INVESTIGATIONS INTO POSSIBLE INTERMEDIATES IN PHOTOREDUCTION OF CONJUGATED CYCLOPROPYL KETONES IN 2-PROPANOL

The photoreductions of a series of bicyclo[4.1.0]- and bicyclo[3.1.0]alkan-2-ones (1) in 2-propanol and the ground-state radical rearrangements of the corresponding bicyclo[4.1.0]- and bicyclo[3.1.0]alkan-2-ols (6) (using di-t-butyl peroxide as initiator) were compared. The product ratios, in either the photoreductive process or the radical rearrangement, change markedly with temperature. The similarity in product distributions of the two processes at different temperatures indicates that a common intermediate is involved, i.e., the α-hydroxycyclo propylcarbinyl radical 7. Overlap between the p orbital of the radical and the outside cyclopropane bond leads to selective cleavage of this bond at room temperature. However, at elevated temperatues appreciable amounts of the product originating from inside bond opening may be found, provided that in this cleavage a thermody namically preferred species is formed. The similarity of specific ring opening of the outside bond of the cyclopropane ring in photoisomerization and photoreduction stems from the energy features of the ring system rather than a common intermediate. Initial studies on the rearrangement of the related carbonium ion are given for comparison. © 1969, American Chemical Society. All rights reserved.