Silanethiones

No silanethiones R2Si=S have yet been reported to be stable in pure form or in solutions of any kind and even the evidence for their transient existence is still limited19, although recent developments are very promising2221. Only a few calculations have been reported, all for the parent silanethione, H2Si=S137 147,220. 

Various unimolecular reactions of H2Si=S, such as the 1,2-hydrogen shift to produce HSiSH and the dissociation reactions to H2 + SiS or to H + HSiS, have been studied220. The calculated potential energy surfaces of analogous reactions of H2Si=S and of H2Si=Q 

FIGURE 38. Energy profiles (in kcal mol 1) for the addition of H20 to H2Si=S (full line) and H2Si=0 (dotted line), at MP3/6-31G //6-31G. Reprinted with permission from Organometallics, 5, 1207 (1986). Copyright (1986) American Chemical Society220. 

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The adducts 41 from 1 and ketones or thiobenzophenone undergo interesting photochemical cycloreversion to afford a silanone or silanethione intermediate 42 in addition to silene 43 both of these intermediates are trapped by ethanol, as shown in Eq. (14).68 71 In the reaction with the thiobenzophenone adduct 41 (R = Ph, X = S), the intermediate silene 43 (R = Ph) was detected by Si NMR.71... 

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. 

On the other hand, the calculation for the relative stabilities of H2Si=S shows that the silanethione form (H2Si=S, 0.0 kcal mol-1) is more stable than both s-cis-HSiSH (12.0 kcal moP1) and s-trans-HSiSH (9.3 kcal moP1) isomers.12 Thus, the relative stability of heavy ketones to their carbene type isomers is considerably influenced by the combination of the Group 14 and Group 16 elements. 

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

Although there have been few reports on the chemistry of transient silanethiones,113 some stable examples have been successfully isolated by thermodynamic stabilization. In 1989, Corriu et al. reported the first synthesis of an isolable silanethione 31a (mp 170-171°C) by the reactions of the pentacoordinated functionalized silane 30a with elemental sulfur or carbon disulfide (Scheme 8).17... 

In contrast to the ready trimerization of the silanone 4 having the same ligands, silanethione 31a was found to be relatively long lived in solution (t 1/2 3 d in CDC13 at 25°C) as a monomeric species, though extreme precautions must be taken to... 

Corriu et al. also described an alternative synthetic method for internally coordinated silanethiones starting from the pentacoordinated diaminosilanes.28 As shown in Scheme 9, the pentacoordinated diaminosilanes 32 are allowed to react with sulfur-containing heterocumulenes such as carbon disulfide or phenyl isothiocyanate to give the corresponding insertion products 33, which undergo thermal decomposition to produce the corresponding silanethiones 31, 34, and 35.28... 

As in the case of silanone 9, the reaction of the silylene bis[2-(dimethylamino-methyl) phenyl]silanediyl (8) with phenyl isothiocyanate was examined.29 In this reaction the expected silanethione 36 was obtained as a single product even in the presence of (Me2SiO)3, no insertion product of 36 into a Si-O bond of (Me2SiO)3 was observed (Scheme 10). 

Silanethione 38 was characterized by H, 13C, and 29Si NMR, Raman, and UV-vis spectroscopic methods. The 29Si NMR chemical shift of 38 (8Si 166.56/C6D6) for the silathiocarbonyl unit is much downfield shifted from those of the thermodynamically stabilized silanethiones, 31, 34, 35,28 and 36,29 mentioned in the previous sections, clearly indicating a genuine Si=S double bond in 38 without any intra- or intermolecular coordination. The molecular structure of 38 was successfully established by X-ray crystallographic analysis, and the detailed structural parameters are discussed in the following section. 

Although it is possible to trap the silanethione 40 by intermolecular addition reactions at —78°C (Scheme 12), the combination of Tbt and Mes groups is not efficient enough to stabilize the silathiocarbonyl unit as stable compounds at ambient temperature. 

In contrast to the remarkable progress in the chemistry of silanones and silanethiones, very little is known for the chemistry of their heavier chalcogen... 

First, crystallographic structural analysis of the thermodynamically stabilized silanethione was established for the bulky silanethione 31b (see Scheme 8). 

Therefore, the elucidation of the intrinsic structural parameters of heavy ketones has to be done with kinetically stabilized systems. Most of the heavy ketones synthesized by steric protection with the Tbt group have provided single crystals suitable for X-ray structural analysis. The results for silanethione 38, germanethione 71b, germaneselones 75, 84, germanetellones 77,85, and stannaneselone 127, are summarized in Table III. 

The method (i) can be applied to the synthesis of almost all heavy ketones (Tables 3-5). Silanethiones and a silaneselone stabilized by the coordination of a nitrogen group have been synthesized by the method (ii) (Table 4). The method (iii) is effective to the synthesis of kinetically stabilized tricoordinate heavy ketones, although it cannot be applied to the synthesis of double-bond compounds between heavier group 14 elements and tellurium due to the instability of polytellurides (Table 3). The method (iv) can be used only when the unique dilithiometallanes can be generated (Table 3). The synthesis of heavy ketones by the method (v) demands the isolation of the corresponding heavy acyl chlorides as stable compounds (Table 5). 

The first silicon-organophosphorus betaine with a thiolate center (15a) was synthesized by the reaction of stable silanethione (14) with trimethyl-methylenephosphorane (Scheme 8) and characterized by multinuclear NMR spectroscopy.14 Compound 15a is formed under kinetic control and is transformed, under the thermodynamically controlled conditions, into the silaacenaphthene salt (16). The processes presented in this scheme reflect the competition of the basicity and nucleophilicity of phosphorus ylides. Betaine 15b prepared from less nucleophilic and less basic ylide with phenyl substituents at the phosphorus atom is much less resistant toward retro-decomposition compared to the alkyl analog. Its equilibrium concentration does not exceed 6%. 

The composition of thermolysis products in ethanol, which acted as a chemical trap, allows the estimation of the contributions of these processes under different conditions (Scheme 24). On heating of an alcohol solution of 20a in a sealed tube, the short-lived silanethione Me2Si=S (50) and sila-thiirane 46a are trapped immediately by ethanol. The first compound forms Me2Si(OEt)2, and the second one gives silylated mercaptan Me2Si(OEt) CMe2SH (51). At 150°C these products are formed in a ratio of 1 1. The Corey-Chaykovsky type reaction becomes predominant at 245 °C. The Me2Si(OEt)2 51 ratio at this temperature is already 1 3. 

Thermolysis at 180°C of dithiaphosphadigerminane 1929 leads to transient 2-thia-l,3-digermetane 20, which gives a [4] —> [2 H- 2] decomposition with formation of both dimethylgermene 15 and dimethylgermanethione 21 [Eq. (4)]. Note that a silene and a silanethione were also simultaneously obtained by West et al in a rather similar decomposition by photolysis of a 3-thia-1,2-disiletane,30 a four-membered ring with a Si-Si-S-C linkage. 

Recent advances in the chemistry of silicon-heteroatom multiple bonds 1065 B. Theoretical Calculations 1. Silanones and silanethiones... 

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