Some spectroscopic, kinetic and equilibria studies of cyano- and nitro- substituted anisoles and phenols and their reactions with nucleophiles
The reactions of ring-activated alkyl aryl ethers with nucleophiles have been examined. The main pathways involve attack by the nucleophiles at the alkyl carbon to give substituted phenoxide ions, or at ring carbon atoms to give σ-adducts. The dealkylation reactions have been observed by both (^1)H nuclear magnetic resonance spectroscopy and spectrophotometrically using dimethyl sulphoxide as solvent. The reactivity order for nucleophiles in this solvent is SCN- < I- < Br- < C1- < CN(_3)- and is markedly different from that found for Sn2 reactions in protic solvents. These differences probably reflect changes in the relative salvation of the nucleophiles on charge of solvent. Dealkylation is also observed in dimethyl sulphoxide alone and is thought to yield S-alkylated DMSO. In methanol reaction of 4-cyano-2,6-dinitroanisole and 2-cyano4,6-dimtro- anisole with methoxide ions results in competition between attack at ring carbon and at the cyano-group. Kinetic and equilibrium data are reported for reaction of 4-cyano-2,6-dinitroanisole where rapid formation of the 1,1-dimethoxy adduct is followed by slower equilibration with the imido-ester solvate. There is evidence for strong association with cations of the 1,1-dimethoxy adduct, and values of the association constants decrease in the order of cations, Ba(^2+) > Ca(^2+) > K(^+) > Na(^+) > Li(^+). This explains a longstanding discrepancy in the literature between the values of equilibrium constants for attack of lithium methoxide and of sodium methoxide. In concentrated solutions of sodium or potassium methoxide in methanol nitro-activated anisoles form adducts with 2:1 and 3:1 stoichiometry. Here methoxide addition occurs at two or three ring positions respectively. Such equilibria have been examined spectrophotometrically for 2,4,6-trinitroanisole, the isomeric cyanodinitroanisoles and the spiro-adduct formed by cyclisation of picryl glycol ether. For these equilibria the "basicity" of sodium methoxide solutions jn methanol is appreciably greater than that of corresponding potassium methoxide solutions. This is in contrast with other measures of basicity and is attributed to the association of the multi-charged adducts with cations which is stronger with sodium ions than with potassium ions. The infrared spectra of the parent and nitro-cyanoanisoles and their Meisenheimer complexes have been examined in the N0(_2) group, benzene ring and CN group regions. In order to attempt to understand their chemical and spectral behaviour the corresponding phenols and their Na(^+) and K(^+) salts have also been studied both in solution in dimethyl sulphoxide and in the solid state. Band shifts, widths and intensities have been interpreted in terms of the nature and extent of intra-molecular electronic rearrangements caused by the intermolecular interactions (hydrogen-bonding and cation coordination). Interesting, although not fully understood, differences in behaviour between the 2 and 4-CN substituted dinitrophenols and anisoles have been noted.