Silicon silanethione

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)... 

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%. 

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

The reaction of 59 with sulfur is somewhat more complicated (Scheme 19). At low temperatures, an intermediate is observed with a 29Si NMR chemical shift of +122.6 ppm this deshielded value is consistent with the silanethione, 69. Wanning of the solution produces the 4-ring dimer, 70. But when the reaction is carried out at room temperature this intermediate is not observed, and the major product is the trisilicon compound 71 in which a molecule of diimine has been lost from the central silicon374. The structure of 71 is somewhat similar to that of solid silicon disulfide. 

With isothiocyanates, the silanethione (84) is obtained, without oligomerization. The 29Si NMR of 84, at —21.0 ppm, shows that the silicon atom is highly shielded, consistent with a pentacoordinate structure as shown in equation 131. The chemical shift can be compared with that for singly N-coordinated silanethiones, +22 to +41 ppm, and with that for a true tricoordinate silanethione, +166.6 ppm. 

Cases in which the attacking nucleophile is present in the silene molecule itself have also been reported. The ring closure of the silicon analogues of an acrolein derivative and thioacrolein to silaoxetenes232 and silathietenes233, which immediately undergo a 2 + 2 cleavage to yield an acetylene and a silanone or silanethione, has been proposed to account for the results of flash vacuum pyrolysis of silylated ketenes and thioketenes (equation 110). 

Donor-acceptor interactions between the electrophilic Si atoms of three-coordinate silicon derivatives and suitable bases lead to an effective stabilization of these usually extremely reactive systems. Thus, for example, silenes and silanimines have been shown to be stabilized by tertiary amines [1]. Extraordinary stabilizations have been achieved by intramolecular Si-N coordinations, making possible the isolation of stable silanethiones [2] and silylium salts [3]. Intramolecularly donor-stabilized silenes have been unknown till now, possibly because of the lack of suitable synthetic methods. 

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