Silanone 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... 

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 . 

It was established that the reaction of the methyl radical with the silanone group occurs via two steps first it adds to the silicon atom to form the oxy radical, and then the radical is isomerized via a shift of the H atom to oxygen (reaction 2 in Table 7.11). The estimated activation energy of this monomolecular reaction was 16kcal/mol. This suggests that the addition of other alkyl radicals (reactions 3 and 4) to the silanone groups also occurs via steps. The mechanism of the addition of an H(D) atom to the silanone group remains unclear. 

Silanone groups easily undergo the addition to unsaturated molecules (reactions 6-9) to form the corresponding cyclic structures [86,87], According to the results of quantum-chemical calculations, in these reactions, comparatively stable ( 10kcal/mol) intermolecular complexes are formed, which then yield the final products. The silicon atom of the silanone group mainly participates in complex formation. 

C02 undergoes the addition to a silanone group (reaction 7 in Table... 

The silanone route of functionalization of siloxanes turned out to have a high fimctional group tolerance - not only alkoxy and vinyl group can thus be inserted into siloxane molecules, but also NH2, Cl, CF3 and CN (Table 1). In the following, these groups can be used for additional specific functionalization, grafting, or special cure of these siloxanes. 

The first reduction step provides superoxide anion, which was shown to react with added silanone precursors as is seen fiom the decrease up to a total disappearance of the oxidation peak of O2. Using the ratio of these signals, /p(02 ) tp(02), and the kinetic treatment proposed in [10] for reversible electron transfer followed by an irreversible reaction of ion-radicals and modifying it to pseudo-first order reactions, we determined absolute rate constants of nucleophilic addition of electrogenerated superoxide anion on several silanone precursors (Scheme 5). 

In the case of the reaction of 1 with COS the intermediate silathione dimerizes to the dithiadisiletane derivative III. In the reaction of 1 with RNCS the silathione does not dimerize, but it is trapped by the respective isothiocyanate in form of the formal [2+2] cycloaddition product IV. The intermediates Cp 2Si=X (X = O, S) (2) could not be isolated but derivatized by trapping reactions. In the presence of t-butyl methyl ketone the silanone (Cp 2Si=0) reacts in an ene-type reaction [9] to give the addition product 5 the silathione (Cp 2Si=S) is transformed in a first step to an addition product analogous to 5, in a further step this species reacts with 1 in an oxidative addition process to form the final product 4 (see Scheme 2). 

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. 

It has been proposed that irradiation of 2,2,3,3-tetramesityl-4,4-dimethyl-2,3-disilaoxetane, obtained by addition of acetone to tetramesityldisilene, leads to dimesityl-silanone and 1,1-dimesityl-2,2-dimethylsilene (equation 185)76. 

Three additional processes are believed to involve the thermal generation of silanones in solution (i) cycloreversion, (ii) silanolate fragmentation and (iii) oxygen atom abstraction by a silylene. 

Addition of lone-pair carrying double bonds to silanones has not been explored much. Silanone oligomerization involves such a process as an initial step, and a 2 + 2 addition to a silanimine to form four-membered ring has also been described244. 

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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