Cope interconversions of the 9-methoxy-endo-tricyclo[4.2.1.0]nona-3,7-dienyl cation and its [4.3.0.0]tautomer. Evidence of enhanced ionic stability by delocalization of neighbouring σ bonds

Abstract

Dissolution of 9-chloro-9-methoxy-endo-tricyclo[4.2.1.0^2.5]nona-3,7-diene (7b, c) in liquid SO₂ gives rise to a dissociation equilibrium which involves the rearranged 7-methoxy-endo-tricyclo[4.3.0.0^2.5]nona-3,9-dienyl cation (11). This allylic ion was formed quantitatively by dissolution of (7b, c) in FSO₃H–SO₂ or AlCl₃–CH₂Cl₂. Its structure was established by ¹H and ¹³C n.m.r. spectroscopy. Quenching of the ionic solutions with methanol afforded the starting acetal (7a). In contrast, reaction of (11) with pyridine produced its corresponding pyridinium salt (17a). Ionization of (7b, c) under short-life conditions, i.e. reaction with pyridine in CH₂Cl₂, proceeds without skeletal rearrangement. The structure of the reaction products of the reference compounds (8)–(10) appeared to be independent of the conditions used. In all cases the unrearranged structures were observed. The ready skeletal rearrangement of (7b, c) under acidic conditions is ascribed to the involvement of a homoaromatic Cope transition state. From the enhanced reaction rate of (7b, c) relative to (8b, c)–(10b, c) it is concluded that the σ bond involved in the Cope rearrangement provides stabilization of the incipient cation in (7). This kind of stabilization is also present in the rearranged cation (11), as indicated by its enhanced stability relative to the corresponding allylic ion (3) and by comparison of their ¹H and ¹³C n.m.r. data.