CIDNP study of photodealkylation in ortho-substituted aryl methyl sulphoxides

Muszkat, K. A.; Praefcke, Klaüs; Khait, Igor; Lüdersdorf, Reinhard

doi:10.1039/c39790000898

Cited by 10 publications

(7 citation statements)

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“…EPR work by Leaver (1965), a CIDNP study by Muszkat and Weinstein (1975), and product studies by Pfoertner and Bose (1970) on the aerobic photooxidation of phenols indicate radical intermediates are involved. The products from dye sensitized photooxidation are often identical to those observed from radical oxidations (Kharasch and Joshi, 1957;Gersmann and Bickel, 1959;Coppinger, 1964;Howard and Ingold, 1967).…”

Section: Introductionmentioning

confidence: 99%

CHEMISTRY OF SINGLET OXYGEN—XXVI. PHOTOOXYGENATION OF PHENOLSy†

Thomas

Foote

1978

Photochem & Photobiology

196

112

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Abstract— The aerobic dye‐sensitized photooxygenation of monohydric phenols proceeds by way of singlet oxygen under the conditions studied. Various phenols give different proportions of reaction with and quenching of singlet oxygen. Para‐substituted 2,6‐di‐t‐butylphenols show a linear correlation between the log of the total rate of singlet oxygen removal and their halfwave oxidation potentials; the same correlation is given for certain phenol methyl ethers. A Hammett plot using s̀+ gives ρ ‐ 1.72 ± 0.12, consistent with development of some charge in the quenching step. Reaction of photo‐chemically generated singlet oxygen with 2,4,6‐triphenylphenol gives 2,4,6‐triphenylphenoxy radical as an intermediate in singlet oxygen quenching, although no overall reaction occurs. Kinetic analysis indicates that the radical is derived exclusively from the interaction of 2,4,6‐triphenylphenol with singlet oxygen. A charge‐transfer mechanism for quenching of singlet oxygen by phenols is proposed.

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Section: Introductionmentioning

confidence: 99%

CHEMISTRY OF SINGLET OXYGEN—XXVI. PHOTOOXYGENATION OF PHENOLSy†

Thomas

Foote

1978

Photochem & Photobiology

196

112

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“…This took the form of weak EPR signals of sulfinyl radicals at low temperature [34][35][36][37] and chemically induced dynamic nuclear polarization (CIDNP) signals attributed to the intermediacy of sulfinyls. [38][39][40] However, overinterpretation of such data without corroborating evidence can be misleading as they do not necessarily indicate radical pathways for the majority of material. Only a brief and qualitative report of a transient absorption spectrum attributed to phenylsulfinyl has appeared previously.…”

Section: Introductionmentioning

confidence: 99%

Generation and Decay of Aryl Sulfinyl and Sulfenyl Radicals: A Transient Absorption and Computational Study

et al. 1997

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Absorption spectra and extinction coefficients of phenylsulfinyl and phenylsulfenyl (thiyl) radicals are determined by nanosecond laser photolysis in various solvents. Direct observation and characterization of arylsulfinyl radicals from the photolysis of several aromatic sulfoxides provides the strongest evidence to date for R-cleavage as the predominant primary photochemical process for these compounds. The absorption spectrum of phenylsulfinyl, with λ max ) 300 and 450 nm and ) 1.1 × 10 4 and 1.3 × 10 3 M -1 cm -1 , is practically independent of solvent. Quantum yields of free sulfinyl radicals range from 0.09 to 0.18 in various solvents. Recombination rate constants very near diffusion control indicate that there is a large spin-orbital coupling in the radical pair. Rate constants for the reactions of arylsulfinyl radicals with stable nitroxide radicals are among the fastest known, but reactivity with O 2 is very modest. Computations indicate that the singly occupied molecular orbital is a π* orbital largely localized on the sulfur and oxygen atoms.

