Kinetics of nucleophilic attack on co-ordinated organic moieties. Part 15. Addition of p-toluidine to tricarbonyl(1–5-η-dienyl)iron cations

Abstract

The reactions of the cations [Fe(η5-dienyl)(CO)3]+(1; dienyl = C6H7′ C6H6OMe-2, or C7H9) with p-toluidine in CH3CN have been shown to give the neutral adducts [Fe(η4-diene·NHC6H4CH3)(CO)3](2) according to the equation (1)+ 2CH3C6H4NH2→(2)+[CH3C6H4NH3]+. The products have been characterised by analyses, 1H n.m.r., i.r., and field-desorption mass spectroscopy. The reactions could be reversed by the addition of acid. A stopped-flow kinetic study of the above reactions in CH3CN yields the general rate law, Rate =k[Fe][CH3C6H4NH2]. These results are rationalised in terms of a two-step mechanism involving initial reversible addition (k1, k–1) of p-toluidine to the dienyl rings to form the cationic intermediates [Fe(η4-diene·NH2C6H4CH3)]+(3), followed by rapid proton loss, k2(either solvent- or amine-assisted). For such a mechanism the kobs. values refer to k1, which is consistent with the observed order of reactivity of the cations at 0 °C (C6H7 > C6H6OMe-2 > C7H9, 17 : 2.6 : 1), and the low ΔH‡ and negative ΔS‡ values.