Kinetics of nucleophilic attack on co-ordinated organic moieties. Part 19. Addition of anilines to tricarbonyl(1–5-η-dienyl)iron cations

Abstract

Spectroscopic and kinetic studies of the reactions below (X = H, 2-Me, 3-Me, 4-Me, 2-Cl, 3-Cl, 4-Cl, 4-OMe, or 4-NO₂) in CH₃CN have provided quantitative information on the importance of basicity and steric factors in controlling amine nucleophilicity towards co-ordinated π-hydrocarbons. Similar reactions with cations [Fe(1–5-η-C₆H₇)-(CO)₃]⁺(1a) and [Fe(1–5-η-C₇H₉)(CO)₃]⁺(1c) have also been investigated. For the reactions of 4-methoxy-and 4-methyl-aniline with (1b), which proceed to completion, k_obs.=k₁[amine]. However, for the equilibrium [graphic omitted] reactions of the other less basic amines with cations (1a)–(1c) the two-term expression k_obs.=k₁[amine]+k_–1[H⁺]/([H⁺]+K₂K_a) is indicated. The general rate trend C₆H₇ > 2-MeOC₆H₆ > C₇H₉ and the low ΔH‡₁ and large negative ΔS‡₁ values are consistent with direct addition to the dienyl rings. For attack by non-sterically crowded anilines on (1b), a Brönsted plot of log k₁versus pK_a of the amine conjugate acid (in H₂O) has a high slope of 1.0, indicating a very strong dependence of k₁ on amine basicity. A Hammett plot for this reaction gives a slope of –2.7, indicating significant bond formation and build-up of positive charge on the aniline nitrogen atom in the transition state. The steric retardation of k₁ caused by blocking substituents in the 2-position of the anilines is considerably smaller than that previously found for the related additions of pyridines to cations (1a)–(1c).