Silicon compared with carbon

In the more distant future it is quite possible that nanotubes (and other carbon based structures for that matter) may find their way into electronics [165, 145,148,183-185]. This would of course be a spectacular development, in an area which has been dominated by silicon for so long. On the other hand nanotubes will have to demonstrate vastly superior properties compared with silicon to be a serious contender in this game. At the moment, nanotubes have been little more than rather curious objects from an electronics point of view. In fact, very much... 

These two compounds 1 and 2 cannot always be considered as synthetic equivalents of diazomethane for example, they do not react with carboxylic acids to give esters. In fact, they play a very different role in synthesis, and it is demonstrated in this review that they are powerful synthetic building blocks. Since tin chemistry is often compared to that of silicon, a few results involving bis(trimethylstannyl)diazomethane 3 [4] will also be presented (Scheme 1). Juggling the relative steric bulk of the triisopropylsilyl versus the trimethylsilyl group, and the enhanced reactivity of tin-carbon compared to silicon-carbon bonds, the three diazomethane derivatives 1-3 are complementary synthons. Not only are they attractive precursors for the synthesis of stable compounds hitherto believed to be only transient intermediates, but also of new heterocycles or even of polymer crosslinking agents. 

A recent example of bioisosteric replacement of a carbon atom with silicon was reported by Warneck and colleagues at Paradigm Therapeutics, who synthesized the silicon derivative of the p38 MAP kinase inhibitor BIRB-796 (Figure 2.4), which is in clinical evaluation for several inflammatory diseases, such as rheumatoid arthritis, Crohn s disease, and psoriasis [14]. This silicon isostere, compound 1, was found to be unusually less lipophilic than BIRB-796, of comparable potency, and more metabolically stable in human hver microsomes. 

The presence of active sulphate-reducing bacteria usually results in graphitic corrosion and this has led to a useful method of diagnosing this cause of corrosion. The leaching out of iron from the graphitic residue which is responsible for the characteristic appearance of this type of corrosion leads to an enriched carbon, silicon and phosphorus content in the residue as compared with the original content of these elements in the cast iron. Sulphur is usually lost to some extent but when active sulphate-reducing bacteria are present, this loss is offset by the accumulation of ferrous sulphide in the residue with a consequent increase in the sulphur content of the residue out... 

Despite the increasing interest in understanding the phenomena of bonding in silicon compounds, there are, until now, no well established and commonly accepted theories. Silicon compounds are mainly discussed in terms of carbon chemistry. Thus, specific properties of silicon compounds are usually compared with those of the corresponding carbon homologues. In this report some important features of silicon compounds are developed by means of ab initio calculations. From this a set of basic rules will be presented by which more complex phenomena can be explained in turn. 

Fig. l.The large radius of the silicon 3p orbital, compared with that of the carbon 2p orbital, is the origin of the extended Si=Si bond length, compared with that of C=C. 

The destabilizing effect of a silyl group compared with an alkyl group in trivalent carbocations was explained by the weaker hyperconjugation of the Si-R a-bond (R = alkyl) relative to a C-R cr-bond (R = H or alkyl) and by electrostatic repulsion between the adjacent positively charged cationic carbon and the electropositive silicon (10). 

Protonation of 4b leads to the symmetrically substituted 3b (Scheme 3.2-3) and methylation of 4b at temperatures higher than —60 °C gives 3c (Scheme 3.2-5) [19]. In the latter reaction, 6a can be identified as an intermediate at —80 °C by 13C NMR spectroscopy [19]. Its planar-tetracoordinate carbon atom is strongly de-shielded 3 13 C = 144 ppm) as compared with tetrahedrally-coordinated carbon atoms connected to three boron and one silicon center (d 13C = 70-100 ppm). Computations for the model compounds 6A and 6B give 144 and 104 ppm, re-... 

Note the much larger enthalpy of formation of silicon dioxide as compared with carbon dioxide this arises in part because of greater strength in the Si—O bonds and also because the Si—Si bond in silicon is much weaker than the C—C bond (p. 162). 

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