Generation and Trapping of the 4-Biphenylyloxenium Ion by Water and Azide: Comparison with the 4-Biphenylylnitrenium Ion

Generation of the 4-biphenylyloxenium ion, "1a", from hydrolysis of 4-acetoxy-4-phenyl-2,5-cyclohexadienone, "2a", is demonstrated by common ion rate depression and azide trapping. The ion is less selective with a shorter lifetime (12 ns at 30°C) in aqueous solution than the corresponding nitrenium ion, "6a" (ca 0.3 μs at 30°C). Its lifetime is considerably longer than those of structurally related carbenium ions. Unlike "6a", "1a" reacts with N₃-, in part, at the distal ring to generate 4'-azido-4-hydroxybiphenyl, "5". These results are rationalized by calculations at the HF/6-31G* and pBP/DN*//HF/6-31G* levels that show that "1a" is planar but is less stable than "6a" relative to their hydration products by ca. 12 kcal/mol. The p-tolyloxenium ion, "1b", has not been unequivocally demonstrated to be formed in H₂O, but kinetic data show that it must be at least 10⁴-fold less stable than "1a" relative to their respective 4-acetoxy derivatives. It is also calculated to be ca. 19 kcal/mol less stable than the corresponding nitrenium ion, "6b", relative to their hydration products.