The effects of metal salts on the photosensitized (electron transfer) carbon–carbon bond cleavage of the 2,2-diphenylethyl ether system

Abstract

Irradiation of an acetonitrile solution of para-dicyanobenzene (1), 3,3-diphenyltetrahydrofuran (2), and magnesium perchlorate leads to the formation of 4,4-diphenyl-1,3-dioxane (7). Similarly, irradiation of an acetonitrile solution of 1, methyl 2,2-diphenylcyclopentyl ether (5), and magnesium perchlorate leads to the formation of 6,6-diphenyl-2-methoxytetrahydropyran (8). The mechanism proposed for these reactions involves formation of the radical cations [Formula: see text] and [Formula: see text] with the first excited singlet state of 1 acting as the photosensitizer (electron transfer). The radical cations [Formula: see text] and [Formula: see text] then cleave to give 1,5 radical cations. Reaction of the cationic site with water, followed by futher oxidation of the radical site by the perchlorate anion, gives the diphenylalkyl carbocation that can cyclize via a six-membered intermediate to the observed products 7 and 8. While the addition of other perchlorate salts (lithium and tetra-n-butylammonium) also lead to the formation of 7 and 8, lithium trifluoroacetate is ineffective. The proposal that water is involved as the nucleophile is supported by incorporation of 17O in the products 7 and 8 when 17O-enriched water was added to the reaction mixture. Keywords: photosensitization, electron transfer, radical cations, bond cleavage, 2,2-diphenylethyl ether.