Silicon, -bonding

Fig. 2. The distribution of silicon—oxygen—silicon bond angles in vitreous siUca (22,25). The function V(a) is the fraction of bonds with angles normalized to the most probable angle, 144°. This distribution gives quite a regular stmcture on the short range, with gradual distorting over a distance of 3 or 4 rings (2—3 nm). Crystalline siUca such as quartz or cristobaUte would have a narrower distribution around specific bond angles.

At one time it was felt that it would be possible to produce silicon analogues of the multiplicity of carbon compounds which form the basis of organic chemistry. Because of the valency difference and the electropositive nature of the element this has long been known not to be the case. It is not even possible to prepare silanes higher than hexasilane because of the inherent instability of the silicon-silicon bond in the higher silanes. 

The view has also existed in the past that the carbon-silicon bond should be similar in behaviour to the carbon-carbon bond and would have a similar average bond energy. There is some measure of truth in the assumption about average bond energy but because silicon is more electropositive than carbon the C—Si bond will be polar and its properties will be very dependent on the nature of groups attached to the carbon and silicon groups. For example, the CH3—Si group is particularly resistant to oxidation but H13—Si is not. 

Figure 29.5. Effect of aging at 250°C on the power factor of silicone-bonded, glass-cloth laminates.

The rotational barrier in methylsilane (Table 3.4, entry 5) is significantly smaller than that in ethane (1.7 versus 2.88 kcal/mol). This reflects the decreased electron-electron rqjulsions in the eclipsed conformation resulting from the longer carbon-silicon bond length (1.87 A) compared to the carbon-carbon bond length (1.54 A) in ethane. 

Computational investigations of vinylsilanes indicate that there is a groimd-state interaction between the alkene n oibital and the carbon-silicon bond which raises the energy of the n HOMO and enhances reactivity. Furthermore, this stereoelectronic interaction favors attack of the electrophile anti to the silyl substituent. 

The silyl group directs electrophiles to the substituted position. That is, it is an ipso-directing group. Because of the polarity of the carbon-silicon bond, the substituted position is relatively electron-rich. The ability of silicon substituents to stabilize carboca-tion character at )9-carbon atoms (see Section 6.10, p. 393) also promotes ipso substitution. The silicon substituent is easily removed from the c-complex by reaction with a nucleophile. The desilylation step probably occurs through a pentavalent silicon species ... 

Epoxides are regio- and stereoselectively transformed into fluorohydrins by silicon tetrafluoride m the presence of a Lewis base, such as diisopropyleth-ylamme and, m certain instances, water or tetrabutylammonium fluoride The reactions proceed under very mild conditions (0 to 20 C in 1,2-diohloroethane or diethyl ether) and are highly chemoselective alkenes, ethers, long-chain internal oxiranes, and carbon-silicon bonds remain intact The stereochemical outcome of the epoxide ring opening with silicon tetrafluoride depends on an additive used, without addition of water or a quaternary ammonium fluoride, as fluorohydrins are formed, whereas m the presence of these additives, only anti opening leading to trans isomers is observed [17, 18] (Table 2)... 

Difluorocarbene generated by the thermolysis of trimethyltnfluoromethylsilane reacts with disilanes by insertion into the silicon-silicon bond [S] (equation 9) Thermolysis of pentafluoroethyltnfluorosilane at 200 °C gives tetrafluoro ethylidene carbene, which reacts with phosphorus trifluonde to give trifluoro vinyltetrafluorophosphorane [9] (equation 10) and with perfluorotnmethylphos-phine to give perfluorodimethyhsopropylphosphine and perfluoro-2-butene [9] (equation 10)... 

Additions Forming Carbon-Carbon and Carbon-Silicon Bonds... 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

Silicon analogues of imidazole-2-ylidenes are stable carbenes that form adducts where the metal-silicon bond is relatively weaker than that between metal and carbon atoms. 

Finally, oxidative cleavage of the remaining aryl-silicon bond with lead tetrakis(trifluoroacetate), [Pb(OCOCF3)4]19, furnishes ( )-estrone [( )-1 ] in nearly quantitative yield. 

A short digression is in order at this juncture. The electrophilic substitution of a vinylsilane is a very useful process because it is generally both regio- and stereospecific. The carbon-silicon bond is strongly polarized due to the high electronegativity of carbon (2.35) relative to silicon (1.64).67b A unique and important conse-... 

An interesting variant of metal-silicon bond formation is the combination of metal halides with silyl anions. Since silyl dianions are not available, only one metal-silicon bond can be formed directly. The silylene complexes are then accessible by subsequent reaction steps [113], An example of this approach is given by the reaction of cis-bistriethylphosphaneplatinumdichloride 25 with diphenylsilylli-thium, which yields, however, only dimeric platinadisilacyclosilanes 26a-c [114]. 

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. 

This excellent method of oxidative cleavage (/) of carbon-silicon bonds requires that the silane carry an electronegative substituent (2), such as alkoxy or fluoro. Either hydrogen peroxide or mcpba may be used as oxidant, and the alcohol is produced with retention of configuration (3). Fluoride ion is normally a mandatory additive in what is believed to be a fluoride ion-assisted rearrangement of a silyl peroxide, as shown below ... 

Reactions (1), (2), and (3) are generally carried out at atmospheric pressure and produce films up to 100 im thick. These reactions, which are used extensively in production, are reversible since the formation of HCl promotes the etching off of impurities during deposition due to the high energy states of silicon bonding at the sites of impurities,... 

Apart from silyl shifts, other reactions that are also characteristic of this class of compounds or their derivatives are due to the easy formation of halogen-silicon bonds. Phosphonium salt 34, resulting from the addition of bromine to 33, undergoes spontaneous desilylation by the action of the bromide anion to give the P-bromophosphazene 35 [138,139] (Scheme 33). 

Activation of Silicon Bonds by Transition Metal Salts and Complexes... 

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