Sulfur-silicon double bonds

Since genuine [4-1-2] cycloaddition products had previously not been prepared, it was surprising to find that the action of sulfur on 24 resulted in a formal [4-I-1] cycloaddition to furnish the first ftve-membered ring with an endocyclic silicon-silicon double bond 25. The heavier chalcogens selenium and tellurium did not react with 24. However, in the presence of small amounts of triethylphosphane smooth reactions with these elements did occur to furnish two further ftve-membered ring compounds 26, 27, each with an endocyclic silicon-silicon double bond (Eq. 8) [14]. 

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... 

As a result of comparing the properties of silanethione with silanone and formaldehyde, Nagase and Kudo obtained an important finding that silicon is much less reluctant to form double bonds with sulfur than with oxygen.12,13 Thus, silanethione is more stable and less reactive than silanone. They concluded that the maj or obstacle to the successful isolation of silanethione is its relatively high reactivity. 

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

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 . 

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. 

Following the classical double-bond rule, multiple bonds between silicon and sulfur, both elements of the third period, should be very difficult to obtain. Except for SiS119,120,... 

Three additional rings systems containing endocyclic Si=Si double bonds were obtained from the tetrasilabutadiene 139. Since Diels-Alder products of this compound have as yet not been synthesized, probably because of the steric overcrowding and the large 1, 4-separation of the terminal silicon atoms, it was surprising to find that the action of sulfur on 139 resulted in a formal [4 + 1] cycloaddition to furnish the thiatetrasilacyclopentene 154 in high yield (equation 39)142. 

The class of phosphaalkenes with isolated P=C double bonds was first synthes ized by Becker.33 His synthetic strategy starting from trimethylsilylphosphines and acyl chlorides is still the most versatile (Protocol 3). The principle is based on the easily achievable, 1,3-silatropic migration of a silyl group bonded to phosphorus to a doubly bonded element such as nitrogen, oxygen, or sulfur. The process is favoured energetically by the construction of the P=C double bond with concomitant formation of a very stable silicon-element bond. 

The formation of heterobutadiene 9 can be explained by an insertion-elimination reaction Insertion of sulfur into the silicon-hydrogen bond yields in the first step (2-Me2NCH2C6H4)(CH=CH2)Si(SH)2 as intermediate. This species eliminates H2S, producing the 1-thia-2-sila-1,3-diene 9, which is one of the rare examples possessing a silicon-sulfiir double-bond unit... 

Oxidation of alcohol, carbonyl and acid functions, hydroxylation of aliphatic carbon atoms, hydroxylation of alicyclic carbon atoms, oxidation of aromatic carbon atoms, oxidation of carbon-carbon double bonds, oxidation of nitrogen-containing functional groups, oxidation of silicon, phosphorus, arsenic and sulfur, oxidative N-dealkylation, oxidative O- and S-dealkylation, oxidative deamination, other oxidative reactions... 

These rubbers are based on atoms of silicon chains rather than carbon atoms. Their unique structure is responsible for their extreme temperature properties. The most common types of silicone rubbers are specfically polysilaxanes. The Si-O-Si bonds can rotate much more freely than the C-C bond or the C-O bond. So the silicone chain is much more flexible and less affected by temperature. Silicone rubber is vulcanised by the action of peroxides which crosslink the chains by abstracting hydrogen atoms from the methyl side groups, allowing the resulting free radicals to couple into a crosslink. Some varieties of polysiloxanes contain some vinyl methyl siloxane units, which permit sulfur vulcanisation at the double bonds. 

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