- Radiolytic studies of the mechanism of autoxidation of triphenylphosphine and related compounds
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Triphenylphosphine (Ph3P) undergoes one-electron oxidation in irradiated CH2Cl2 solutions to yield the radical cation Ph3P?+. This short-lived species exhibits intense absorption with a maximum at 325 nm and an extended shoulder at 400-500 nm and decays by a second-order process in the absence of O2. In the presence of O2, however, the radical cation reacts with O2 with a rate constant of 7 × 106 L mol-1 s-1 to yield a peroxyl radical, Ph3P+OO?, which exhibits no significant absorbance at λ > 300 nm. Similar results, but with slightly different rate constants, were obtained also in CCl4 solutions and in CH3CN and cyclohexane solutions containing 1% CCl4. Tris(2,4,6-trimethoxyphenyl)phosphine in CH2Cl2 exhibits a behavior similar to that of Ph3P, but the reaction of its radical cation with O2 is an order of magnitude more rapid. On the other hand, the perfluorinated Ph3P?+ in CH2Cl2 reacts with O2 much more slowly, if at all. Diphenyl-2-pyridylphosphine yields a radical cation which exhibits a slightly narrower absorption band but reacts with O2 with the same rate constant as Ph3P?+. The peroxyl radicals propagate a chain reaction by further oxidation of phosphine molecules to Ph3P?+ either directly or indirectly. The final radiolysis product is the phosphine oxide, Ph3PO. The radiolytic yields for oxidation of the phosphines were much higher than the radiolytic yield of the solvent derived radicals, except for the case of the perfluoro derivative, and were dependent on the concentrations of phosphine and O2 and on the dose rate. At low dose rates and high solute concentrations the chain lengths exceeded 1000.
- Alfassi,Neta,Beaver, Bruce
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- Effects of substituents on aryl groups during the reaction of triarylphosphine radical cation and oxygen
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In a previous report (S. Yasui, S. Tojo and T. Majima, J. Org. Chem., 2005, 70, 1276), we presented the results from the laser flash photolysis (LFP) and product analysis of the 9,10-dicyanoanthracene (DCA)-photosensitized oxidation of triarylphosphine (Ar3P) in acetonitrile under air, which showed that the photoreaction results in the oxidation of Ar3P to give the corresponding phosphine oxide (Ar3PO) in a nearly quantitative yield, and that the reaction is initiated by the electron transfer (ET) from Ar 3P to DCA in the singlet excited state (1DCA*), producing the triarylphosphine radical cation Ar3P+. This radical cation decays through radical coupling with O2 to afford the peroxy radical cation Ar3P+-O-O, which we proposed to be the intermediate leading to the product Ar3PO. We now examined this photoreaction in more detail using ten kinds of Ar3P with various electronic and steric characteristics. The decay rate of Ar3P + measured by the LFP was only slightly affected by the substituents on the aryl groups of Ar3P. During the photolysis of trimesitylphosphine (Mes3P), the peroxy radical cation intermediate (Mes3P+-O-O) had a lifetime long enough to be spectrophotometrically detected. The quantum yields of Ar3PO increased with either electron-withdrawing or -releasing substituents on the aryl groups, suggesting that a radical center is developed on the phosphorus atom during the step when the quantum yield is determined. In addition, the o-methyl substituents in Ar3P decreased the quantum yield. These results clearly indicated that Ar3P+-O-O undergoes radical attack upon the parent phosphine Ar3P that eventually produces the final product, Ar3PO. The Royal Society of Chemistry 2006.
- Yasui, Shinro,Tojo, Sachiko,Majima, Tetsuro
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p. 2969 - 2973
(2008/02/11)
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- Reaction of triarylphosphine radical cations generated from photoinduced electron transfer in the presence of oxygen
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(Chemical Equation Presented) The 9,10-dicyanoanthracene (DCA)-sensitized photoreaction of triarylphosphines (1) was carried out in acetonitrile under aerobic conditions. Phosphine 1 was oxidized to the corresponding phosphine oxide with no appreciable side reactions. Product analysis and laser flash photolysis experiments suggest that the radical cation of 1 formed by the electron transfer from 1 to DCA in the singlet excited state ( 1DCA*) reacts with O2 to eventually afford the phosphine oxide.
- Yasui, Shinro,Tojo, Sachiko,Majima, Tetsuro
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p. 1276 - 1280
(2007/10/03)
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- Three-electron ?-Bonded Radicals: An Electron Paramagnetic Resonance Spectroscopic Study of R3PCl Radicals in Freon
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Our main purpose is to draw attention to the relative stability of three-electron ?-bonded radicals (?12, ?21), the large number of such radicals that have been studied by EPR spectroscopists, and their potential importance as reaction intermediates.The topic is illustrated by experiments which show that when R3P.+ radical cations (Me3P.+ amd Ph3P.+) are generated in CFCl3 at 77 K by exposure to 60Co γ-rays, and the system is annealed to ca. 155 K, features characteristic of R3PCl radicals are detected, with well defined hyperfine coupling to 31P and 35/37Cl nuclei.It is argued that these must have been formed from chloride ions released from (CFCl3).- radical-anions following electron capture.
- Abu-Raqabah, Atieh,Symons, Martyn C. R.
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p. 3293 - 3298
(2007/10/02)
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- Preparation and Structure of Certain Phosphorus-centred Radical Cations
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Exposure of dilute solutions of various trivalent phosphorus derivatives, PL3, in fluorotrichloromethane to (60)Co γ-rays at 77 K gave the corresponding cations, (1+), characterised by their e.s.r. spectra.The spectra establish that the SOMO for these cations is considerably localised on phosphorus, which contributes extensive atomic 3s and 3p character.The results confirm and extend previous studies using sulphuric acid matrices.The tendency for these cations to react with corresponding neutral molecules to give ?* dimer-cations PL3>(1+) is also confirmed. The structures of these species are discussed and compared with those of related radicals.
- Hasegawa, Akinori,McConnachie, Glen D. G.,Symons, Martyn C. R.
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p. 1005 - 1016
(2007/10/02)
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