L. Paquette, A. Varadarajan, L. D. Burke
Dec 1, 1986
Citations
0
Influential Citations
16
Citations
Journal
Journal of the American Chemical Society
Abstract
Seven 1-substituted 6-methoxybenzonorbornadienes were prepared and subjected to acetophenone-sensitized photoisomerization. In the -COOCH/sub 3/, -CONH/sub 2/, -COCH/sub 3/, and -CN examples, the excited-state 1,2-aryl shift was overwhelmingly controlled by the bridgehead substituent. As the electron-withdrawing character of this group is reduced as in -C(CH/sub 3/)/sub 2/OH, -NHCOCH/sub 3/, and -CH/sub 3/, the extent of proximal di-..pi..-methane rearrangement falls off to a level as low as 55%. Notably, the otherwise well-established preference of m-methoxy to engage that aryl carbon positioned meta to it never dominates in this series. Quantum yield measurements have also been performed on 17 benzonorbornadienes. The results reveal particularly efficient processes at one end of the spectrum to 0% reaction in the 2-bromo example. The full range of the data provides an important glimpse into how triplet deactivation can be realized in rigid bichromophoric systems possessing well-defined geometry. Finally, the secondary deuterium isotope effect associated with triplet state di-..pi..-methane rearrangement of benzonorbornadiene-d/sub 2/ has been determined to be greater than unity and quite large (k/sub H//k/sub D/ = 1.34). This partitioning, together with the global quantum efficiency and regioselectivity patterns, is discussed in mechanistic terms.