Silenate adducts, formation

Scheme 8. HOMO and LUMO which characterize adduct formation of silenes and donors...

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. 

In the first stereochemical study, Brook has observed nonstereospecific addition of methanol to certain isolable silenes65,66. Although the precise stereochemistry of the products was not established, a 1/3 mixture of syn/anti or anti/syn isomers was obtained (equation 13). The results indicate a nonconcerted process for the addition of alcohol to silenes. Wiberg has proposed a two-step mechanism involving an initial formation of a silene-alcohol complex, in accord with the formation of nucleophilic adducts, followed by proton migration from the alcohol to the carbon of the silene (equation 14)59,61. This mechanism may be compatible with the results obtained by Brook and coworkers, if rotation around the silene s Si—C bond occurs faster than the proton migration. 

The silene 124 is probably formed as its THF adduct and can be trapped by, e.g., 1,3-dimethyl 2,3-butadiene to give a [4+2] cycloadduct. The attempt to liberate the silene 124 from its donor adduct results in the formation of a disilacyclobutane 125. This is ascribed to the prolonged life-time of the intermediate 359 formed by the methyl migration in the silene (equation 96), which allows for a hydrogen migration to take place. 

With the dihydropyranyl-substituted phenyldisilane 240 Ishikawa and coworkers found the formation of both possible types of silenes 241 and 242 they were formed and trapped by methanol giving adducts 401-403 (equation 120)134. 

Furthermore, the geometry of adducts is influenced by the silicon bonded X groups. It should be pointed out that the rotation about the SiY bond is obviously not much hindered therefore, the substituents at silicon are always oriented in such a way in the crystalline phase as to minimize repulsive intramolecular interactions with substituents at Y. Over and above that, it should be noticed that the formation of n-complexes of silenes stabilize the unsaturated systems. 

Trapping reactions of equimolar amounts of 2/LirBu and norbornadiene [10-12] clearly prove the formation of 3, which is trapped to give 13 in a [2+2+2] cycloaddition. Very similar to 11, 13 may serve as a silene precursor It is able to react slowly with a second equivalent of Li/Bu and norbornadiene to give the double [2+2+2] adduct 14, which is isolated alternatively from the reaction mixture of 2 with two equivalents of LilBu and norbornadiene in a one step procedure. In strong contrast to the stepwise synthesis, the latter reaction is very fast. 

We have already discussed the structure of the stable adduct of 41 and THF in Section III.A.l. The addition of anions to 1,1-dimethylsilene in the gas phase is mentioned in Section III.B.3.h. Also, the solution chemistry of the adducts of 1,1-dimethyl-2,2-bis(trimethylsilyl)silene 54 has been investigated266. This silene dimerizes rapidly even at —100 °C when free but forms an isolable silenate with trimethylamine, stable up to 0°C. Even at room temperature it decomposes only over a period of weeks and yields the dimer of 54. Since excess NMe3 slows down the formation of the dimer, the free silene 54 is clearly involved in this reaction, but it is not yet known whether it attacks a molecule of silenate or whether it dimerizes. The existence of an equilibrium between the silenate and its constituents is also supported by the mass spectrometric detection of the constituents in the vapor of this sublimable material (equation 115). It is possible to replace a weaker donor in a silenate by a stronger one, and the order of strength is F" >NMe3>NEt3>Br >THF (with 12-crown-4 complexed Li+ as the counterion). 

The evidence is the formation of the expected methanol adducts of the silanone and of the silene when methanol is present in the irradiated mixture. In an inert solvent the expected 2 4-2 cycloaddition product of the silanone and the silene is formed quantitatively76. If the suspected silene and silanone transients were present in the solution in a truly free form, one might expect the formation of some silene dimer and some silanone... 

Relative to carbon, silicon has a much smaller tendency to form compounds of coordination number less than four, such as silenes, silanones or silicenium ions, but a pronounced capacity for the enlargement of the coordination sphere. Complex formation by silicon tetrafluoride was first observed1 early in the nineteenth century, when Gay-Lussac and Davy reported the formation of the adduct SiF4 2NH3. A hexacoordinate cationic complex, Si(acac)3 HCI2 was described by Dilthey2 in 1903, and represented a new structural type for the element. 

The photocycloreversion of the cage compounds (103) and (104) affords the corresponding naphthalenes quantitatively. Noh et al. have described the photochemical dissociation of the adducts (105). Benzophenone-sensitized irradiation of the cyclobutene derivative (106) in acetonitrile solution results in the formation of a diene. The reaction mixture is subsequently heated in xylene at 150 °C whereby the dimer (107) was obtained. Irradiation of the silacyclobu-tene (108) at 193 nm brings about ring opening with the formation of the silene... 

The dimerization of 49 (Ar = Mes) yields compound 54, whose X-ray crystal structure could be determined , via diradicals 52 and 53. This reaction may be considered as a special case of silene dimerization. Although the typical course of silene dimerization leads to head-to-tail adducts (cf Section II.A), highly substituted analogues often give the head-to-head isomer. Brook and Baines have proposed that many of these dimerizations also proceed via initial formation of a Si—Si bond to give 1,4-diradicals which, in some cases, isomerize by intramolecular hydrogen abstractions . ... 

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