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

Abstract

. Can. J. Chem. 69, 225 (1991). The reactivity of the radical cations of methyl 2,2-diphenylcyclohexyl ether (7), 6,6-diphenyl-l,4-dioxaspiro[4,5]decane (8), methyl cis-and trans-2-phenylcyclohexyl ether (9cis and trans), and 6-phenyl-l,4-dioxaspiro[4.5]decane (lo), 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-diphenylpenty1)-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. Chem. 69, 225 (1991). On a CtudiC la rCactivitC des cations radicalaires du mkthoxy-2,2,-diphCnylcyclohexane (7), du 6,6-diphCnyl-l,4-dioxaspiro [4,5] Introduction Photosensitized (electron transfer) carbon-carbon bond cleavage of five-membered cyclic compounds generates 1,5-radical cations (2). When the irradiation is carried out in the presence of a nucleophile, often the solvent, further reaction of these intermediates proceeds by nucleophilic attack on the cationic site, and reduction of the radical site by electron transfer from the radical anion of the ~hotosensitizer. followed by protonation of the resulting anion 12). A typical example is shown in reaction [I]: irradiation of an acetonitrile-methanol (3: 1) solution of methyl 2,2-diphenylcyclopentyl ether (1) and 1,4-dicyanobenzene (2) (an electron-accepting photosensitizer)

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

“…The irradiation procedure was similar to that used in earlier studies (2). Solutions of the l-methyl-2-phenylcyclopentane (lb); 1,4-dicyanobenzene (2), an electron-accepting photosensitizer; and 2,4,6-collidine (3), a nonnucleophilic base, in acetonitrile or in a mixture of acetonitrile-methanol-0-d (3: l), were degassed by nitrogen ebullition and then irradiated through a Pyrex filter using a medium-pressure mercury vapour lamp.…”

“…There are several recent reports of photosensitized (electron transfer) carbon-carbon bond cleavage and deprotonation from radical cations generated in this way, and progress has been made toward an understanding of the scope and mechanism of these reactions (1-4). Particularly relevant to this study is the report of contrasting behaviour of the radical cations of trans and cis methyl 2-phenylcyclopentyl ethers ( l a ) (2). Irradiation of an acetonitrile-methanol (3: 1) solution of trans methyl 2-phenylcyclopentyl ether ( l a trans) and 1,4-dicy-'This is part 30 of the series Radical Ions in Photochemistry.…”

“…In marked contrast, the radical cation of the cis isomer ( l a cis:) does not undergo carbon-carbon bond cleavage; deprotonation resulting ultimately in formation of the trans isomer occurs instead (reaction [2]). Evidence for the deprotonation mechanism was provided by the observation that deuterium was incorporated at the benzylic position when the irradiation was carried out in acetonitrile-methanol-0-d.…”

“…The lack of carbon-carbon bond cleavage of the radical cation l a cis: was explained on the basis of an unfavourable conformation of the radical cation: insufficient overlap of a nonbonding pair of electrons on the oxygen with the a* orbital of the vulnerable benzylic carbon-carbon bond. The preferred conformation of l a cist does, however, have zylic carbon-hydrogen bond and deprotonation occurs (step 40) (2).…”

Substituent and charge distribution effects on the redox potentials of radicals. Thermodynamics for homolytic versus heterolytic cleavage in the 1-naphthylmethyl system

“…[15]), will have a significantly smaller effect. Although Arnold et al (36) and Tolbert et al (37) have pointed out that kinetics of these competing pathways is strongly influenced by stereoelectronic factors, the relative thermochemistry for these pathways will have a significant impact on the interpretation of these data. Proton loss is usually more exoergonic than C-C cleavage.…”