Ryoichi Akaba

Mechanistic considerations on photoreaction of organic compounds via excitation of contact charge transfer complexes with oxygen

Abstract Reactions of various organic compounds induced by excitation of their contact charge transfer (CCT) pairs with oxygen were classified into three types of behaviour on the basis of reaction products. It is proposed that the excited state of the CCT pairs gives the products through the most isoenergetic course along the plausible exothermic pathway.

TRIPLET-MEDIATED ELECTRON TRANSFER OXYGENATION OF STILBENE DERIVATIVES WITH 2,4,6-TRIPHENYLPYRYLIUM TETRAFLUOROBORATE

Abstract The triplet state of 2,4,6-triphenylpyrylium tetrafluoroborate (TPP + ) has efficiently induced an electron transfer oxygenation of stilbene derivatives.

Detection of dimeric radical cations of trans-stilbene in 2,4,6-triphenylpyrylium tetrafluoroborate-sensitized electron transfer reactions in dichloromethane

Abstract Laser flash photolysis of 2,4,6-triphenylpyrylium tetrafluoroborate in the presence of trans-stilbene, triphenylethylene, and tetraphenylethylene has been carried out in dichloromethane. It has been revealed that trans-stilbene radical cations form dimeric radical cations of trans-stilbene through a reaction with its neutral while the radical cations of triphenylethylene and tetraphenyl-ethylene do not.

Autoxidation of adamantylideneadamantane in a protic solvent. Facile proton-induced electron transfer from the olefin to oxygen

Abstract Addition of trifluoroacetic acid to adamantylideneadamantane in dichloromethane solution under oxygen in the dark results in a rapid oxygenation of the olefin. The reaction is proposed to proceed through a mechanism involving proton-induced electron transfer from the olefin to oxygen giving its radical cation and a hydroperoxy radical followed by their subsequent reactions.

Triphenylpyrylium-salt-sensitized electron transfer oxygenation of adamantylideneadamantane. Product, fluorescence quenching, and laser flash photolysis studies

Triphenylpyrylium tetrafluoroborate (TPP+BF4–) has been found to sensitize oxygenation of adamantylideneadamantane to its dioxetane in good yield. Product studies, measurements of quantum yields for dioxetane formation, laser flash photolysis, and quenching experiments of TPP+ fluorescence by various electron donors have been carried out to elucidate the mechanism for dioxetane formation. Cyclic voltammetry was also used to examine the reactivity of pyryl radicals (TPP˙) with molecular oxygen. The results are consistent with a mechanism involving electron transfer from the alkene to the excited states of TPP+ to give radical pairs consisting of the alkene radical cations and TPP˙. The alkene radical cation reacts with molecular oxygen to afford the dioxetane. A singlet oxygen mechanism is not likely since the quenching rate of the excited triplet state of TPP+ by the alkene is 400 times faster than by dissolved oxygen under the photooxygenation conditions.

Triphenylpyrylium salt as sensitizer for electron transfer oxygenation not involving superoxide anion

2,4,6-Triphenylpyrylium tetrafluoroborate sensitized electron transfer oxygenation of adamantylideneadamantane proceeds via the reaction of the alkene radical cations with molecular oxygen without participation of superoxide anion.

HIGHLY SPECIFIC ISOMERIZATION OF AROMATIC OLEFIN RADICAL CATIONS PRODUCED BY PHOTOSENSITIZED ELECTRON TRANSFER

The efficiency of unimolecular cis→trans conversion of the stilbene radical cations is enhanced by introduction of two bromine atoms, two methyl or methoxy groups at the 4- and 4′-positions and by addition of salts like lithium or magnesium perchlorate. On 2,4,6-triphenylpyrylium tetrafluoroborate (TPP+ BF 4 -) sensitization, only the triplet radical pair is effective for the isomerization though both of the singlet and triplet excited states of the sensitizer are efficiently quenched. The isomerization efficiency increases in a sodium dodecyl sulfate (SDS) micellar solution.