Disilane radical cations

Figure 12 Observed (—) and simulated (—) ESR spectra of disilane and polysilane radical cations at 77 K in -y-irradiated freon matrices. The simulation for polymers was carried out under the assumption that an unpaired electron is localized on a part of polymer skeleton composed of about six Si atoms.

Nakadaira et al. have reported the photoalkylation and photosilylation of polycyanobenzenes by the use of disilanes [438 140], They also reported that photosubstitution of TCNB by use of ( -Bu)3SnSiMe3 (Scheme 122). In this photoreaction, the Sn—C bond of the radical cation is cleaved in preference to the Sn—Si and Si—C bonds to generate an //-butyl radical. Similar photosubstitution of 2,3-dicyanopyradines has been reported by Mizuno and his co-workers [437],... 

FIGURE 3. (a) Comparison of the He(I) PE spectra of the >3d-symmetric molecules ethane and disilane, which exhibit Jahn/Teller splitting (J/T) of their M (e) states with dominant contributions ( ) to positive hole delocalization, based on their quantitative radical cation state assignment (see text), (b) United Atom correlation for the isoelectronic 18-electron species from Ar to SiFLj including the iso(valence)electronic 10-electron molecule CH4 with average atomic ionization energies... 

FIGURE 12. Low-energy region of the (He I)PE spectrum of hexakis(trimethylsilyl)disilane (a) Gaussian-shape band deconvolution between 7.5 and 9.5 eV and (b) topological SiSi bond interaction model with radical cation state assignment... 

When unsymmetrical disilanes are used as substrates, more bulky silyl groups are predominantly added to the dicyanoethenes. A key step of this photoreaction is also the reaction of the radical anion of the dicyanoethene with silyl radicals which are generated by solvent-assisted cleavage of the radical cations of disilanes (Scheme 41) [125],... 

Mizuno and his co-workers reported the photosilylation of electron-deficient alkenes by use of hexamethyldisilane and unsymmetrical disilanes. The photoreaction of l,l-dicyano-2-phenylethene and disilanes in acetonitrile in the presence of phenanthrene affords sily-lated dicyanoethanes in good yields. A key intermediate in this process is the silyl radical, generated by solvent (nucleophile)-assisted cleavage of the radical cation of disilanes. Silyl radical attack at the radical... 

Ando and co-workers reported that electron transfer photooxygenation of three- and four membered cyclic disilanes gives cyclic perox-ides. DCA-sensitized photooxygenation of 1,2-disiletenes affords cyclic peroxides, 1,2,3,6-dioxadisilins, and 1,2,5-oxadisilole-nes. Similar photooxygenation of oxadisiliranes gives 1,2,4,3,5- tri-oxadisilolanes. For these photoreactions, Si-Si bond cleavage in the radical cations of the cyclic disilanes is postulated to be the key process. 

Pig. 5. Photoelectron spectroscopic radical cation state comparison of the iso(valence)electronic molecules disilane and ethane as well as an approximate se<)uence of effective nuclear potentials for some main group element centers (cf. text) [1]. 

The oxidation of sterically hindered disilanes by trityl cation in pivaloyl nitrile was shown to yield silylium ions which form with the solvent silylnitrilium ions, e.g., 38 (Scheme 14) [53]. The generatitMi of silyl cations was, however, only observed for sterically overloaded disilanes, such as hexa-tcrt.-butyldisilane. Hexaethyldisilane for example was found to be inert under the applied reaction conditions [53], In view of the problems with bulky silanes in the standard hydride-transfer reaction, the oxidation of disilanes is a complementary addendum to the synthetic methodology. The very limited preparative access to stable radicals restricts the oxidation of silyl radicals to give silylium ions to only a few selected examples. The most prominent example is the oxidation of the stable radical 39 to the homoaromatic silylium ion 40 by trityl cation (Scheme 15) [23]. 

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