Silanones photochemical

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... 

The photochemical reactivity goes up with increasing number of methyl groups at the double bond and decreasing ionization potential28. Key intermediates in both the photochemical and the thermal oxidation of silenes 12, 7 and 13 are the siladioxetanes 14. These species are labile even in low-temperature matrices and could not be identified spectroscopically. Evidence for their formation comes from the observed oxidation products such as complexes 15 between silanones and formaldehyde and formylsilanols 16. 

West and coworkers studied the photolysis of several adducts of disilenes with ketones, i.e. 1,2-disiloxetanes83. Based on the products obtained when the photolysis was carried out in ethanol as a trapping agent, it appears that the heterocyclic disiloxetane 179 decomposed to the silanone 180 and the silene 181, each trapped by ethanol to give the adducts 182 and 183, respectively (Scheme 29). In the absence of a trapping agent the silene photochemically rearranged to 184. A related 1,2-disilathietane 185 showed similar behavior (Scheme 29184. 

A recent study of the photolysis of simple diazoalkanes 314 or diazirines 315, compounds known to lead to the formation of silenes under inert conditions, led, in oxygen-doped argon matrices, via the silene 316 to the siladioxirane 317. While previously postulated as an intermediate in silene oxidations, this is important experimental evidence for this intermediate. Continued photolysis of the system led to a compound identified as the silanone-formaldehyde complex 318, which on further irradiation led to the silanol-aldehyde 319. The latter compound itself underwent further photochemical oxidation leading to the silanediol 320160. The reactions are summarized in Scheme 58. Detailed infrared studies, including the use of isotopes, and calculations, were used to establish the structures of the compounds. 

The red-shift of the C=0 and Si=0 stretching vibration of 100 and 49 cm" , respectively, the calculated bond energy of complex 10 of more than 20 kcal/mol (RHF/6-31G(d,p)), and the calculated non-bonding Si-O distance of 1.98 A, being only slightly longer than the normal Si-0 bond distance, indicate the unusual electronic properties of 10. Evidently dioxasiletane 9 is formed as a thermally or photochemically unstable intermediate which rapidly decomposes to the silanone-formaldehyde complex. 

Other examples are of interest. When a compound having a dimethylsiloxy group bridging a six-membered ring was photolyzed, the extrusion of dimethylsilanone (the first photochemical synthesis of a silanone) was observed. It was not possible to definitively distinguish between two proposed mechanisms, but the second mechanism was favored92 (equation 59). 

Silanone produced by the silane-ozone photochemical reaction in Ar matrices gives rise to a band at 1202 cm in the IR spectrum that is assigned to the v(Si=0) [766] ... 

Thus, silanones are generated from (141) by thermal or photochemical means. 145 as js hexamethyl-1.4-disiiabenzene from (142). 146 The adducts... 

As primary products of the reaction of silylenes with molecular oxygen the formation of either silanone (9-oxides 3 or the dioxasiliranes 4 can be expected. The homologous carbenes react exclusively to give carbonyl (9-oxides 5 which rearrange photochemically to dioxiranes 6 (Scheme 5.1). ... 

Thus, silanones are generated from (141) by thermal or photochemical... 

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