Oxygen Acidity of 1-Arylalkanol Radical Cations. 4-Methoxycumyloxyl Radical as −C(Me)2−O--to-Nucleus Electron-Transfer Intermediate in the Reaction of 4-Methoxycumyl Alcohol Radical Cation with OH-

1998 ◽  
Vol 120 (44) ◽  
pp. 11516-11517 ◽  
Author(s):  
Enrico Baciocchi ◽  
Massimo Bietti ◽  
Osvaldo Lanzalunga ◽  
Steen Steenken
Keyword(s):  
Electron Transfer ◽  
Radical Cation ◽  
Radical Cations ◽  
Alcohol Radical

1,n-Radical ions. Photosensitized (electron transfer) carbon–carbon bond cleavage. Formation of 1,6-radical cations

1991 ◽  
Vol 69 (2) ◽  
pp. 225-233 ◽  
Author(s):  
Donald R. Arnold ◽  
Laurie J. Lamont ◽  
Allyson L. Perrott
Keyword(s):  
Electron Transfer ◽  
Radical Cation ◽  
Bond Cleavage ◽  
Radical Cations ◽  
Dimethyl Acetal ◽  
Mercury Vapour ◽  
Carbon Bond ◽  
Carbon Carbon Bond ◽  
Mercury Vapour Lamp ◽  
Mm2 Calculations

The reactivity of the radical cations of methyl 2,2-diphenylcyclohexyl ether (7), 6,6-diphenyl-1,4-dioxaspiro[4.5]decane (8), methyl cis- and trans-2-phenylcyclohexyl ether (9cis and trans), and 6-phenyl-1,4-dioxaspiro[4.5]decane (10), generated by photosensitized (electron transfer) irradiation, has been studied. Solutions of the ethers and acetals in acetonitrile–methanol (3:1), with 1,4-dicyanobenzene (2) serving as the electron acceptor, were irradiated with a medium-pressure mercury vapour lamp through Pyrex. The diphenyl derivatives 7 and 8 were reactive; 7 gave 6,6-diphenylhexanal dimethyl acetal (11) and 8 gave 2-methoxy-2-(5,5-diphenylpentyl)-1,3-dioxolane (12). These are the products expected from the intermediate 1,6-radical cation, formed upon carbon–carbon bond cleavage of the cyclic radical cation. The monophenyl derivatives 9cis and trans and 10 were stable under these irradiation conditions. The mechanism for the carbon–carbon bond cleavage and for the cis–trans isomerization is discussed. An explanation, based upon conformation, is offered for the lack of reactivity of 9 and 10. Molecular mechanics (MM2) calculations were used to determine the preferred conformation of 9cis and trans, and 10. Key words: photosensitization, electron transfer, radical cation, carbon–carbon bond cleavage, conformation.

Radical ions in photochemistry. 18. The photosensitized (electron transfer) tautomerization of alkenes; the 1,1-diphenyl alkene system

1987 ◽  
Vol 65 (9) ◽  
pp. 2312-2314 ◽  
Author(s):  
Donald R. Arnold ◽  
Shelley A. Mines
Keyword(s):  
Hydrogen Bond ◽  
Electron Transfer ◽  
Radical Cation ◽  
Radical Anion ◽  
Relative Rate ◽  
Radical Cations ◽  
Radical Ions ◽  
Back Electron Transfer ◽  
Ambident Anion ◽  
Determining Factor

The photosensitized (electron transfer) irradiation of several conjugated 1,1-diphenyl alkenes, in acetonitrile with 1,4-dicyanobenzene or 1-cyanonapthalene as electron accepting sensitizer and 2,6-lutidine as base, leads essentially quantitatively to tautomerization to the less stable unconjugated isomer(s). The proposed mechanism for this reaction involves formation of the alkene radical cation and sensitizer radical anion followed by deprotonation of the radical cation, reduction of the resulting radical to the ambident anion by back electron transfer from the radical anion, and reprotonation. There are several steps in this mechanism that could control the ratio of isomers. Evidence is provided that, at least in some cases, it is the relative rate of deprotonation from the isomeric radical cations that is the determining factor. This rate is influenced by the conformation of the radical cation; the carbon–hydrogen bond involved in the deprotonation step must overlap with the singly occupied molecular orbital.

