Direct methoxylation of nitroarenes and nitroazaarenes with alkaline methoxides via nucleophilic displacement of an aromatic hydrogen atom

Abstract

Treatment of 1,3-dinitrobenzene and 5-substituted derivatives with excess potassium or sodium methoxide in 1,3-dimethylimidazolidin-2-one (DMI) at room temperature results in the displacement of an aromatic hydrogen at the 4-position by methoxide, affording 2,4-dinitroanisole and its 6-substituted derivatives, respectively, in low to moderate yield. In contrast, an equimolar reaction under similar conditions leads to the replacement of the nitro group in preference to the ring hydrogen. The reaction does not take place with lithium methoxide as a base. Mono- and dinitronaphthalenes and nitroquinolines undergo similar displacement of a hydrogen atom at the position ortho or para to the nitro group, giving the corresponding methoxy derivatives in moderate yield. A slow addition of the nitro compound to a large excess of potassium methoxide under an oxygen atmosphere has been found to enhance the conversion and improve the product yield. On the basis of the product distribution as well as the kinetic isotope effect k_H/k_D = 2.1, direct displacement of a ring hydrogen atom by methoxide ion has been interpreted in terms of the rate-determining release of an ipso-hydrogen atom as a proton from the initially formed Meisenheimer adduct.