Silanethione addition

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... 

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. 

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 this section we will review the computational studies on reactions of silylenes, mainly addition and insertion reactions. Some reactions, in particular the following isomeriz-ations of silylenes to the corresponding multiply bonded species, were discussed above (a) to silaethylene in Section V.A.l.a.v, (b) to substituted silenes in Section V.A.l.b.iv, (c) to disilenes in Sections V.A.2.d and f, (d) to silanimines in Section Y.A.3, (e) to silanephos-phimines in Section V.A.4, (f) to silanones in Section V.A.5, (g) to silanethiones in Section V.A.6, (h) to silynes in Section V.B.l, (i) to disilynes in Section V.B.2, (j) to aromatic compounds in Section VI.A and to antiaromatic compounds in Section VI.D. 

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. 

We look forward to the addition of other chromophores substituted with heavy atoms to the short list compiled in this work. The preparation of gas phase telluro-ketones appears imminent. A variety of silicon substituted compounds have been reported including silanones (RR Si=0), silanethiones (RR Si=S), silaisonitriles (RNSi), silenes (R R"Si=CRjRj) and others. Phosphorus compounds such as phosphaallenes (RC=P=X), phosphaalkenes (RR C=PR") and diphosphenes (RP=PR ) have been synthesized by a number of groups A few compounds containing As, Sb, Bi, and Ge atoms have also been reported We are confident that many of these new species will have properties which are sufficiently at variance with those of conventional organic chromophores to make spectroscopic studies both novel and rewarding. 

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