The effect of meta- and para-methoxy substitution on the reactivity of the radical cations of arylalkenes and alkanes. Radical ions in photochemistry. Part 26

1991 ◽  
Vol 69 (5) ◽  
pp. 839-852 ◽  
Author(s):  
Donald R. Arnold ◽  
Xinyao Du ◽  
Kerstin M. Henseleit
Keyword(s):  
Electron Transfer ◽  
Radical Cation ◽  
Bond Cleavage ◽  
Cation Radical ◽  
Radical Cations ◽  
Addition Product ◽  
Molecular Orbital Calculations ◽  
Carbon Carbon Bond ◽  
Markovnikov Addition ◽  
Cis And Trans

The effect of meta- and para-methoxy substitution on the reactivity of some radical cations has been determined. The compounds chosen for study were 1-(3-methoxyphenyl)-1-phenylethylene (7), 1-(4-methoxyphenyl)-1-phenylethylene (8), 3-(3-methoxyphenyl)indene (9), 3-(4-methoxyphenyl)indene (10), methyl 2-(3-methoxyphenyl)-2-phenylethyl ether (11), methyl 2-(4-methoxyphenyl)-2-phenylethyl ether (12), cis- and trans-2-methoxy-1-(3-methoxyphenyl)indane (13), and cis- and trans-2-methoxy-1-(4-methoxyphenyl)indane (14). The radical cations of these compounds were generated by photosensitization (electron transfer) using 1,4-dicyanobenzene (3) as the electron acceptor. The three reactions studied were: (1) The addition of nucleophiles (methanol) to the radical cation of the arylalkenes, a reaction that yields the anti-Markovnikov addition product. (2) The carbon–carbon bond cleavage of radical cations, which yields products derived from the radical and carbocation fragments. (3) The deprotonation of the radical cation, a reaction that can be used to invert the configuration at a saturated carbon centre. The mechanisms of these reactions are discussed and the factors that need to be considered in order to predict reactivity are defined. Molecular orbital calculations (UHF/STO-3G) were carried out on the radical cations of the model compounds 3- and 4-vinylanisole and 3- and 4-methylanisole. Key words: photochemistry, photosensitize (electron transfer), radical cation, radical.

The importance of conformation in the reactivity of radical cations. Changing configuration at saturated carbon centres

1992 ◽  
Vol 70 (1) ◽  
pp. 272-279 ◽  
Author(s):  
Allyson L. Perrott ◽  
Donald R. Arnold
Keyword(s):  
Electron Transfer ◽  
Excited State ◽  
Molecular Mechanics ◽  
Radical Cation ◽  
Trans Isomer ◽  
Radical Cations ◽  
Singlet Excited State ◽  
Photostationary State ◽  
Configurational Isomerization ◽  
Cis Isomer

Irradiation of an acetonitrile solution of cis 1-methyl-2-phenylcyclopentane (1bcis); 1,4-dicyanobenzene (2), an electron-accepting photosensitizer; and 2,4,6-collidine (3), a nonnucleophilic base, leads to configurational isomerization of the cyclopentane; the photostationary state lies > 99% in favour of the trans isomer. The mechanism proposed for this reaction involves formation of the radical cation of 1bcis by photoinduced electron transfer to the singlet excited state of 2, deprotonation of the radical cation assisted by the base 3, reduction of the resulting benzylic radical by the radical anion [Formula: see text], and reprotonation of the benzylic anion to give both the cis and the trans isomers of 1b. The photostationary state is controlled by the relative rates of deprotonation of the radical cations of 1bcis and trans; these rates are dependent upon the extent of overlap of the SOMO of the radical cation, which is largely associated with the phenyl ring, and the benzylic carbon–hydrogen bond. Molecular mechanics calculations (MM3 and MMP2) are used to calculate the preferred conformations of the isomers. The required orbital overlap is 31% effective with the global minimum conformation of the cis isomer and essentially ineffective for the low-lying conformations of the trans isomer. This proposed mechanism is supported by Stem–Volmer quenching studies, which indicate that both isomers quench the singlet excited state of 2 at the diffusion-controlled rate, and by deuterium incorporation studies. When irradiation of the cis isomer is carried out in acetonitrile–methanol-O-d as solvent, isomerization is accompanied by deuterium exchange at the benzylic position; the trans isomer is stable under these conditions. Keywords: photosensitized electron transfer, radical cation, deprotonation, configurational isomerization, conformation, molecular mechanics (MM3).

