Si=S bonds

Silyl-derived protective groups are also used to mask the thiol function. A complete compilation is not given here since silyl derivatives are described in the section on alcohol protection. The formation and cleavage of silyl thioethers proceed analogously to simple alcohols. The Si—S bond is weaker than the Si—O bond, and therefore sulfur derivatives are more susceptible to hydrolysis. For the most part silyl ethers are rarely used to protect the thiol function because of their instability. Silyl ethers have been used for in situ protection of the — SH group during amide formation. ... 

A novel demonstration of the strength of the Si-O bond relative to the Si-S bond was provided during attempts to obtain the oxide and sulphide of Bu MeP-NfSiMea).. ... 

Rh-catalyzed reaction of hydrosilanes with thiols results in the formation of Si-S bond compounds (Scheme 5).35 37 The use of dihydrosilanes and dithiols as starting materials affords the corresponding polymers having Si-S bonds. The reaction of a trihydrosilane with 2 equivalent of dithiols in the absence of a... 

The crystals of both compounds contain pairs of (A)- and (A)-enantiomers. Selected geometric parameters for 88 and 89 are listed in Table XIV. Similar crystal structures were observed for the corresponding oxygen analogs 53 and 77, respectively (see Sections III,D and III, E). As can be seen from the Si-O [1.7600(14)-1.815(4) A], Si-S [2.144(2)-2.1638(9) A], and Si-C distances [1.900(5)-1.906(2) A], the Si02S2C frameworks of 88 and 89 are best described as being built up by five normal covalent bonds rather than a bonding system in the sense of a 4+1 coordination. The Si-S distances are very similar to those observed for tetracoordinate silicon compounds with Si-S bonds. 

Scheme 120 illustrates aldol-type reaction of aldehydes and silyl ene-thiolates catalyzed by 20 mol % of Sn(II) triflate-chiral diamine combined system in propionitrile or dichloromethane (291). A variety of aldehydes such as aliphatic, ,/3-unsaturated, and aromatic aldehydes are usable. The reaction is facilitated by high affinity of the Sn atom to sulfur atoms and the weak Si—S bond. A binaphthol-containing Ti oxo... 

N, 0, S, F and Cl the polarisation of the covalent C—El bonds is qualitatively the same as that of the corresponding Si—El bonds, with only two exceptions the polarisation of the Si—H bond (Si°s—HSe) is opposite to that of the C—H bond (Cfi9 —HSe), and the polarisation of the Si—S bond (Si5s-S69) is opposite to that of the C—S bond (C6e—S5 ), which is nearly without polarity. The Si—El bonds always exhibit a higher degree of polarisation than the corresponding C—El bonds. 

The Si—S bond has some similarities with the Si—O bond. However, its resistance to cleavage by nucleophilic reagents is considerably smaller. In contrast to the C—S bond the Si-S linkage very readily undergoes solvolytic cleavage in water and alcohols. Silthianes are hydrolyzed even by atmospheric moisture, leading to the corresponding silanols. 

The sum of the covalent radii3 for silicon and sulphur is 2.21 A. The electronegativity of the sulphur atom is smaller than that of its congener oxygen atom and therefore the expected polarization and ionicity of the Si—S bond is smaller than that of the Si—O bond. As a result it is expected that the shortening of the Si—S bond length from the sum of covalent radii will be significantly smaller than that observed for the Si—O bond. 

The crystal structures of over 70 different compounds possessing Si—S bonds have been determined. These structures provide more than 90 bond lengths and bond angles. The complexity and diversity of the compounds do not allow fruitful discussion of all of them. 

FIGURE 19. Histogram of Si—S bond lengths in compounds containing Si—S—Si fragments... 

Under similar reaction conditions, Cl2Si=S (127) was formed in a matrix reaction between SiS and CI2. The formation of 127 was also concluded from some isotopic shifts in the IR spectra. The force constant of the Si—S bond in 127 has a value of... 

TABLE 18. Comparison of Si—S bond parameters and 29Si chemical shifts for cyclic silanes174... 

Si-S bond lengths of 212.3 and 212.9 pm) and (PhSe Si (Si-Se bond lengths are 227.2 and 227.4 pm) includes a comprehensive list of the structures of silicon-sulphur and silicon-selenium compounds. These suggest a decrease in the Si—S bond length from 216 pm to 212 pm as the number of Si—S bonds in the molecule increases5. 

RhH2(SiPh2SAr)(PMe3)3 with a Si—S bond length of 222.8 pm. Since the silyl group in 3 is trans to the thio group and the isomerization is intramolecular, it is suggested that cis-trans isomerization occurs prior to transfer (equation ll)19. 

Ph3SiSH can be S-halogenated using the 7V-halosuccinimide. The bromo and iodo derivatives decompose to release sulphur in polar solvents, but a structure determination of the bromo derivative shows the Si—S bond to be 216 pm, compared with 215 pm in the parent thiol. Surprisingly, the S— Br bond is 217 pm long29. 

Electron diffraction studies of the three methylsilyl sulphides (Me H3 Si)2S (n = 1-3) show Si—S bond lengths of 214.1, 214.6 and 215.4 pm respectively, with SiSSi angles of 97.9°, 100.8° and 105.8°, hindrance indicating a steady increase without undue lengthening of the Si—S bonds35. 

Thiadisilacyclopropanes result from the reaction of disilenes with sulphur and episul-phides. From the mechanistic standpoint, the reaction of RR,Si=SiRR/ is of great interest and the separation of the (E)- and (Z)-isomers (trans and cis) [R = 2,4,6-( -Pr)3C6H2, R = t-Bu] has led to the separate addition of sulphur to each. This occurs within a minute to give trans and ds-isomers, respectively, the latter with slightly different Si—S bond lengths60. Propylene sulphide reacted similarly, and the reaction of (i-Bu)2Si=Si(Bu-t)2 with thiophene leads to sulphur abstraction with the formation of the thiadisilacyclo-propane, with Si—S bonds of 217.1 pm, along with the l,2-disilacyclohexa-3,5-diene and 2,6-disilabicyclo[3.1.0]hex-3-ene (equation 29)61. 

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