Silanone synthesis

Since the silanone intermediates have been postulated as key intermediates in reports on the synthesis and reactions of organosilicon compounds,27 the isolation of a stable silanone and its characterization should be one of the most fascinating subjects in the future chemistry of low-coordinate organosilicon compounds. 

Silylenes 1 are highly reactive homologues of the carbenes, and we have been interested to compare the reactivity and primary products of the oxidation of these divalent species. In principal one can expect two different primary adducts of a silylene and molecular oxygen the formal "end-on" adducts silanone 0-oxide 2 or "side-on" adducts dioxasilirane 3 (Scheme 1). It was shown by numerous matrix studies [8-12], experiments in solution using time resolved spectroscopy [13-16], and a preparative scale synthesis in solution [17] that triplet as well as singlet carbenes yield carbonyl 0-oxides as the primary oxidation products, while dioxiranes are products of secondary photolysis. Ando et al. reported on the synthesis of the silanone 0-oxide 2e by the reaction of dimesitylsilylene le and O2 in solid argon [1]. This is so far the only experimental evidence for a silanone O-oxide. 

Keywords Silanols / Structure / Synthesis / Silanone / Steric Hindrance... 

The synthesis of a second series of compounds, the hydrido silanols (Me3Si)3CSiRH(OH), is shown in Scheme 2 [8, 10-12]. These compounds are again unusual in that they have both H and OH as substituents at silicon, which, with smaller substituents than (Me3Si)3C present, normally leads to species that undergo rapid self-condensation reactions. Such compounds are useful precursors to silanone radical anions as described below. 

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). 

Thus, the silene reaction with ketones represents a formal analogy of the Wittig olefin synthesis. Silanone is believed to be formed, since its trimer and other oligomers are isolated from the final product mixtures. Thermal reactions of the known stable 2-siloxetanes are quite complex, but at least in some cases the expected olefin is formed110,244. 

Alternative routes of nonhydrolytic conversion of chlorosilanes into siloxanes are also known. The most common one uses dimethyl sulfoxide (DMSO) as the oxygen source. Another useful method of the siloxane bond formation is the reaction of dichlorodiorganylsilanes with some metal oxides, such as ZnO (eqn [3]). The reaction is particularly useful for the synthesis of cydotrisiloxanes, which are rather difficult to prepare in any other way. By this method they can be obtained with the yield 30-60%. The mechanism of this transformation was recently studied by theoretical methods indicating that the reaction involves insertion of -Zn-O- into the Si-Cl bond. An alternative pathway involving transient silanone was shown to be much less thermodynamically feasible. 

Reactions of silylenes with unsaturated organic substrates are valuable for the synthesis of novel silicon compounds (1,2). Over the past three decades, the reactions of silylenes with carbonyl compounds have been extensively investigated (3—5). It has been shown that these reactions produce diverse types of products via siloxirane or carbonyl silaylide intermediates while the carbonyl C—O a bond is generally maintained (3—5). Complete cleavage of the C—O bond has only been observed in several cases (4). Furthermore, the synthesis and physical properties of siloles and related unsaturated carbon—silicon heterocycles are of great interest as heteroaromatic molecular precursors and potential molecular optical materials (6). In this experiment, reaction of a NHC-stabilized silacyclopentadienylidene (silole silylene) with aldehydes, leading to the formation of the novel NHC-stabilized a,(3-unsaturated silanone, was described. 

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