Oligosilanes, cyclic

The photochemical addition of some cyclic oligosilanes to Ceo has also been found interesting. Scheme 8.8 shows some examples of such a transformation. Irradiation (X > 300 nm) of a toluene solution of disilirane 36 with Ceo afforded the fullerene derivative 37 in a 82% yield [37]. The reaction mechanism is still unknown. When toluene is replaced by benzonitrile the bis-silylated product of the solvent was obtained in good yields. In these experiments a photoinduced electron transfer between 36 and Ceo is demonstrated, indicating the role of Ceo as sensitizer [38]. The photoinduced reactions of disilirane 36 with higher fullerenes such as C70, Cv8(C2v)and CuiDi) have also been reported... 

Laser flash photolysis experiments showed that the (Reaction 8.17) reacts with cyclic oligosilanes 42, 43 and 44 in benzonitrile by an electron transfer mechanism [43]. The rate constant ( et) for the three-membered cyclic compound 42 is found to be 7.0 x 10 M s, whereas for the other two compounds it was more than two orders of magnitute lower, i.e., (1-2) X 10 M- s-. ... 

Similar results are observed in the reaction of a disilanyllithium with a chlorodisilane (28), as shown in Scheme 3. When a mixture of monosilyl and disilanyl anions is introduced to l-chloro-2-phenyltetramethyldisilane, the usual coupling products, 1,3-diphenyltrisilane and 1,4-diphenyltetra-silane, are formed. Alternatively, when the chlorosilane is added into a mixture of the anions, the scrambling processes via the Si-Si bond cleavage occur, to give a mixture of linear and cyclic oligosilanes. These results indicate that the scrambling processes are very fast, but much slower than the coupling of anions with chlorosilanes (see also Section VI-B). 

Another interesting ring-contraction method is the photochemical extrusion of dimethyl-silylene from cyclic oligosilanes. Ando et al. used this method to cut out two Mc2Si units from dodecamethylhexasilacyclooctyne to yield octamethyltetrasilacyclohexyne (Scheme 8-9) [32, 33]. 

Silyl radicals participate as the silylation agents in the above reactions. However, silyl cations have not yet been characterized as intermediates in these processes. Nakadaira and his co-workers reported intramolecular trapping of silyl cation by use of a OH group. In the photoreaction of cyclic oligosilane, ring-opening or insertion reactions at Si-Si bonds occur via Si-Si bond cleavage. The photore-... 

Summary 1,2-Dichlorodisilanes are prepared by reaction of silyl metal compounds with dichlorosilanes and subsequent chlorination. Alternatively, 1,2-dichlorodisilanes are obtained by insertion of a silylene in the Si-Cl bond of dichlorosilanes. Dehalogenation of 1,2-dichlorodisilanes with magnesium leads to cyclic oligosilanes or vicinally dimetallated species. Ab initio calculations show that coordinated disilenes are plausible intermediates in some of these reactions. 

A second useful precursor for the synthesis of sterically overcrowded oligosilyl anions is octakis(trimethylsilyl)tetrasilacyclobutane (8) [3], a cyclic oligosilane which can easily be converted into HTSB-K (9) by selective cleavage of the Si-SiMes bond with rBuOK in THF [4], After removal of the solvent, a highly moisture- and air-sensitive yellow-orange powder was obtained, which could be identified by NMR spectroscopy as the THF adduct of 9 (Scheme 2). 

In the 1980s and 1990s, West, Masamune and coworkers,and Hengge and Janoschek completed a comprehensive review of cyclosilanes. In recent years, the chemistry of cyclic oligosilane compounds has received much attention in experimental and theoretical studies. Matsumoto et al. developed the ladder-like oligosilanes consisting of fused cyc/o-tetrasilanes due to the uniqueness of the isopropyl groups (Chart 13.11). " ... 

Kitamura, A. Photoinduced Electron-Transfer Reaction of Cyclic Oligosilanes and Polysilanes in Solution. Heteroatom Chem. 2001,12, 269-275. 

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