PHOTOCHEMISTRY OF ALKYL-HALIDES .12. BROMIDES VS IODIDES

Conditions have been developed for optimizing ionic photobehavior and good material balances from alkyl bromides. Hydroxide ion serves as an efficient scavenger for the byproduct HBr while giving minimal competing photoreduction via electron transfer to the alkyl bromide. The photobehavior of bromides 1, 11, 25, and 40 has been examined and compared with that of the corresponding iodides 2, 12, 26, and 41 under these conditions. In each case, the bromide gave higher yields of products derived from out of cage radical intermediates than the corresponding iodide. However, labeling studies with the 2-norbornyl bromides 11 and iodides 12 showed that, of products not formed from the out of cage 2-norbornyl radical 13, the bromides 11 gave a higher percentage of products from the ionic intermediates 15 and 16 than did the iodides. Thus, electron transfer within the radical pair cage 14 is apparently more rapid for bromides than iodides, as expected on the basis of the relative electronegativities of bromine and iodine. It is suggested that the substantially higher yields of out of cage radical products from alkyl bromides may be due in part to formation of the radical pair with greater excess energy, which results in more rapid escape from the cage. The epimeric 2-norbornyl bromides 11x and 11n underwent no detectable interconversion and afforded somewhat different product ratios. The more hindered epimer 11n underwent conversion to products at a slower rate than 11x. By contrast, iodides 12x and 12n underwent substantial interconversion via out of cage transfer of an iodine atom from iodide 12 to radical 13. Epimerization was significantly attenuated in the more viscous solvent tert-butyl alcohol.