1,n-Radical ions. The photosensitized (electron transfer) formation of 1,5-radical cations

1987 ◽  
Vol 65 (12) ◽  
pp. 2734-2743 ◽  
Author(s):  
Donald R. Arnold ◽  
Brian J. Fahie ◽  
Laurie J. Lamont ◽  
Jacek Wierzchowski ◽  
Kent M. Young
Keyword(s):  
Electron Transfer ◽  
Radical Cation ◽  
Phenyl Ring ◽  
Bond Cleavage ◽  
Relative Rate ◽  
Radical Cations ◽  
Carbon Bond ◽  
Trans Isomerization ◽  
Carbon Carbon Bond ◽  
Cis And Trans

The photosensitized (electron transfer) reactions of 3-phenyl-2,3-dihydrobenzofuran (8a), 5-methyl-3-phenyl-2,3-dihydrobenzofuran (8b), cis and trans-2-methoxy-1-phenylindane (9, cis and trans), 3,3-diphenyltetrahydrofuran (10), and 2,2-diphenyl-1-methoxycyclopentane (11) have been studied using 1,4-dicyanobenzene as an electron-accepting photosensitizer and acetonitrile–methanol (3:1) as solvent. These reaction conditions cause carbon–carbon bond cleavage of analogous acyclic β,β-diphenylethyl ethers to give products derived from the diphenylmethyl radical and the α-oxycarbocation intermediates. The purpose of this study was to determine if this reaction could be applied to five-membered cyclic derivatives to give 1,5-radical cations.The primary products from 8a and 8b were the dehydrogenated, aromatized 3-phenylbenzofurans 14a and 14b. These products react further; continued irradiation gave the methanol adducts, cis and trans-2-methoxy-3-phenyl-2,3-dihydrobenzofuran (15a and 15b, cis and trans). The only observed reaction of the indanes (9, cis and trans) was cis-trans isomerization. Deuterium was incorporated at the bis-benzylic position of 8 and 9 when the irradiation was carried out in acetonitrilemethanol-O-d. These results are consistent with reversible deprotonation from the radical cations. There was no evidence for carbon–carbon bond cleavage with either 8 or 9. The relative rate, deprotonation faster than carbon–carbon bond cleavage, is explained in terms of the conformation of the bond that cleaves in relation to the singly occupied molecular orbital (SOMO) of the radical cation. Oxidation potential measurements support the conclusion that the SOMO of 8 and 9 is largely associated with the fused phenyl ring and is therefore orthogonal to the benzylic carbon–carbon bond. Irradiation of cis or trans-2-methoxy-3-phenyl-2,3-dihydrobenzofuran (15a, cis or trans), under these conditions, leads to cis–trans isomerization. The mechanism in this case involves the reversible loss of methanol. There is evidence that the addition of methanol to 14 involves the sensitizer radical anion – 14 radical cation pair.In contrast with the fused bicyclic systems, the monocyclic tetrahydrofuran 10 and the methoxycyclopentane 11 both cleave under these conditions; the products are the expected acetals 22 and 29 formed from the intermediate 1,5-radical cations. In 10 and 11 the SOMO, which is largely associated with the diphenylmethyl moiety, can overlap with the adjacent carbon–carbon bond and cleavage occurs as in analogous acyclic systems. Both 10 and 11 are relatively stable to irradiation under conditions that are identical except with acetonitrile as solvent (without methanol). We found no evidence for cyclization of the intermediates (1,5-radical cation or 1,5-diradical) into the terminal phenyl ring.

scholarly journals Triplet 1-Nitronaphthalene and Competitive Energy and Electron Transfer Reactions With Trans-Stilbene

1999 ◽  
Vol 19 (1-4) ◽  
pp. 397-401
Author(s):  
T. Fournier ◽  
G. D. Scholes ◽  
I. R. Gould ◽  
S. M. Tavender ◽  
D. Phillips ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Radical Cation ◽  
Transient Absorption ◽  
Radical Cations ◽  
Time Resolved ◽  
Polar Solution ◽  
Trans Stilbene ◽  
Energy And Electron Transfer ◽  
High Concentrations ◽  
Dimer Radical Cation

A time resolved resonance Raman (TR3) study of 1-nitronaphthalene (1NN) first triplet excited state in various solvents is combined with ab initio calculations to determine the nature and the geometry of this state in environments of different polarity. A transient absorption and TR3 study of the reactions of triplet 1NN with trans-stilbene (tS) in solution in both non-polar and polar solvents is reported. In polar solution, 31NN acts as an electron acceptor whereas in non-polar solution only energy transfer to tS is observed. Moreover, at high concentrations of tS, the radical cation tS+ reacts with ground-state tS to form a dimer radical cation (tS2)+. Differences between the Raman spectra of the monomeric and the dimeric radical cations are reported in the 1500–1650 cm-1 region. The switch from energy to electron transfer when going from nonpolar to polar solutions is discussed in terms of the Marcus-Hush theory of electron transfer.

