Unexpected rearrangements induced by the metal cation in the reduction of cycloocta[b]naphthalene with alkali metals in ethers. An EPR and ENDOR study

Abstract

A set of structurally different radicals has been detected by EPR in the reduction of cycloocta[b] naphthalene (1) by potassium in 1,2-dimethoxyethane. The results differ from those previously reported for the reduction in hexamethylphosphoric triamide, where the radical anion 1˙^– yields the dianion 1^2– by further reaction with potassium. In a second reduction step, an apparently symmetry-forbidden ring closure leads to benzo[b]biphenylene (2), shown by EPR spectrum of 2˙^–. By subsequent reduction, a bond fragmentation takes place in the cyclobutadiene ring, leading to the 2-phenylnaphthalene radical anion 3˙^–. Finally, the EPR spectrum of the radical anion of an unknown compound 4 is observed, the structure of which is tentatively proposed to be a dipotassium salt of dibenzoheptafulvene. It was possible to show that 4˙^– arises from 2˙^– by a different opening process of the cyclobutadiene ring, followed by rearrangement. This study was extended to other alkali metals (lithium and sodium), and to other ethereal solvents (2-methyltetrahydrofuran and tetrahydrofuran). Strong ion association determines the course of the reduction. Mechanisms for the successive reactions observed are proposed.