The free-radical substitution of ferrocene and the stability of ferrocenylmethyl radical.

Abstract

The electron transfer between ferrocene and triphenylmethyl tetrafluoroborate has been shown to be moderately fast, and to be completely independent of the subsequent substitution of ferrocene. Methyl-, isopropyl-, benzyl-, and phenyl-ferrocene have been mono-benzoylated by free-radical and electrophilic methods. For a particular ferrocene the isomer distributions of the products in both methods have been found to be identical. It was concluded that free-radical substitution of ferrocenium ion and electrophilic substitution of ferrocene pass through a common transition state. Some benzoylferrocenes which are inaccessible by electrophilic substitution may be synthesized by the free-radical route; the synthesis of p-benzoylferrocene in moderate yields was achieved. The reaction of ferrocenium ion with alkyl radicals generated from triorganotin hydrides and alkyl halides was investigated. A value of c.7x10⁵ 1. mol ⁻¹s ⁻¹ at 25°C was derived for the rate constant for ferrocenium ion substitution by alkyl radicals. The stability of ferrocenylmethyl radical has been investigated by studying the products obtained from the one-electron reductions of a series of alkyl (ferrocenylmethyl) quaternary ammonium salts. The stability of this radical is intermediate between that of benzyl and t-butyl radicals. A method of synthesizing ferrocenylmethylamines from ferrocenylmethanol and amines has been explored, and shown to depend upon the pH of the solution, and on the nature of the amine.