INTERANNULAR PROTON EXCHANGE IN PROTONATED LONG- CHAIN l,w-DIPHENYLALKANES

The interannular proton exchange in (w-phenylalkyl)-benzenium ions, 1, (2 < n < 20) is an extremely fast process. It gives rise to the statistical distribution of the eleven protons at the two rings within the microsecond time scale even if twenty methylene groups are located between the rings. 1 ions and various deuterium-labelled analogues have been generated by EI-induced fragmentation of the corresponding l-( w-phenylalkyl)-1,4-dihydrobenzoic acids, 2, and/or by CI-( i-butane) of the corresponding l,w-diphenylalkanes, 3. Proton randomization is not affected by the internal energy of the ions. The results suggest that an association complex is formed gaining energy from the interaction of the benzenium and the benzene ring. Whereas the 2-butyne-1,4-diyl moiety in 5 ions mediates the fast proton transfer between the rings in spite of its steric rigidity, the trans-cyclohexane-1,4-diyl unit in 7 ions suppresses the proton exchange almost completely. Kinetic model calculations yield rate constants for the interannular proton transfer in the order of k, > 1 X lo6 s-’ for metastable 1 ions, but k, = 5 X104 s-’ for metastable 7 ions. A kinetic isotope effect of k,/k, = 5 seems to operate during the interannular proton transfer in sterically hindered (w-phenylalkyl)-benzenium ions.