title: Spectroscopic and calorimetric studies on the mechanism of methylenecyclopropane rearrangements triggered by photoinduced electron transfer.
abstract: 2-(dideuteriomethylene)-1,1-bis(4-methoxyphenyl)cyclopropane (d(2)-1) undergoes degenerate rearrangement in both singlet- and triplet-sensitized electron-transfer photoreactions. Nanosecond time-resolved absorption spectroscopy on laser flash photolysis of the unlabeled 1 with 9,10-dicyanoanthracene, 1,2,4,5-tetracyanobenzene, or N-methylquinolinium tetrafluoroborate as an electron-accepting photosensitizer gives rise to two transients with lambda(max) at 500 and 350 nm assigned to the dianisyl-substituted largely twisted trimethylenemethane cation radical (6.+) and the corresponding diradical (6..), respectively. These intermediates are also detected, respectively, by steady state and nanosecond time-resolved EPR with chloranil or anthraquinone as a sensitizer. The degenerate rearrangement of d(2)-1 thus proceeds via these two different types of intermediates in a cation radical cleavage-diradical cyclization mechanism. Energetics based on nanosecond time-resolved photoacoustic calorimetry support this mechanism. A comparison of the reactivities and the spectroscopic results of 1, 1,1-bis(4-methoxyphenyl)-2-methylenespiro[2.2]pentane (2), and 1-cyclopropylidene-2,2-bis(4-methoxyphenyl)cyclopropane (3) suggest that the reversible methylenecyclopropane rearrangement between 2 and 3 proceeds via a similar mechanism.
authors.affiliation: Contribution from the Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Ikeda
authors.forename: Kimio
authors.initials: K
authors.lastname: Akiyama
authors.forename: Yasutake
authors.initials: Y
authors.lastname: Takahashi
authors.forename: Tatsuo
authors.initials: T
authors.lastname: Nakamura
authors.forename: Satoshi
authors.initials: S
authors.lastname: Ishizaki
authors.forename: Yukio
authors.initials: Y
authors.lastname: Shiratori
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Ohaku
authors.forename: Joshua L
authors.initials: JL
authors.lastname: Goodman
authors.forename: Abdelaziz
authors.initials: A
authors.lastname: Houmam
authors.forename: Danial D M
authors.initials: DD
authors.lastname: Wayner
authors.forename: Shozo
authors.initials: S
authors.lastname: Tero-Kubota
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Miyashi