Electron donor-acceptor complexes photochemical activation

Electron donor-acceptor complexes, electron transfer in the thermal and photochemical activation of, in organic and organometallic reactions, 29, 185 Electron spin resonance, identification of organic free radicals, 1, 284 Electron spin resonance, studies of short-lived organic radicals, 5, 23 Electron storage and transfer in organic redox systems with multiple electrophores, 28, 1... 

Electron transfer, in thermal and photochemical activation of electron donor-acceptor complexes in organic and organometallic reactions, 29,185 Electron-transfer, single, and nucleophilic substitution, 26,1 Electron-transfer, spin trapping and, 31,91 Electron-transfer paradigm for organic reactivity, 35, 193... 

Electron Transfer in the Thermal and Photochemical Activation of Electron Donor-Acceptor Complexes in Organic and Organometallic Reactions... 

Photochemical electron-transfer can be effected by irradiation of the charge-transfer absorption band of the electron donor-acceptor complex.15 Alternatively, photochemical electron-transfer may proceed by actinic activation of RH followed by quenching with A, or by the reverse sequence involving activation of A and quenching with RH. 

Having shown that the enol silyl ethers are effective electron donors for the [D, A] complex formation with various electron acceptors, let us now examine the electron-transfer activation (thermal and photochemical) of the donor/ acceptor complexes of tetranitromethane and quinones with enol silyl ethers for nitration and oxidative addition, respectively, via ion radicals as critical reactive intermediates. 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

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