Photoinduced reactions of chloranil with 1,1-diarylethenes and product photochemistry-intramolecular [2+2] (ortho-)cycloadditions of excited enedione's C=C double bond with substituted benzene

Photoinduced reactions of chloranil (CA) with 1,1-diarylethenes 1 [(p-X-Ph) (2)C=CH2, X = F, Cl, H, Me] in benzene afforded products 4-14, respectively , with the bicyclo[4.2.0]oct-3-ene-2,5-diones 4, the 6-diarylethenylcyclobe xa-2,5-diene-1,4-diones 5, and 2,3,5,6-tetrachlorohydroquinone 13 as the ma jor primary products. The cyclobutane products 4 are formed via a triplet d iradical intermediate without involvement of single electron transfer (SET) between the two reactants, while 5 is derived from a reaction sequence wit h initial SET interaction between (3)CA* and the alkene. The 9-arylphenanth rene-1,4-diones 6 and its 10-hydroxy-derivatives 7 are secondary photochemi cal products derived from 5. The isomeric cage products 9-11 are formed fro m 4 via intramolecular benzene-alkene [2 + 2] (ortho-)photocycloadditions i nduced by the triplet excited enedione moiety. The relative amount of the t wo groups of products (4 and its secondary products 9-11 via non-SET route vs 5 and its secondary products 6, 7, 8, 12, and 14 via SET route) shows a rather regular change, with the ratio of non-SET route products gradually i ncreasing with the increase in oxidation potential of the alkenes and in th e positive free energy change for electron transfer (Delta G(ET)) between ( 3)CA* and the alkene, at the expense of the ratio of the products from the SET route. The competition between the SET and non-SET routes was also foun d to be drastically influenced by solvent polarity, with the SET pathways m ore favored in polar solvent. Photo-CIDNP investigations suggest the interm ediacy of exciplexes or contact ion radical pairs in these reactions in ben zene, while in acetonitrile, SET process led to the formation of CA(.-) and cation radical of the alkene in the form of solvent separated ion radical pairs and free ions.