Endo addition to dienylium metal complexes

Abstract

Various mechanistic studies have been carried out on reactions of 5-exo-substituted tricarbonyl(η-cyclohexa-1,3-diene)iron and the tricarbonyl(1–5-η-cyclohexadienylium)iron cation. These have shown that the corresponding endo complexes can be prepared provided that formation of the exo complex is reversible in the presence of acid, that the endo complex is sufficiently stable thermodynamically, and that formation of the endo complex is not unduly inhibited by bulkiness of the nucleophile. Formation and decomposition of the endo complexes are generally slower than those of the exo complexes, partly for steric reasons, the nucleophile having to pass close to Fe(CO)₃ moiety on its way in or out. Reaction of the exo complex may be assisted by Fe–C bond formation coincidental with loss of the nucleophile.