Silanethiones reactivity

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. 

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 . 

Little is known about the reactivity of silanethiones. The reactions that have been invoked to explain the results observed so far and listed above are (i) nucleophilic attack on Si=S leading to an addition, in particular addition of Si-O and Si-S bonds, and (ii) nucleophilic attack on Si=S leading to cycloaddition, in particular 2 4- 2 dimerization and cycloaddition to a silene. 

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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