2-(Dideuteriomethylene)-1,1-bis(4-methoxyphenyl)cyclopropane (
d2-
1) undergoes degeneraterearrangement 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
max at 500 and 350 nm assigned to the dianisyl-substituted largelytwisted trimethylenemethane cation radical (
6
+) and the corresponding diradical (
6), respectively. Theseintermediates are also detected, respectively, by steady state and nanosecond time-resolved EPR withchloranil or anthraquinone as a sensitizer. The degenerate rearrangement of
d2-
1 thus proceeds via thesetwo different types of intermediates in a cation radical cleavage-diradical cyclization mechanism. Energeticsbased on nanosecond time-resolved photoacoustic calorimetry support this mechanism. A comparison ofthe 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.
