Kinetics of nucleophilic attack on co-ordinated organic moieties. Part 4. Mechanisms of addition of indole and substituted indoles to tri-carbonyl( cyclohexadienyl)-iron and -ruthenium cations

Abstract

A kinetic study of the addition of indoles to the dienyl cations [M (C6H7)(CO)3]+(M = Fe or Ru) and related species in nitromethane reveals a change in mechanism with metal. The iron complexes obey the rate equation Rate =k[complex][indole]. This observation togetherwith substituent effects (e.g. deuteriation) and product stereochem-istry supports direct electrophilic attack of the dienyl cation at C3 of indole followed by rapid proton loss. On the other hand, kinetic evidence has been obtained for an intermediate prior to the rate-determining step in the ruthenium case. These differences are most readily rationalised in terms of rate-determining formation of a π complex for iron, whereas with ruthenium pre-equilibrium formation of a π-complex intermediate is followed by slow formation of a Wheland-type σ complex.