Meisenheimer complexes: some structural, equilibrium and kinetic studies

Abstract

Structural and equilibrium studies of Meisenheimer complexes formed from activated anisoles and from l-X-3,5-dinitrobenzenes have been carried out. Specific effects of cations on the stabilities of such a complexes were observed and an explanation has been put forward. The results of (^1)H n.m.r. and visible spectroscopy for the interaction of l-chloro-2,6.dinitro-4-X-benzenes (X = Cl, CO(+2)Me, CF(_3)) with sodium methoxide in dimethyl sulphoxide solutions show that initially base addition takes place at an unsubstituted aromatic carbon, forming a O-complex. However the faster addition of base at the unsubstituted ring position is followed by nucleophilic replacement of chloride ion forming the corresponding anisole. Visible spectroscopy was used for the equilibrium and kinetic study of a-complex formation from activated anisoles and methoxide ion in methanol. The values of stoichiometric equilibrium constants so obtained show an increase with base concentration when sodium, potassium or tetra-n-butyl-ammonium methoxide is used as the base, while no such increase is observed with lithium methoxide. It is suggested that the observed increase in stoichiometric equilibrium constants is due to the stabilisation of the complexes by association with the cations present. As a result of this observation it seems more appropriate to use lithium methoxide for the measurements of thermodynamic equilibrium constants. The specific effects of cations are also observed with bivalent barium and calcium ions, in whose presence large increases in the stoichiometric equilibrium constants for methoxide addition to 2_methoxycarbonyl_4, 6_dinitroanisole, 4-methoxycarbonyl-2, 6-dinitroanisole and 2-chloro-4,6-dinitroanisole are found. This effect, attributed to cation association with the Meisenheimer complexes, is demonstrated in the case of the methoxide σ-complex from 2,4,6-trinitro-anisole, by change in visible spectrum in the presence of various cations. However the σ-complex formed from 1,3,5-trinitrobenzene and methoxide ions shows little tendency to form ion associates with cations. It is therefore suggested that when ion pairs are formed the cation is held by a cage effect by the two oxygen atoms of the methoxyl group at the position of addition and by the electronegative substituent at the ortho-position. Structural and equilibrium studies for the formation of Meisenheimer complexes from some l-X-3,5-dinitrobenzenes and methoxide ions were also made. (^1)H n.m.r. results in conjunction with visible spectroscopy indicateinitial addition of base at an aromatic carbon para to the substituents X, however rearrangement occurs so that at equilibrium the isomeric adduct formed from addition of base at a ring carbon para to a nitro-group usually dominates. The measurements of indicator ratios in methanol-dimethyl sulphoxide mixture containing (~10(^-1) sodium methoxide allowed the simultaneous determination of the acidity function and the thermodynamic equilibrium constants, K, for complex formation. Similar measurements of indicator ratios with the less reactive anisoles in methanol-dimethyl sulphoxide mixtures lead to the determination of an alternative J(_M) acidity function and the thermodynamic equilibrium constants for the complex formation from anisoles and methoxide ions. The two scales as well as the stability constants of the adducts formed from l-X-3,5-dinitrobenzenes and similarly activated anisoles are compared.