Sulfur-silicon bonds

Element-element bonds, addition to G-G multiple bonds arsenic—selenium bonds, 10, 782 boron—boron bonds, 10, 727 boron—sulfur bonds, 10, 778 B-S and B-Ge bonds, 10, 758 chalcogen—chalcogen additions, 10, 752 germanium—germanium bonds, 10, 747 germanium-tin bonds, 10, 780 overview, 10, 725-787 phosphorus—phosphorus bonds, 10, 751 phosphorus—selenium bonds, 10, 782 phosphorus-sulfur bonds, 10, 781 Se-Si and Se-Ge bonds, 10, 779 silicon-germanium bonds, 10, 770 silicon-phosphorus bonds, 10, 780 silicon-silicon bonds, 10, 734 silicon-sulfur bonds, 10, 779 silicon-tin bonds, 10, 770 tin-boron bonds, 10, 767 tin-tin bonds, 10, 748... 

This study was complemented by a selective synthesis of the tautomeric enethiols [77]. Aliphatic thioketones were deprotonated by LDA, silylated, and the resulting silyl vinyl sulfides were smoothly converted to enethiols by simple addition of methanol. These are stable compounds which do not equilibrate with thioketones, this behaviour probably related to the extremely mild conditions of the (easy) cleavage of the silicon-sulfur bond. 

The silicon-halogen, silicon-oxygen, and silicon-sulfur bonds of the haiogenosil-anes, siiyl ethers, and siiyi thioethers are cleaved by reaction with LAH, AIH3, or DIBAH, and the corresponding siiyl hydrides are obtained [CGI, CG2]. Ultrasound activation can be applied [LG2] (Figure 5.9), Anionic pentacoordinated silicon compounds are reduced to hydrogenosilanes by LAH or DIBAH [BC6]. 

Silicon—sulfur bonds may be formed by the same methods utilized to generate Si—N bonds -. Four-, six- and eight-membered cyclosilthianes rings based on a regular alternation of Si and S are known. 

Harrison DJ, McDonald R, Rosenberg L (2005) Borane-catalyzed hydrosilylation of thiobenzophenone a new route to silicon-sulfur bond formation. Organometallics 24 1398... 

The methods available for synthesis have advanced dramatically in the past half-century. Improvements have been made in selectivity of conditions, versatility of transformations, stereochemical control, and the efficiency of synthetic processes. The range of available reagents has expanded. Many reactions involve compounds of boron, silicon, sulfur, selenium, phosphorus, and tin. Catalysis, particularly by transition metal complexes, has also become a key part of organic synthesis. The mechanisms of catalytic reactions are characterized by catalytic cycles and require an understanding not only of the ultimate bond-forming and bond-breaking steps, but also of the mechanism for regeneration of the active catalytic species and the effect of products, by-products, and other reaction components in the catalytic cycle. 

Furthermore, it was found that I I2Si=S is thermodynamically stable compared with H2Si=0. In an attempt to assess the strength of a silicon-sulfur double bond, a comparison was made of the hydrogenation energies released upon addition of H2... 

Silicon-Sulfur Double Bond Compounds (Silanethiones)... 

The chemistry of silacyclopentanes resembles closely that of acyclic alkylsilanes. Strong bases are needed to cleave the silicon-carbon bond, while electrophilic attack occurs readily with halogens and strong acids, e.g. sulfuric. In addition, aluminum chloride will induce polymerization of silacyclopentanes unless one of the groups on silicon is carbofunctional (Scheme 154) (67MI12000). If this is chloromethyl, then ring expansion occurs (Scheme 155). 

Among the silicon-chalcogen double-bond compounds, the silicon-sulfur doubly-bonded compounds (silanethiones) are considered to be easier to synthesize, since it has been predicted by the theoretical calculations that a silicon-sulfur double bond is thermodynamically and kinetically more stable than a silicon-oxygen double bond (silanone)13,14. According to the calculations, the lower polarization of Si=S compared to Si=0 should lead to a lower reactivity of Si=S. In addition, H2Si=S (1) is calculated to be by 8.9 kcal mol-1 more stable than its divalent isomer, H(HS)Si , whereas H2Si=0 (2) is by 2.4 kcal mol-1 less stable than H(HO)Si . 

As described in the preceding reviews on this field, most of the early work on silicon-sulfur doubly-bonded compounds was restricted to simple dialkylsilanethiones, which are all transient in solution or in the gas phase4. However, in contrast to the successful matrix isolation and spectroscopic identification of dimethylsilanone 1023, no spectroscopic detection of transient dialkylsilanethiones in matrices has been reported up to now, although the matrix isolation of Cl2Si=S50 and Cl(H)Si=S51, the silicon analogues of thiophosgene and thioformyl chloride, has been reported. 

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. 

SiOS121 and a sterically hindered silanethione122 no compounds with silicon-sulfur multiple bonds have been isolated1. 

The selective cleavage of the silicon-silicon bond in the disilacyclopentane and -hexane is probably due to the concentration of internal angular strain at this bond in such smaller polygonal molecules. In case of the disilacyclo-heptane, however, it seems likely that the strain is smaller and uniformly distributed all over the ring, and hence cleavage occurs preferentially at the silicon-carbon bond by the accepted mechanism involving both an electrophilic attack on carbon and nucleophilic attack on silicon by the sulfuric acid molecule(s) 169). 

You have now seen how enols and enolates react with electrophiles based on hydrogen (deuterium), carbon, halogens, silicon, sulfur, and nitrogen. What remains to be seen is how new carbon-carbon bonds can be formed with alkyl halides and carbonyl compounds in their normal electrophilic mode. These reactions are the subject of Chapters 26-29. We must first look at the ways aromatic compounds react with electrophiles. You will see similarities with the behaviour of enols. 

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