by Dubac and coworkers75. 1 -Fluoro-1,3,4-trirncthylsilolc (66) has been identified spectroscopically and chemically,... 

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