Rates of hydride transfer from hydrosilanes HSiR1R2R3 with widely varying substitution to para-substituted diarylcarbenium ions have been measured in dichloromethane solution. Generally the reactions follow a second-order rate law, -d[Ar,CH+]/dt = k2[Ar2CH+] [HSiR1R2R3], and k2 is independent of the degree of ion-pairing and the nature of the counterion (exceptions are reported). The reaction rates are almost independent of solvent polarity. Kinetic isotope effects exclude an SET-type mechanism and are in accord with a polar mechanism with rate-determining formation of silicenium ions. The reactivities of para-substituted aryldimethylsilanes are linearly correlated with up ( p = -2.46), not with up+. In the series H3SiHex, H2SiHex2, HSiHex3, the relative reactivities are 1.00:155:7890, and in the corresponding phenyl series the reactivity increase is much smaller (H3SiPh:H2SiPh2:HSiPh3 = 1.00:17.2: 119). As a consequence, trihexylsilane is approximately two orders of magnitude more reactive than triphenylsilane though hexylsilane and phenylsilane show similar reactivities. Tris(trimethylsily1)silane is just slightly more reactive than trimethylsilane. Replacement of hydrogen by chlorine reduces the reactivity by one order of magnitude. Variation of the electrophilicities of the hydride abstractors does not affect the relative reactivities of the silanes, Le., constant selectivity (Ritchie-type) relationships are encountered. Correlation equations are given, which permit the calculation of hydride transfer rates from hydrosilanes to any carbenium ion on the basis of pKR+ values or the ethanolysis rate constants of the corresponding alkyl chlorides.

and carbon-centered "multiple warhead" tetraradical 10, a potentially powerful species for DNA damage. In the event, however, none of the products expected to arise from 10 were formed. Instead, the two novel dihydroperoxides 8 and 9 were isolated in 22 and 10% yield, respectively. The same transformations were also effected, although less cleanly, by thermolysis in the presence of hydrogen atom donors. Quite intriguing is the possibility of the intermediacy of a dioxetener'] which is presumed to eject the equivalent of ethenedione,"] perhaps as two carbon monoxide molecules.[91 Compounds2 and 3 caused significant DNA cleavage when incubated with +XI74 supercoiled DNA at pH 8.5 and 50 "C (Fig. 2). Interestingly, dione 4 caused only slight DNA damage under the same conditions, whereas hydroperoxides 8 and 9 showed, as expected, quite strong DNA-cleaving activities. Based on the chemistry of these systems we presume that whereas the ten-membered ring enediynes 2 and 3 may exert their DNA-cleaving power by cyclization to an aromatic diradical, the open-chain enediynes 8 and 9 may operate by an entirely different mechanism involving the generation of oxygen-centered radicals.

ANGEWANDTE

Abstract:Rates of hydride abstractions from j3-silyl-, j3-germyl-, and B-stannyl-substituted silanes [HSiMeZ(CHzMMe3), M = Si, Ge, Sn] and related compounds by diarylcarbenium ions have been measured. The intermediate silicenium ions, produced in the rate-determining step, are stabilized by hyperconjugation, but the effects are much smaller than in comparable carbenium ions.

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