Silene adducts

The silicon atom in Wiberg s silenes is a Lewis acidic center and can be coordinated with donors. These donors may be halide ions [like, e.g., in 104 LiX(T2-C-4)831, ethers or nitrogen bases84. Coordination with the donor stabilizes the silenes and, when the basicity suffices (LbOcBr, THF, NMe3, F ), the adducts serve as stock compounds from which the silenes may be liberated (equation 32). 92 decomposes at —100 °C, but its trimethylamine adduct is stable at room temperature85. Noticeable amounts of 97 are available from 97-LiBr already at — 78 °C, whereas from 97 LiF the temperature has to be raised to +30 °C. Structures of such silene adducts will be discussed in Section I.C.l. The silene adduct 124-TIIF has been discussed in Section I.A.2. 

The proton and fluoride silene adducts were produced according to equations 198-201. 

Silene adducts probably react through the free silaethene, although the reaction with bromine may be an exception. Addition of bromine to 9 gives the expected dibromide, but the adduct, 9 THF, gives a mixture of the dibromide and the product >Si[0(CH2)4Br]C(Br)<. The formation of the latter suggests electrophilic addition to the Si=C bond before the dissociation of THF211. 

FIGURE 5. Molecular structures of silene adducts 9 and 10. Redrawn from Reference 68... 

Since the starting lithiated fluorosilane is available as a THF solvate, this procedure leads initially to a silenate adduct with THF which, however, can be removed by distillation with benzene. It is also possible to start with an Et2Q solvate, which yields... 

An intensely red intermediate silenate adduct Me2Si=C(SiMe3)2- Py has been observed at — 78 °C, but it decomposes already below 0°C to yield the insertion product266, possibly by a transfer of the acidic a-protons to the basic silenate carbon followed by a 1,2-shift in the resulting ylide (equation 118). 

The addition of N-(trimethylsilyl)benzophenone imine to 1,1-dimethyl-2,2-bis(trimethylsilyl)silene (54)243 proceeds in a fashion quite analogous to the reaction of benzophenone, but is much slower. Both the 2 + 2 (56) and the 4 + 2 (55) cycloadducts have been isolated (equation 74). The former is formed at only about half of the rate of the latter but dominates in the equilibrium. As described in Section III.A.2.C, the small amount of 54 present in the equilibrium can be trapped by other added reagents so that 56 represents a convenient means of storing the silene 54, which cannot be stored neat since it rapidly dimerizes244,246. Another such store is provided by the silenate adduct of 54 with NMe3, which reacts with N-(trimethylsilyl)benzophenone imine to yield 55 and 56266. 

A few routes to new silenes, usually involving flash vacuum pyrolysis at high temperatures, have been reported. Silenes were proposed as the result of the thermal expulsion of trimethylmethoxysilane, or a similar volatile fragment, from the starting material but frequently, proof that the silenes proposed to account for the observed products were in fact formed was not provided.116 119 The other thermal route employed was the retro-Diels-Alder regeneration of a silene from an adduct with an aromatic compound—often a 9,10-anthracene or 1,4-naphthalene adduct or, in some cases, a 1,4-benzene adduct, as illustrated in Eq. (19).120... 

Reactions favoring [2 + 2] cycloaddition tended to be those that had strongly electronegative groups on the sp2-hybridized silicon but only H and the neopentyl group on the sp2-hybridized carbon atom. Butadiene and cyclohexadiene generally favored [2 + 2] cycloaddition with these silenes. The [2 + 2] adducts with cyclohexadiene appear to be kinetic products, since they cleanly isomerized to the Diels-Alder adducts over time.182... 

As noted earlier, the [2 + 2] adduct of the silene Me2Si=C(SiMe3)2 with Af-trimethylsilylbenzophenonimine serves as an important source of the silene on warming to about 100°C, and the [2 + 4] adduct of the same silene with benzophenone can also serve as a store for the silene.166... 

Finally, it has recently been shown207 that some silenes can add sulfur or selenium to give silathiiranes 110, as confirmed by a crystal structure, or silaseleniranes 111, respectively (Eq. 37). Only sterically crowded silenes such as 112 gave stable solid adducts, and in the case of the selenium species the adducts visibly decomposed in the solid state over a few days. 

Sakurai et al. have provided what is probably the most important mechanistic finding in the area of intermolecular additions of silenes in recent years, namely a detailed proposal for the mechanism of alcohol addition to the silicon-carbon double bond.68 A cyclic silene 116 was synthesized in the presence of various amounts of methanol and other alcohols, and varying proportions of methanol adducts 117 and 118 were obtained. It was concluded that the methanolysis involved two steps, the first being the association of the oxygen lone pairs with the sp 2-hybridized silicon atom of the silene. The second step, proton transfer, could occur in two ways. If the proton was transferred from the complexed methanol molecule (path a) its delivery would result in syn addition. However, if a second molecule of methanol participated (path b), it would deliver its proton... 

A molecule of methanol may begin to complex with the silicon atom of the adduct 119, which is about to be sp 2-hybridized (A) as the silene frees... 

Jones218 has described an unusual photochemically initiated rearrangement of a silene-anthracene adduct to a silene which is part of an eight-membered ring (Eq. 60). Photolysis of the adduct 190 was believed to form the silaallylic diradical 191, whose canonical form 192 affords the... 

Wiberg has studied the kinetics of several systems involving the silene Me2Si=C(SiMe3)2. The kinetics for the complex system of the silene with jV-trimethylsilylbenzophenonimine, namely [2+4] adduct <=> silene + imine <=> [2+2] adduct as shown in Eq. (62), were measured174,198 as were the data for the corresponding system with benzophenone, viz. [2+4] adduct <=> silene + benzophenone <=> [2+2] adduct.220... 

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