Intramolecular Electron Transfer and SN2 Reactions in the Radical Anions of 1-(4-Biphenylyl)-ω-haloalkane Studied by Pulse Radiolysis

Article abstract of DOI:10.1021/ja00407a037

One-electron reduction of 1-(4-biphenylyl)-ω-haloalkane (BP_X-n) by solvated electrons and the intramolecular reactions of the radical anion thus formed have been investigated using the pulse-radiolysis technique.The spectrum observed immediately after the pulse has an absorption maximum at 410 nm which is assigned to the corresponding biphenyl radical anion.The decay behavior of all these 410-nm bands follows first-order kinetics and the rates are dependent on the methylene chain length, n.With BP_Cl-0 and BP_Cl-2, which provide large rate constants (7.5E7 and 1.7E8 s^-1, respectively) and large G values of 4-alkylbiphenyl formation, an intramolecular electron transfer from a biphenyl radical anion to a C-Cl bond is presumed.The rate constants of the radical anions decrease in the order BP_Cl-1 > BP_Cl-2 > BP_Cl-0 which is roughly parallel to the C-Cl bond energy of these compounds and does not correlate with the chain length n, which corresponds to the distance required for the electron transfer.On the other hand, in the case of BP_Cl-3 and BP_Cl-4, the decay of the 410-nm band decreased about two orders of magnitude (5.5E5 and 1.2E6 s^-1, respectively) and was characterized by a simultanous formation of a 330-nm band which is assigned to a spirocyclic radical.The G values of the corresponding 4-alkylbiphenyl formation were low for these compounds.On the basis of these observations, an intramolecular carbanionic displacement of the biphenyl radical anion on the chlorine center, which is a novel type of intramolecular S_N2 reaction, has been concluded.Intramolecular reactions of bromo and iodo derivatives were also investigated and compared with those of corresponding chlorides.