Comparative capabilities of 1-phenylpropane-1-13C and 1-phenylbutane-1-13C toward the alkylbenzene automerization

1-Phenylbutane-1-13C failed to undergo rearrangement of the isotopic label from C-1 to C-2 of the side chain (automerization) when heated with AlCl3 or AlBr3 under conditions which produce complete equilibration of isotopic carbon between C-1 and C-2 in labeled 1-phenylpropane (n-propylbenzene). When equimolar mixtures of 1-phenylbutane-1-13C and 1-phenylpropane-1-13C were treated with AlCl3 and AlBr3, neither hydrocarbon underwent automerization when the usual concentration of catalysts was used; with a much higher concentration of AlBr3, 1-phenylpropane-1-13C showed isotopic automerization but 1-phenylbutane-1-13C could not be recovered. Various compounds, known to be produced from 1-phenylbutane under the conditions of the reaction, were tested as potential inhibitors of the isotopic automerization. Di- and tri-n-butylbenzenes appeared to be the most likely candidates. The failure of 1-phenylbutane-1-13C to undergo isotopic automerization may be ascribed partly to alternative reactions which it undergoes but which are not available to 1-phenylpropane-1-13C. These alternative reactions are cyclialkylations to indan and indene derivatives via the secondary carbocation produced by hydride abstraction at the 3 position in the side chain of 1-phenylbutane. In accord with this explanation, significant amounts of indans and indenes have been identified among the products from 1-phenylbutane. A second factor in the failure of 1-phenylbutane-1-13C to undergo automerization is the lesser tendency of a 1,2-ethyl shift in an intermediate carbocation compared with a 1,2-methyl shift. This was demonstrated in the reaction of 1,2-diphenylbutane-2-13C with aluminum chloride, which gave only 8 to 10% isotopic rearrangement in recovered 1,2-diphenylbutane and the 1-phenylbutane produced from it