Addition reactions of silyl radicals to unsaturated compounds

Alberti, Angelo; Pedulli, Gian Franco

doi:10.1007/bf03156140

Cited by 28 publications

(6 citation statements)

References 142 publications

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“…The reaction of (TMS) 3 Ge • with diaryl ketones gave rise to EPR spectra of moderate intensity, indicative of steady-state concentrations of radical adducts lower than those observed when adding (TMS) 3 Si • or Et 3 Si • radicals to the same compounds. , The hyperfine splitting constants of these radicals are close to those reported for the related adducts of other germyl and silyl radicals and have been assigned on a similar basis …”

Section: Resultssupporting

confidence: 66%

Addition Reactions of Tris(trimethylsilyl)germyl Radicals to Unsaturated Compounds. An EPR and Product Study

et al. 1997

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Tris(trimethylsilyl)germyl radicals, (TMS)(3)Ge(*), thermally or photochemically generated by means of suitable radical initiators, were reacted with a number of unsaturated compounds. The preparative scale reaction of (TMS)(3)GeH with a variety of alkynes proceeded stereo- and regioselectively affording exclusively 2-alkenylgermanes in excellent chemical yields. In the analogous reactions with alkenes, no addition products were obtained except in the case of 4-vinylpyridine. This behavior is explained in terms of the reversibility of germyl radical addition to olefines. The reaction with arylalkenes, ketones, quinones, azines, and nitroalkanes was instead carried out in the cavity of an EPR spectrometer. In most cases the corresponding radical adducts were observed; however, quite often the intensity of the spectra was lower than expected and with azines no EPR signals could be detected. These results have been interpreted as an indication that the addition reaction of tris(trimethylsilyl)germyl radicals to multiple bonds is less exothermic than that of trialkylgermyl radicals. With some of the investigated compounds, the addition of (TMS)(3)Ge(*) is thermoneutral or even endothermic and is therefore readily reversible.

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Section: Resultssupporting

confidence: 66%

Addition Reactions of Tris(trimethylsilyl)germyl Radicals to Unsaturated Compounds. An EPR and Product Study

et al. 1997

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“…As it will be shown later, E γ ′ centers surviving the warming at room temperature do not react with adsorbed polybutadiene. This lack of reactivity is at variance with the chemistry of Si radicals, which are known to be even more reactive toward oxygen and double bond addition than their C analogues. ,, A possible explanation is that the stable fraction of E′ γ centers is buried out of the reach of reactants within the bulk of the silica structure.…”

Section: Results and Discussionmentioning

confidence: 99%

Reaction Mechanisms in Irradiated, Precipitated, and Mesoporous Silica

et al. 2013

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A matrix EPR spectroscopy study of the low temperature γ radiolysis of precipitated (Zeosil) and mesoporous high surface silica has afforded evidence of the formation of trapped H-atoms, H-atom centers, siloxy radicals ≡Si-O(•), anomalous silyl peroxy radicals ≡Si-OO(•) with reduced g tensor anisotropy, siloxy radical-cations (≡Si-O-Si≡)(+•), E' centers, and two species from Ge impurity. Coordination of peroxyl radicals with diamagnetic ≡Si(+) centers is proposed and tested by DFT computations in order to justify the observed g tensor. Coordination of H-atoms to ≡Si(+) centers is also proposed for the structure of the H-atom centers as an alternative model not requiring the intervention of Ge, Sn, or CO impurities. The DFT method has been employed to assess the electronic structure of siloxy radical-cations and its similarity with that of the carbon radical-cation analogues; the results have prompted a revision of the structures proposed in the literature for ST1 and ST2 centers. The comparison between the two types of silica has afforded evidence of different radiolysis mechanisms leading to a greater yield of trapped H-atoms and H-atom centers in zeosil silica, which is reckoned with the 4-fold greater concentration of silanol groups. Parallel radiolysis experiments carried out by using both types of silica with polybutadiene oligomers as adsorbate have afforded evidence of free valence and energy migration phenomena leading to irreversible linking of polybutadiene chains onto silica. Reaction mechanisms are proposed based on the detection of SiO2-bonded free radicals whose structure has been defined by EPR.

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“…The bond angles θ determined at the UHF/VTZP level are compared in Table with the values obtained from the C-2s character s in the SOMO estimated from the experimental 13 C α hfs constants, a C exp , of imidoyls Y−C(•)NBu t (Y = N(SiBu t 3 ) 2 , OSiMe 3 , OBu t , SiEt 3 , SBu t 3 , SCF 3 ) bearing alkyl groups instead of γ-hydrogens. Still now, ESR spectroscopists , assume that in localized radicals the contributions to the hfs constant at the radical center due to spin-polarization and spin-delocalization are negligible and determine the s-character by dividing simply the experimental hfs constant by the value ( A 0 ) computed for a unitary atomic s-population 1 setting in eq 3 the total spin population at the radical center (ρ C ) and the polarization contribution a C pol equal to unit and zero, respectively. In the present case the contribution due to spin-polarization has not been neglected in eq 4 because it can be approximated by the value (29.8 G) of a C exp in the Et 3 Si−C(•)NBu t radical since the direct contribution vanishes in a π-electron radical.…”

Section: Resultsmentioning

confidence: 99%

Theoretical Evidence from UQCISD Calculations against the Linear π-Configuration of α-Silylimidoyl Radicals As Determined by ESR Spectroscopy

Guerra

1996

J. Phys. Chem.

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Geometries of imidoyl radicals Y−C(•)NMe have been determined by means of ab initio UHF/VTZP calculations. The local radical structure is found to be essentially indipendent of the electronic nature of the α-substituent, the ∠YCN bond angle being about 128°. This structural information is in contrast to that obtained from the magnitude of the experimental isotropic 13C hyperfine splitting (hfs) constants indicating that imidoyls bearing electropositive α-substituents such as silyl groups are nearly linear about Cα. UQCISD/VTZP calculations on H3Si−C(•)NH confirm that α-silylimidoyl radicals should be largely bent at Cα. At variance with what was previously found in other localized radicals, UMP2/DZP//UHF/VTZP calculations do not closely reproduce the hfs constants at the radical center but underestimate largely their values owing not only to slow convergence of the doubles amplitudes in Møller−Plesset perturbation theory but mainly to neglect of single excitations. Excellent agreement with experiment has been achieved at the UQCISD/DZP//UHF/VTZP level.

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Addition reactions of silyl radicals to unsaturated compounds

Cited by 28 publications

References 142 publications

Addition Reactions of Tris(trimethylsilyl)germyl Radicals to Unsaturated Compounds. An EPR and Product Study

Addition Reactions of Tris(trimethylsilyl)germyl Radicals to Unsaturated Compounds. An EPR and Product Study

Reaction Mechanisms in Irradiated, Precipitated, and Mesoporous Silica

Theoretical Evidence from UQCISD Calculations against the Linear π-Configuration of α-Silylimidoyl Radicals As Determined by ESR Spectroscopy

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