Convenient Synthesis of p-Aminobenzoic Acids and their Methyl Esters

Amino groups are of significant biological and industrial importance as they are fundamental components of living organisms and of amino acids. The properties of amines depend on the type of substituents on the nitrogen, steric hindrance, and solvation effects. In general, the basicity of an amine increases with the number of alkyl groups on the amine while aryl substituents generally delocalize the lone pair of electrons into the ring leading to decreased basicity. These factors may cause amino groups to have a major influence on the spectroscopic and redox properties of such compounds as well as on the possibility of the occurrence of the interand intramolecular electron transfer (ICT) processes. Aromatic amines are generally quite toxic and readily absorbed through the skin, some are potent carcinogens. However, appropriate modifications of their structure make it possible to access compounds having the desired properties such as desired in biologically active compounds, polymers, radical photo-initiators, dyes, photo-voltaic cells, lightemitting diodes, spectroscopic probes, modern medical techniques and in many other fields. p-Aminobenzaldehydes constitute convenient and frequently used stating reagents for the preparation of various amines. Babu andBalasubramaniam proposed a simplemethod for the synthesis of p-dimethylaminobenzoic acid by oxidation of an aldehyde. However, it requires sodium chlorite, which is hazardous and less universally available.Moreover, this process, like the methods using hydrogen peroxide as oxidant, were only autilized for the preparation of p-dimethylaminobenzoic acid. No evidence of their scope and versatility with respect to other amine derivatives of benzoic acid was provided. Our need of various p-aminobenzoic acids and their esters for the preparation of a few boron-dipyrromethene (BODIPY) dyes, and other dyes containing difluoroborane moiety as spectroscopic probes prompted us to re-investigate and modify the reaction reported by Ott and his group nearly fifty years ago after our unsuccessful attempts to obtain these acids in good yields by oxidation of aldehydes with silver nitrate or via the carboxylation of the corresponding Grignard reagents. We now report this route as a very convenient preparation of these aminobenzoic acids, requiring only the use of simple and readily available reagents. It involves no solvents, and produces the desired acids efficiently and in high purity with no troublesome