Acid Catalysis vs. Electron-Transfer Catalysis via Organic Cations or Cation-Radicals as the Reactive Intermediate. Are These Distinctive Mechanisms?
摘要:
Proton transfer to aromatic and olefinic donors (D) lends to the facile interchange of transient carbocations (DH+) and cation-radical (D+.). The same types of cation and cation-radical are reactive intermediates in the acid catalysis and the electron-transfer catalysis of such organic transformations as benzylic coupling, epoxide.-pinacol rearrangements and cis-trans isomerization of stilbenes when they are both carried out under otherwise identical reaction conditions, However, the rapid exchange of diamagnetic cations and paramagnetic cation-radicals blurs the traditional view of sepal ate electrophilic and homolytic processes, and rigorous experimental evidence is required to establish whether acid catalysis and electron-transfer catalysis actually represent distinct mechanistic categories.
Direct Measurement of Ultrafast Carbon−Carbon Cleavage Rates via the Subpicosecond Charge-Transfer Activation of Pinacols
作者:T. M. Bockman、S. M. Hubig、J. K. Kochi
DOI:10.1021/ja980097d
日期:1998.7.1
Highly transient (benzpinacol) cation radicals (D2+•) and their ultrafast mesolytic fragmentations to the diarylhydroxymethyl radical (D•) and cation (D+) are directly observed on the early picosecond time scale upon the charge-transfer photoactivation of the intermolecular donor−acceptor complexes of pinacol donors with methyl viologen. Ultrashort lifetimes of the cation radicals with τ ≈ 10 ps obtain
Photoinduced electron transfer by charge-transfer and singlet-sensitized activation. Facile retro-pinacol via fragmentation of cation radicals
作者:S. Sankararaman、J. K. Kochi
DOI:10.1039/c39890001800
日期:——
Direct comparison of the efficiency of photoinducedelectrontransfer by charge-transferactivation of electron donor–acceptor (EDA) complexes and diffusional quenching of excited singlet acceptors is allowed by a series of silylated pinacols derived from the p-methoxytoluene donor.