1.2- Disilirane

Strained Si-Si a bonds are also known to act as good electron donors [263-265]. Photoinduced electron transfer from strained disiliranes such as 1,1,2,2-tetramesityl-1,2-disilirane to Ceo occurs in benzonitrile to give a transient absorption spectrum at 1080 nm due to Ceo obtained by 532 nm laser flash photolysis of Ceo in the presence of disilirane [266]. The photochemical reaction of Cso with the disilirane in benzonitrile afforded the 1 1 and 1 2 adducts of disilirane and benzonitrile in good yields. The structures of the corresponding adducts were determined by X-ray crystallographic analysis [266]. These adducts were also obtained in good yields in the presence of catalytic amounts of Ceo- Thus, Ceo acts as a photoinduced electron-transfer catalyst as shown in Scheme 9. 

Akasaka T, Nagase S, Kobayashi K, Suzuki T K, Kikuohi K, Aohiba Y, Yamamato K, Funasaka FI and Takahashi T 1995 Synthesis of the first adduots of the dimetallofullerene La2 Cgg and Se2 Cg by addition of a disilirane Angew. Chem. Int. Edn. Engl. 34 2139... 

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... 

Photochemical reaction with disiliranes leads to a [3-i-2]-cycloaddition (Section 4.3.9). Disilylcyclobutanes [67, 68] and cyclotetrasilanes [68, 69] react in a similar fashion. In both cases the four-membered ring is photolytically cleaved and a diradical is formed. This diradical adds in a formal cycloaddition to the [6,6] double bond of CgQ. The cycloadduct 20 (R = 4-MeCgH4) can be obtained from 19 in 13% yield, but it is not the major product (Scheme 6.12). Rearrangement of an H atom leads to 21 in 46% yield. The product distribution depends strongly on R. Changing R from 4-MeCgH4 to phenyl leads to an exclusive formation of 21. 

Since strained silicon-silicon a-bonds are mnch more reactive than the linear Si—Si bond toward atmospheric oxygen, most stndies were carried ont for strained disiliranes 82. 

An oxygen-saturated acetonitrile solution of l,l,2,2-tetramesityl-l,2-disilirane 82a was irradiated to give dioxygenated product, 3,3,5,5-tetramesityl-l,2-dioxa-3,5-disilolane 83a, and monooxygenated product, 3,3,5,5-tetramesityl-l,3-cyclodisiloxane 84a, in 75 and 25% yields, respectively. Similar results were obtained with 2,2,3,3-tetrakis(2,6-diisopropyl-phenyl)oxadisilirane 82b and 2,2,3,3-tetrakis(2,6-diethylphenyl)oxadisilirane 82c, affording the corresponding l,2,4-trioxa-3,5-disilolanes 83b and 83c (equation 90) . ... 

TABLE 17. Disilirane oxidation with molecular oxygen... 

The singlet oxygenation of disiliranes 82a-f in benzene with tetraphenylporphyrin (TPP) as sensitizer gave cyclic peroxides 83a-f in 40-92% yields with a small amount of dioxasiletane 84a-f (Table 17)94,98,99... 

Clearly, TPP-sensitized oxygenation of disilirane proceeds in a stereospecific manner via peroxonium ion intermediate 89 (Scheme 23). The potential energy surface of the... 

Disilirane 87 Conditions Product 88 Yield(%) cis/trans Ratio... 

Compounds with a Si=Si or Ge=Ge bond (i.e., disilenes and digermenes) can be isolated when the double bond contains bulky substituents. Only a few cycloaddition reactions with diazo compounds are known, and two reaction modes have been observed. One of the paths leads to the formation of disiliranes 184 (242) and digermiranes 185 (243) (Scheme 8.42), probably by ring contraction of an initially formed [3 + 2] cycloaddition product. The other path involves a 1,1-cycloaddition of the diazoalkane to give disilaaziridine 186 (244) and digermaazir-idine 187 (245). This nitrene-like reactivity is rather uncommon although some intramolecular examples are known (see Section 8.6.1). 

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