The effect of meta- or para-cyano substitution on the reactivity of the radical cations of arylalkenes and alkanes. Radical ions in photochemistry, Part 34

1995 ◽  
Vol 73 (3) ◽  
pp. 307-318 ◽  
Author(s):  
Donald R. Arnold ◽  
Xinyao Du ◽  
Jing Chen
Keyword(s):  
Electron Transfer ◽  
Molecular Orbital ◽  
Radical Cation ◽  
Radical Cations ◽  
Molecular Orbital Calculations ◽  
Single Electron Transfer ◽  
Radical Ions ◽  
Benzylic Carbon ◽  
Carbon Carbon Bond ◽  
Cis And Trans

The effect of electron-withdrawing substituents, meta- or para-cyano, on the reactivity of the radical cation of arylalkenes and alkanes has been determined. The radical cations were generated by single electron transfer (set) to an electron-accepting photosensitizer. Three reactions were studied: (i) the addition of nucleophile to the radical cation of arylalkenes, (ii) cleavage of the benzylic carbon–carbon bond of the radical cation of arylalkanes; and (iii) the deprotonation of the benzylic carbon–hydrogen bond of the radical cation of arylalkanes. The radical cations of 4-(1-phenylethenyl)benzonitrile (1b), 3-(1-phenylethenyl)benzonitrile (1c), 4-(2-methoxy-1-phenylethyl)benzonitrile (2b), 3-(2-methoxy-1-phenylethyl)benzonitrile (2c), cis- and trans-5-cyano-2-methoxy-1-phenylindane (6b-cis and -trans), and 6-cyano-3-phenylindene (7b) were generated, by single electron transfer to the lowest excited singlet state of 1,4-dicyanobenzene (3), in acetonitrile–methanol. The radical cations of 1b, 1c, and 7b react with methanol to yield the anti-Markovnikov adducts (2b, 2c, and 6b-cis and 6b-trans). The radical cations of 2b, 2c, and 6b-trans cleave at the benzylic carbon–carbon bond to give products derived from the radical and carbocation fragments. The radical cation of 6b-cis deprotonates from the benzylic position with subsequent formation of the diastereomer, 6b-trans. This behaviour can be explained/predicted on the basis of the proposed mechanisms for these reactions. Molecular orbital calculations (AM1) support the conclusions. Keywords: photosensitized, electron transfer, radical ions, radicals, molecular orbital calculations (AM1).

Photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction: methanol, beta-myrcene, and 1,4-dicyanobenzene. Intramolecular cyclization of an ene-diene radical cation

1998 ◽  
Vol 76 (9) ◽  
pp. 1238-1248 ◽  
Author(s):  
Donald R Arnold ◽  
Kimberly A McManus
Keyword(s):  
Electron Transfer ◽  
Radical Cation ◽  
Radical Cations ◽  
Oxidation Potential ◽  
Molecular Orbital Calculations ◽  
Evidence Based ◽  
Aromatic Substitution ◽  
Spin Distribution ◽  
Intermediate Radical ◽  
High Level

The photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction of methanol, 7-methyl-3-methylene-1,6-octadiene ( β-myrcene, 1), and 1,4-dicyanobenzene yields five 1:1:1 adducts:cis-2-(4-cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane (15), trans-2-(4-cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane (16), 1-(4-cyanophenylmethyl)-4-(1-methoxy-1-methylethyl)cyclohexene (17), 4-[4-methoxy-3,3-dimethylcyclohex-(E)-1-ylidenyl]methylbenzonitrile (18), and 4-(1-vinyl-4-trans-methoxy-3,3-dimethylcyclohexyl)benzonitrile (19). All of these adducts are cyclic; variation in the product ratio as a function of methanol concentration indicates cyclization is occurring, 1,6-endo, with both the initially formed radical cation and with the intermediate β-alkoxyalkyl radicals. Evidence based upon comparison of the ionization and oxidation potential of β-myrcene with model alkenes and with conjugated dienes indicates the initial electron transfer involves the trisubstituted mono alkene moiety; the diene moiety, mono-substituted at a nodal position, has a higher oxidation potential. High-level ab initio molecular orbital calculations (MP2/6-31G*//HF/6-31G*) provide useful information regarding the nature (relative energies and charge and spin distribution) of the intermediate radical cations, which supports the proposed reaction mechanism. Key words: photoinduced electron transfer, radicals, radical cations, β-myrcene, cyclization.

