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.