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

Abstract

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 ( l l ) , methyl 2-phenylcyclopentyl ether (13), and 2-phenylmethyl-l,3-dioxolane (16) cleave. Cleavage in the monophenylethyl series is limited to formation of a carbocation at least as stable as the secondary a-oxyalkyl or di-a-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-l,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.Key words: photochemistry, radical cation, electron transfer, bond cleavage, radical. (8), le cation radical du mCthoxy-1-2-phtnylCthane (9) est stable dans ces conditions. En opposition au fait que le cation radical du composC 9 n'est pas reactif, les cations radicaux du 2-mCthoxy-2-phCnylpropane ( l l ) , du I-mtthoxy-2-phtnylcyclopentane (13) et du 2-phCnylmtthy1-1,3-dioxolane (16) se clivent. Le clivage dans la sCrie des composCs monophtnylCthylts est limit6 2 la formation d'un cation au moins aussi stable que les a-oxyalkyles ou di-a-oxyalkyles secondaires. On a dCfini les bases permettant de pridire ce type de reactivitt des cations radicaux. Les vitesses de clivage de la liaison carbone-carbone augmentent lorsqu'on augmente le potentiel d'oxydation de la molCcule, lorsqu'on diminue la force de la liaison carbone-carbone et (ou) lorsqu'on diminue le potentiel d'oxydation du fragment qui deviendra le carbocation. Les rtsultats obtenus lors des rtactions des 2,2-diphCnylmtthyl-l,3-dioxolane (14) et 2-phtnylmtthyl-l,3-dioxolane (16) permettent de mettre en doute les rCsultats qui ont Ct C publits relativement au potentiel d'oxydation du radical 1,3-dioxolan-2-yle.

Effects of Alkoxy Groups on Arene Rings of Lignin β-O-4 Model Compounds on the Efficiencies of Single Electron Transfer-Promoted Photochemical and Enzymatic C–C Bond Cleavage Reactions

“…β-Hydroxyketone 29b , formed in trace quantities in the photoreaction of 10E , arises by benzylic C1–H deprotonation of radical cation 33 . These mechanistic considerations are consistent with the results of proposals in studies conducted earlier by Baciocchi, , Arnold, − and Maslak , using lignin models and 1-aryl-2-ethyl ethers.…”

Side-Chain Fragmentation of Arylalkanol Radical Cations. Carbon−Carbon and Carbon−Hydrogen Bond Cleavage and the Role of α- and β-OH Groups

“…The drastically different behavior of 4 •+ and 1 •+ with respect to the competition between C−H and C−C bond cleavage is in full agreement with our results concerning the photooxidation of 4 and 1 sensitized by TiO 2 in the presence of Ag + : 1 gave only products of C−H bond cleavage, whereas from 4 only products of C−C bond cleavage were observed . It should be noted that exclusive C−C bond cleavage in 4 •+ has also been observed by Arnold in the photooxidation of 4 sensitized by 1,4-dicyanobenzene …”

Photocatalysis with organic dyes: facile access to reactive intermediates for synthesis

“…They appear as attractive intermediates for a direct alkene difunctionalization or arene C-H functionalization. In particular, radical cations are strong electrophiles and undergo nucleophilic additions with two-electron donors (Scheme 21a) [93,94]. In contrast, radical anions can act as very strong single-electron nucleophiles and are often subject to fragmentation to give a neutral free radical and a charged species (Scheme 21b) [77,95].…”