Radiation Chemical Studies of the Oxidation of Aniline in Aqueous Solution

1985 ◽  
Vol 40 (10) ◽  
pp. 1026-1039 ◽  
Author(s):  
Ling Qin ◽  
G. N. R. Tripathi ◽  
Robert H. Schüler
Keyword(s):  
Aqueous Solution ◽  
Electron Transfer ◽  
Radical Cation ◽  
Time Scale ◽  
Radical Cations ◽  
Oh Radicals ◽  
Basic Solution ◽  
Direct Oxidation ◽  
Time Resolved ◽  
Radiation Chemical

Oxidation of aniline in aqueous solution by ·OH radicals proceeds predominantly via addition to the ring followed by elimination of OH- on the microsecond time scale to form the aniline radical cation. Direct oxidation by electron transfer accounts for less than 4% of the ·OH reactions. The different hydroxycyclohexadienyl isomers produced by ·OH addition decay at different rates with this decay catalyzed both by protons and phosphate. In basic solution the resultant radical cation deprotonates to form the anilino radical. The p Ka for the equilibrium between the acidic and basic forms of this radical is 7.05 ± 0.05. Secondary radicals such as Br2·- or N3· oxidize aniline directly by electron transfer and allow rapid preparation of the radical cation even in basic solution, as is conclusively demonstrated by observation of the Raman spectrum of the radical cation on the nanosecond time scale at pH 10.4. The deprotonation process can be followed directly by time resolved absorption spectrophotometry in the pH range of 9 - 11 and is shown to occur via reaction of the radical cation with OH- at a rate constant of 2.2 x 1010 M-1 s-1. Reaction of anilino radical with water is relatively slow (k ~ 2 x 103 s-1). Time resolved Raman methods show that benzidine radical cation is an important tertiary transient, indicating that second order reaction between radical cations results to an appreciable extent from coupling at ring positions.

Substituent effects in oxime radical cations. 1. Photosensitized reactions of acetophenone oximes

2003 ◽  
Vol 81 (6) ◽  
pp. 575-585 ◽  
Author(s):  
HJ Peter de Lijser ◽  
Jason S Kim ◽  
Suzanne M McGrorty ◽  
Erin M Ulloa
Keyword(s):  
Electron Transfer ◽  
Steady State ◽  
Radical Cation ◽  
Radical Anion ◽  
Substituent Effects ◽  
Radical Cations ◽  
Ionization Potentials ◽  
Semiquinone Radical ◽  
Am1 Calculations ◽  
Iminoxyl Radical

A variety of ortho-, meta-, and para-substituted (-H, -F, -Cl, -CF3, -CN (meta and para only), -CH3, -OCH3, and -NO2) acetophenone oximes were synthesized and studied using laser flash photolysis (LFP) and steady-state photolysis experiments in acetonitrile with chloranil as the photosensitizer. In addition, semi-empirical (AM1) calculations were performed on the neutral species, the radical cations, and the corresponding iminoxyl radicals. The data was analyzed in terms of the electrochemical peak potentials of the oximes, the quenching rates of triplet chloranil (LFP), the calculated ionization potentials, and the measured conversions of the oximes in the steady-state photolysis experiments. Photolysis of the oximes in the presence of chloranil results in the formation of the chloranil radical anion, which reacts rapidly with the oxime radical cation to form the semiquinone radical and an iminoxyl radical. Evidence for the formation of the chloranil radical anion and the semiquinone radical was obtained from LFP studies. The measured quenching rates from the LFP studies represent the rates of electron transfer from the oximes to triplet chloranil. This data was correlated to various radical and polar substituent constants. The Hammett studies suggest that steric, polar, and radical effects are important for ortho-substituted acetophenone oximes, polar effects are important for para-substituted oximes, and radical stabilization is more important than polar effects for the meta-substituted substrates. The calculated ionization potentials of the oximes show an excellent correlation with the measured quenching rates supporting the electron transfer pathway. On the basis of calculated charge densities, we conclude that the measured substituent effects are transition state effects rather than ground state effects. At this point all of the available data suggests that the conversion of the oximes is controlled by two energetically opposing reactions, namely oxidation of the neutral oxime, which is favorable for oximes with electron-donating substituents, and deprotonation of the oxime radical cation, which is favorable for oximes with electron-withdrawing substituents. The overall result is a reaction with little selectivity as far as substituent effects are concerned.Key words: oxime, radical cation, iminoxyl radical, electron transfer, substituent effect.

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