Hydrogen carbon—silicon bonds

The silyl radicals formed in the initial scission appear to undergo further reactions, which may be complex. A possible secondary reaction is hydrogen transfer from an alpha carbon atom to give Si-H and a silicon carbon double bond (21)... 

Burger. P. and Bergman, R.G. (1993) Facile intermolecular activation of carbon-hydrogen bonds in methane and other hydrocarbons and silicon-hydrogen bonds in silanes with the iridium(III) complex Cp (PMe3)Ir (CH3)(OTf). J. Am. Chem. Soc., 115 (22). 10462-10463. 

Fig. 9 may be viewed, also, as a localized molecular orbital representation of, e.g., a hydrocarbon (cf. Fig. 13, ref. 7). Thus, replacement of (i) the domains of the Si4+ cations (the atomic cores of silicon atoms) by the domains of C4+ cations (the atomic cores of carbon atoms r = 0.15 A 2>), (ii) the domains of the bridging (i.e., bonding) oxide ions by the domains of the electron-pairs of aliphatic carbon-carbon single bonds (r 0.6e A 40)), and (iii) the domains of the non-bridging oxide ions by the domains of the protonated electron-pairs of carbon-hydrogen bonds... 

Since silene is an unstable species, various transition metal-silene complexes coordinated by the silicon-carbon double bond have been reported. In 1970, Pannel reported the formation of silene by irradiation of an iron complex (Eq. 6) [8]. He obtained an iron-TMS complex that was apparently formed from silene and an iron-hydride complex generated from the starting iron complex by /3-hydrogen elimination [8]. Wrighton confirmed the existence of tungsten-and iron-silene complexes by examination of NMR spectra obtained at low temperature (Eqs. 7 and 8) [9]. 

Advantage can be taken of the enhanced polarity of a carbon-silicon bond over that of a carbon-hydrogen bond in the displacement of 2-tri-methylsilyl groups by reaction with a number of carbonyl reagents to give imidazoles substituted at C-2 by secondary alcohol, acyl, aroyl, ester, and amide functions. ... 

Though bonds between silicon and other elements are usually stronger than the corresponding ones involving carbon, this is not true for the silicon-hydrogen bond (76.0 kcal/mole) which is weaker than the carbon-hydrogen bond (99.3 kcal/mole). 

Carbon-Hydrogen Bond Insertion In the early 1960s the activation of alkanes by metal systems was realized from the related development of oxidative addition reactions " " in which low-valent metal complexes inserted into carbon-heteroatom, silicon-hydrogen, and hydrogen-hydrogen bonds. The direct oxidative addition of metals into C-H bonds was found in the cyclometallation reaction [Eq. (6.61)].The reverse process of oxidative addition is called reductive elimination, which involves the same hypercoordinate carbon species. 

Typically, chemically modified surface layers involve thicknesses ranging from less than 1 micrometer(H) up to 20H-, so the overall mechanical properties of the treated objects are hardly affected by the process. Even if one limits the discussion to materials containing C-H bonds, practically all engineering plastics are covered except pure fluorocarbons and some silicones. Clearly, various gases can react with the carbon-hydrogen bonds on the surface of a plastic article and can reduce the diffusion coefficient of penetrants in the material. The choice of sulfonation as the preferred treatment, therefore, is not based solely on the ability to modify transport properties. 

A recent analysis of thermochemical data based on relationships between enthalpies of formation and the unshielded core potential of a moiety X, V ( ), throws some light into the behavior of silicon-hydrogen bonds in methyls lanes. Consider first the case of carbon-hydrogen bonds. The enthalpy of Reaction 12, which is shown to be a linear function of V iZ ) can be expressed in terms of bond dissociation enthalpies. Equation 13. 

Table V. Silicon-Hydrogen and Carbon-Hydrogen Bond Dissociation Enthalpies in Phenylsilanes and Phenylmethanes Data in kJ/mol...

The silicon-carbon double bond in intramolecularly donor-stabilized silenes is also extremely reactive. This is demonstrated by the reaction of (dichloFometfayl)tris(trimethylsilyl)silane with 8-dimethylaminomethyl-l-naphthyllithium, which deviates from the general reaction pattern. We expected to get another stable silene, but we obtained in a yield of 51 % the 1-sila-acenaphthene 13. Obviously, the silene 12 is unstable, and the silicon-carbon bond is inserted into one carbon-hydrogen bond of the benzylic methylene group (Scheme 5). 

---