A kinetic study of the mechanisms of SNAr reactions of neutral and cationic metal-complexed halogenoarenes with methoxide ion

Abstract

Rate studies have shown that the activation of halogenobenzenes towards nucleophilic attack by methoxide ion in methanol increases through the series of π-attached residues: (OC)₃Cr < (OC)₃Mo (η⁵-C₅H₅)Fe⁺ < (OC)₃Mn⁺. For a given (π-arene)metal system, the reactivity of halogenoarene ligands towards methoxide decreases through the series: fluorobenzene > chlorobenzene > chlorotoluene. Rates of reaction with methoxide are retarded in the presence of NaClO₄ or LiCl, but accelerated when the methanol solvent is diluted with tetrahydrofuran or 1,4-dioxan. Fluorobenchrotrene (OC)₃Cr(η⁶-PhCl) is slightly more reactive towards ethoxide in ethanol than towards methoxide in methanol. Evidence has been obtained that the cationic (halogenoarene)metal complexes readily form ion-pairs with methoxide, resulting in a reduction in their reactivity towards nucleophilic attack. Methoxide adds to [(π-methoxyarene)Mn(CO)₃]⁺ complexes to give (η⁵-cyclohexadienyl) products which are reconverted into the cations by acid treatment.