Photolytic Cleavage of Remote Functional Groups in Polyfunctional Molecules. Photolysis of exo- and endo-2-Benzonorbornenyl Chlorides and Methanesulfonates

Article abstract of DOI:10.1021/ja00346a049

The photochemistry and photophysics of the title compounds are reported.The exo chloride exhibits a facile cleavage of the C - Cl bond upon excitation of the aryl chromophore (eq 1 - 3).The reaction has a rate > 700-fold larger than that observed with the endo isomer, derives from the singlet excited state, and leads to the formation of "hot" carbocations as well as free radicals.The exo mesylate shows similar characteristics (eq 4).A mechanism (Scheme II) is suggested which incorporates both heterolytic fission as well as sequential homolytic fission (to a polarized radical pair), followed by electron transfer to form an ion pair.The stereoelectronically controlled activation of the C - Cl and C - OMs moieties is explaned by a correlation diagram (Figure 5) which depicts the ?,?* state as correlating with the diradical product state through an avoided crossing with the C - X ?,?* state.Calculations indicate that this avoided crossing is considerably larger (and the barrier to reaction therefore considerably smaller) when the C - X unit in the 2-benzonorbornenyl series is exo.