Silanones rearrangement

The rearrangement of silicon compounds 97 does not involve the silanone 100 as an intermediate (Scheme 24). 

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. 

The Barton laboratory has found many other rearrangments of silylenes3,5 including strong evidence for the thermal isomerization of silylenes to silanones (equation 83)193. Results were also found that were consistent with silanone-to-silylene194 and silylene-to-silanone-to-silylene143 rearrangements, but alternative mechanisms could be written, e.g. 

Carbene 13 is extremely photolabile, and 570 nm irradiation results in the rearrangement to acylsilanes 14. The calculated activation barrier for this highly exothermic rearrangement is only 4 7 kcal/mol. If we assume that the analogous reaction of silanone and formaldehyde also leads to a labile siloxycarbene, the formation of formylsilanol 11 is easily explained. 

Only one of the adducts decomposed thermally to an olefin and the expected trimer of bis(trimethylsilyl)silanone. This was the adduct of 2-mesityl-2-trimethylsiloxy-l,l-bis(trimethylsilyl)silene with fluorenone all others underwent an intramolecular rearrangement and the authors suggest that this is primarily due to steric reasons110. 

There is evidence that the reverse rearrangement of a silanone to a silylene might have been observed as well374. 

Also, 2-siloxetenes serve as thermal precursors to silanones. The gas-phase pyrolysis of bis(dimethylsilyl)ketene or (trimethylsilyl)(dimethylsilyl)ketene has been proposed to proceed by a retro-Wolff rearrangement followed by a 1,2-hydrogen shift and electro-cyclic closure to a 2-siloxetene, which then fragments to dimethylsilanone and an acetylene (equation 192)231. 

The only reaction type that can be considered well established for silanones is attack by nucleophiles, which is extremely facile. Although some evidence compatible with a high-temperature silylsilanone-to-siloxysilylene rearrangement exists, an alternative interpretation has not been ruled out so that the occurrence of this rearrangement cannot be considered as established374. 

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). ... 

DFT calculations predict a very small barrier (ca. 1 kcal/mol) for the rearrangement of silanone oxide 3f to dioxasilirane Although ab initio calculations at the MP2 level of theory predict a somewhat larger barrier (ca. 6 kcal/mol), it is safe to say that the barrier for the 3 4 rearrangement is much smaller than the barrier for the rearrangement of 5 to 6, while the former rearrangement is considerably more exothermic. This explains why, even under the conditions of matrix isolation, only the siladioxiranes 4, and not the proposed primary adducts of molecular oxygen and silylenes 1, the silanone oxides 3, are observed. 

Milstein and coworkers [43] have also contributed to group 10 metal PSiP chemistry, having attempted to stabilize Pt silanone (R2Si=0) species in the framework of a PSiP pincer. The attempted deprotonation of the silanol Pt" complex 120 with a strong base resulted in an unusual rearrangement process that afforded the dinuclear Pt hydride complex 125 (Scheme 6.26). A proposed... 

In 1989 Jutzi and coworkers reported the reaction of decamethylsilicocene 41 with tri- -butylphosphine selenide in benzene at room temperature leading to almost quantitative formation of 1,3,2,4-diselenadisiletane derivative 138, a head-to-tail [2 + 2] cycloaddition reaction product of the initially formed silaneselone 137. The intermediacy of silaneselone 137 was supported by the fact that the reaction in the presence of 2,3-dimethyl-l,3-butadiene resulted in the formation of the corresponding [2-1-4] cycloaddition reaction product 139 (Scheme 41). As in the cases of silanone 44 and silanethione 80, the ligands on silicon undergo a haptotropic rearrangement from jj -CsMes in 41 to ij -CsMes in 138 or 139. Apparently, silaneselone 137 is not kinetically stable enough to be isolated under normal conditions. 

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