Silicon double bond, examples

Successes in producing reactive intermediates like o-xylylene and carbene and in preparing bimetal lies in high yields using ultrasound led us to attempt to generate West s novel compound, tetramesityldisilene the first example of a stable species with a silicon-silicon double bond(32). We prepared this species in one step and trapped it with methanol(33). The disilene is reactive towards lithium, however, and we have found it very difficult to obtain consistent results. Most often, hexamesitylcyclotrisilane is isolated in very good yield(34). 

Fig. 3. Some examples of postulated concerted [2 + 2] and [4 + 2] cycloaddition reactions of alkenes with silicon double bonds, as well as alternate descriptions in terms of a [1 + 2 + 1] reaction with silicon free radicals that would be expected to proceed in a non-concerted fashion through the intermediate [1 + 1] adduct shown.

Notable advances in the last decade are isolation of kinetically stabilized silicon-containing donble-bonded species. As we have seen already, a variety of sterically bulky substituents have been utilized to stabihze silicon-centered reactive intermediates. This kinetic stabilization is the key issue also to isolate silicon double-bonded species. For example, the synthesis of isolable silicon analog of benzene and other aromatic compounds has been a target for a long time. One example is shown below for the generation of transient silabenezene. ... 

In the previous reviews,1 la,d f syntheses of many examples of transient silicon-oxygen double bond compounds such as MeHSi=0, Me2Si=0, H2Si=0 (2), (H0)HSi=0 (silanoic acid), and (H0)2Si=0 (silicic acid) have been described, and they are reportedly isolated as stable species in the low temperature matrices. However, the stabilization of this extremely reactive double bond species is very difficult, and no stable example of silanone (RR/Si=0) has been isolated until now even by the methods of thermodynamic or kinetic stabilization. 

Despite the differing levels of calculations, the same general conclusions were reached. The silicon-carbon double bonds in 1-silaallene (1.69 A) and 2-silaallene (1.70 A) are shorter than in isolated silenes at the same level of theory. This trend is also observed in the analogous carbon series. 1-Silaallene is thermodynamically more stable than 2-silaallene by 21 kcal/mol (22). Intuitively, this is what would have been expected, realizing the low ability of silicon to participate in multiple bonds. As may be expected from simpler systems (i.e., H2Si=CH2)(i97), silylene isomers (for example, structures 8 and 9) are considerably more stable (approximately IS kcal/mol) than their silaallene counterparts. 

An extension of the latter type of interaction may be found in, for example, the suggestion that in the Silicon tetrahalides the central atom 3d orbitals act as r-acceptors for the lone pair electrons on the halogen atoms. In resonance terms this amounts to the contribution of the ionic double-bond structures to the ground state of the molecule (Fig. 4). 

If the analogy that is drawn between the Si=Si dimer on the Si(100)-2 x 1 surface and an alkene group is reasonable, then certain parallels might be expected to exist between cycloaddition reactions in organic chemistry and reactions that occur between alkenes or dienes and the silicon surface. In other words, cycloaddition products should be observed on the Si(100)-2 x 1 surface. Indeed, this prediction has been borne out in a number of studies of cycloaddition reactions on Si(100)-2x1 [14], as well as on the related surfaces of Ge(100)-2 x 1 (see Section 6.2.1) and C(100)-2 x 1 [192-195]. On the other hand, because the double-bonded description is only an approximation, deviations from the simple picture are expected. A number of studies have shown that the behavior differs from that of a double bond, and the asymmetric character of the dimer will be seen to play an important role. For example, departures from the symmetry selection rules developed for organic reactions are observed at the surface. Several review articles address cycloaddition and related chemistry at the Si(100)-2 x 1 surface the reader is referred to Refs. [10-18] for additional detail. 

This is generally true, but it should be noted that silicon-carbon double bonds with inverse electron demand behave differently. Silatriafulvene, for example, did not react with alcohol but underwent 1,2-silyl migration followed by ring expansion reaction to silacyclobutadiene63. Silacalicene also adds alcohol in a reverse fashion64. 

In recent years, however, impressive progress has been made in the field of silicon- sulfur double-bond chemistry the first examples of kinetically stabilized and electronically stabilized silanethiones were successfully synthesized and fully characterized by spectroscopic and X-ray crystallographic data9,10. These results together with the theoretical studies have revealed the intrinsic nature of this unique double bond to silicon. 

As mentioned in this chapter, in recent years much progress has been made in the chemistry of silicon-chalcogen multiple bonds. For silicon-sulfur doubly-bonded compounds, we have now several isolated examples, both kinetically stabilized and thermodynamically stabilized. Furthermore, there have been reports of the synthesis and characterization of stable compounds with silicon-nitrogen double bonds (i.e. silanimines or iminosilanes) as well as their heavier group 15 element analogues such as phosphasilenes and arsasilenes. 

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