Photosensitized (electron transfer) carbon–carbon bond cleavage of radical cations: the 2-phenylethyl ether and acetal systems

The scope of the photosensitized (electron transfer) carbon–carbon bond cleavage involving radical cations has been defined for 2-phenylethyl ethers and acetals. The thresholds for reactivity of the monophenylethyl and gem-diphenylethyl derivatives are compared. While the radical cation of methyl 2,2-diphenylethyl ether (7) cleaves to give ultimately diphenylmethane (2) and dimethoxymethane (8), the radical cation of methyl 2-phenylethyl ether (9) was stable under these conditions. In contrast to the lack of reactivity of the radical cation of 9, the radical cations of methyl 2-phenyl-2-propyl ether (11), methyl 2-phenylcyclopentyl ether (13), and 2-phenylmethyl-1,3-dioxolane (16) cleave. Cleavage in the monophenylethyl series is limited to formation of a carbocation at least as stable as the secondary α-oxyalkyl or di-α-oxyalkyl. The basis for predicting this type of reactivity of radical cations is defined. The rate of carbon–carbon bond cleavage is increased by increasing the oxidation potential of the molecule, by decreasing the carbon–carbon bond strength, and (or) by decreasing the oxidation potential of that fragment that will become the carbocation. The results obtained from the reactions of 2-diphenylmethyl-1,3-dioxolane (14) and 2-phenylmethyl-1,3-dioxolane (16) cast doubt on the published oxidation potential for the 1,3-dioxolan-2-yl radical. Keywords: photochemistry, radical cation, electron transfer, bond cleavage, radical.

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

Canadian Journal of Chemistry ◽

10.1139/v91-036 ◽

1991 ◽

Vol 69 (2) ◽

pp. 225-233 ◽

Cited By ~ 11

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.

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

Canadian Journal of Chemistry ◽

10.1139/v87-455 ◽

1987 ◽

Vol 65 (12) ◽

pp. 2734-2743 ◽

Cited By ~ 12

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.

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Radical ions in photochemistry. 16. The photosensitized (electron transfer) carbon–carbon bond-cleavage reaction of radical cations

Canadian Journal of Chemistry ◽

10.1139/v85-387 ◽

1985 ◽

Vol 63 (8) ◽

pp. 2341-2342 ◽

Cited By ~ 8

Author(s):

Akio Okamoto ◽

Donald R. Arnold

Keyword(s):

Electron Transfer ◽

Bond Cleavage ◽

Radical Cations ◽

Oxidation Potential ◽

Redox Potentials ◽

Carbon Bond ◽

Bond Cleavage Reaction ◽

Carbon Carbon Bond ◽

Central Bond ◽

Radical Fragment

The photosensitized (electron transfer) carbon–carbon bond cleavage of 1,1,2,2-tetraphenylethane (1b), 2-(4-methoxy-phenyl)-1,1-diphenyl-2-niethylpropane (1c), 1,1,2-triphenyl-2-methylpropane (1d), and 2-(4-trifluoromethylphenyl)-1,1-diphenyl-2-methylpropane (1e) has been studied with 1,4-dicyanobenzene (2) serving as the electron-accepting sensitizer. The oxidation potential of 1b–e have been measured by cyclic voltammetry. Estimation of the free-energy associated with the electron transfer between these donors (1b–e) and the first excited singlet state of 2, using the Weller equation, indicates the process is favorable in every case. There is, in every case, a one-to-one correspondence of products derived from reaction of the carbocation fragment and from the carbanion derived from the initially formed radical fragment. The efficiency of the reaction of 1b, which gives a good yield of diphenylmethane (3) and methyl diphenylmethyl ether, has been studied as a function of temperature. The observed activation energy (7.2 kcal mol−1) is equated to the bond dissociation energy of the central bond of the radical cation (1b+•). The ratio of products from the unsymmetric compounds is dependent upon the redox potentials of the fragment radicals and carbocations.

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The importance of conformational effects on the carbon–carbon bond cleavage of β-phenethyl ether radical cations

Canadian Journal of Chemistry ◽

10.1139/v97-044 ◽

1997 ◽

Vol 75 (4) ◽

pp. 384-397 ◽

Cited By ~ 14

Author(s):

A.L. Perrott ◽

H.J.P. De Lijser ◽

D.R. Arnold

Keyword(s):

Electron Transfer ◽

Molecular Mechanics ◽

Radical Cation ◽

Global Minimum ◽

Bond Cleavage ◽

Radical Cations ◽

Carbon Bond ◽

Conformational Effects ◽

Carbon Carbon Bond ◽

Good Agreement

The photosensitized (electron transfer) bond cleavage of some β-phenylethyl ether radical cations has been investigated. In previous studies the feasibility of the bond cleavage was thought to depend on the bond dissociation energy (BDE). However, this simple hypothesis led to several incorrect predictions and therefore additional criteria, conformational effects, were added to the hypothesis. This study has now been extended and additional examples of the importance of the conformation on the carbon–carbon bond cleavage of radical cations are provided. The four β-phenylethyl ethers studied are 2-methoxy-3-phenylbutane (9, both diastereomers), and cis- and trans-2-methyl-3-phenyltetrahydropyran (10c, 10t). Generally, bond cleavage will occur if the (calculated) BDE in the radical cation is less than 55 kJ/mol, and if there is significant overlap between the singly occupied molecular orbital (SOMO) and the vulnerable (C—C or C—H) bond. In the case of a β-phenylethyl ether radical cation, the alkoxy group must also be oriented so that an oxygen lone pair of electrons can overlap with the C—C sigma antibonding (σ*) orbital. The calculated BDE values of the vulnerable C—C bond in the radical cations of the four ethers studied here are well below the threshold value, 55 kJ/mol, and C—C cleavage will therefore be governed by conformational effects. Molecular mechanics (MM3) calculations were used to identify the most stable conformers of the neutral molecules. Based on the calculated angles and overlap between orbitals it was predicted that the global-minimum conformers of the ethers 9 and 10c would not give C—C bond cleavage products or deprotonation to any significant extent. The global minimum of ether 10t is well oriented for C—C cleavage but not for deprotonation. Irradiation of an electron-accepting photosensitizer, 1,4-dicyanobenzene (2), in the presence of the ethers showed that the ethers 9 did not cleave efficiently; no deprotonation or isomerization was observed. This is in good agreement with the predictions based on the MM3 calculations. Both ethers 10c and 10t gave reasonable yields of the C—C cleavage products; in fact, ether 10c cleaved more efficiently than 10t. This can be explained by the fact that a conformer of 10c, only 4.35 kJ/mol higher in energy than the global minimum, is perfectly aligned for cleavage. Ether 10t did not show any evidence for deprotonation whereas 10c did. This is also in good agreement with the calculations. Keywords: photoinduced electron transfer, radical cations, bond cleavage, conformation, molecular mechanics calculations.

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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

Canadian Journal of Chemistry ◽

10.1139/v91-124 ◽

1991 ◽

Vol 69 (5) ◽

pp. 839-852 ◽

Cited By ~ 20

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.

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