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“…One poorly understood area of fundamental importance is the mechanism by which sulfoxides quench sensitizers that have been used to carry out various chemical reactions. − The triplet energies of simple aromatic sulfoxides are fairly high, in the range of 75−81 kcal/mol, with the higher values reserved for alkyl aryl sulfoxides . These exceed the triplet energies of molecules which have been reported as sensitizers, such as benzophenone ( E T = 69 kcal/mol) and naphthalene ( E T = 61 kcal/mol), though chemical transformations occur nonetheless.…”

Section: Introductionmentioning

confidence: 99%

“…[5][6][7] The triplet energies of simple aromatic sulfoxides are fairly high, in the range of 75-81 kcal/mol, with the higher values reserved for alkyl aryl sulfoxides. 8 These exceed the triplet energies of molecules which have been reported as sensitizers, such as benzophenone (E T ) 69 kcal/mol) and naphthalene (E T ) 61 kcal/mol), though chemical transformations occur nonetheless.…”

Section: Introductionmentioning

confidence: 99%

Mechanisms of Excited State Quenching by Aromatic Sulfoxides

1996

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Aromatic sulfoxides quench singlet excited states of sensitizers whose singlet energies render energy transfer unlikely as a mechanism. Well over 50 rate constants for singlet quenching of various sensitizers by a series sulfoxides have been obtained, as have estimates of the redox potentials for the series of sulfoxides. These data strongly suggest that the mechanism for quenching involves electron transfer and/or exciplex formation. Charge (electron) transfer is from the sensitizer to the sulfoxide. IntroductionBecause of its utility in asymmetric synthesis, the sulfoxide is a functional group of increasing interest in the organic chemistry community. Its thermal chemistry is an active area of research and has been recently reviewed. 2,3 In contrast, the photochemistry of the sulfoxide is underdeveloped. 4 Though evidence of a number of intriguing phototransformations is ample, mechanistic information is meager, and we have undertaken a systematic study.One poorly understood area of fundamental importance is the mechanism by which sulfoxides quench sensitizers that have been used to carry out various chemical reactions. [5][6][7] The triplet energies of simple aromatic sulfoxides are fairly high, in the range of 75-81 kcal/mol, with the higher values reserved for alkyl aryl sulfoxides. 8 These exceed the triplet energies of molecules which have been reported as sensitizers, such as benzophenone (E T ) 69 kcal/mol) and naphthalene (E T ) 61 kcal/mol), though chemical transformations occur nonetheless. For example, Shelton and Davis found that tert-butyl phenyl sulfoxide decomposed under sensitization with acetophenone (E T ) 74 kcal/mol), triphenylene (E T ) 66 kcal/mol), and anthraquinone (E T ) 63 kcal/mol). 9 Anomalous sensitization can also occur from singlet states. Mislow and Hammond found that aryl methyl sulfoxide photoracemization could be brought about by naphthalene (Np) singlets. [10][11][12][13] This remains the best documented case of sulfoxide sensitization. Rate constants of 3 × 10 7 to 2 × 10 8 M -1 s -1 were determined for the quenching of Np by several aryl methyl sulfoxides. Elegant experiments correlated the rate constants with sensitized racemization of the sulfoxide. Recognizing the apparently endothermic energy transfer, Cooke and Hammond suggested that Np-sensitized racemization of tolyl methyl sulfoxide might result from exciplex formation. 12,13 Yet, there was no direct evidence for the exciplex and "use of electron-donating and -withdrawing substituents on the benzene ring failed to show a clear trend or a large variation in the quenching rate constants which would support the hypothesis that charge-transfer interactions are dominant." 13 Given the usually dramatic effect of small energetic changes on rate constants in the reported range, such insensitivity is surprising. To more rigorously examine the mechanism(s) of sulfoxide quenching, a greater spread of related compounds was needed, and the rate constants so obtained needed to be correlated with other physical parameters.We have...

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CIDNP study of photodealkylation in ortho-substituted aryl methyl sulphoxides

Abstract: A magnetic polarization study of the photochemistry of ortho-substituted methyl phenyl sulphoxides indicates t h a t ortho-substituents promote efficient photodealkylation, through a triplet methyl-arylsulphinyl radical pair, escape and recombination paths of which were both observed.

Cited by 10 publications

References 0 publications

CHEMISTRY OF SINGLET OXYGEN—XXVI. PHOTOOXYGENATION OF PHENOLSy†

CHEMISTRY OF SINGLET OXYGEN—XXVI. PHOTOOXYGENATION OF PHENOLSy†

Generation and Decay of Aryl Sulfinyl and Sulfenyl Radicals: A Transient Absorption and Computational Study

Mechanisms of Excited State Quenching by Aromatic Sulfoxides

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