Spectroscopic and calorimetric studies on the mechanism of methylenecyclopropane rearrangements triggered by photoinduced electron transfer.
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.
Ikeda
Hiroshi
H
Contribution from the Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.
Akiyama
Kimio
K
Takahashi
Yasutake
Y
Nakamura
Tatsuo
T
Ishizaki
Satoshi
S
Shiratori
Yukio
Y
Ohaku
Hitoshi
H
Goodman
Joshua L
JL
Houmam
Abdelaziz
A
Wayner
Danial D M
DD
Tero-Kubota
Shozo
S
Miyashi
Tsutomu
T