The 7-Cycloheptatrienylmethyl/7-Norcaradienylmethyl Cation System

Reaction of 7-carbomethoxycycloheptatriene with either BCl 3 or BBr 3 gave a 1:1 complex in which the Lewis acid was complexed to the carbonyl oxygen. The low temperature p.m.r. and u.v. spectra of these zwitterions indicated that they existed almost entirely in the norcaradiene forms 10 and 14, respectively. In contrast, reaction of N,N-dimethyl-1,3,5-cycloheptatriene-7-carboxamide with BCl 3 was found to result in the formation of a mixture of the cycloheptatriene 12 (65%) and norcaradiene 13 (35%). Comparison of the chemical shifts of the cyclopropyl proton resonances of 10 and 14 with model compounds showed that their positions were anomolous, the C 7 protons resonating at much higher and the C 1 and C 6 protons resonating at much lower field than expected and to account for this it is suggested that these norcaradienes are aromatic systems. Both 10 and 14 were thermally unstable and underwent an irreversible rearrangement to form the complex of methyl phenylacetate, the phenylmethylacylium cation and the appropriate counterion, [Formula: see text] respectively. The rate of these rearrangements and the product distribution was found to be dependent upon both the ratio of Lewis acid to ester used and the nature of the Lewis acid.