Pyrazinecarboxylic acid and analogs: Highly efficient co-catalysts in the metal-complex-catalyzed oxidation of organic compounds

Abstract

The development of new metal complex catalysts and efficient protocols for the mild and selective oxidation of alkanes, arenes, olefins, alcohols, and other organic substrates is a challenging topic in areas of homogeneous catalysis, coordination, and organic chemistry. In these oxidation reactions, the activity of many metal complex catalysts is dramatically improved upon addition of certain co-catalysts or promoters, which can also act as ligands in simple catalytic systems generated in situ. Given the fact that 2-pyrazinecarboxylic acid (Hpca) and analogous heteroaromatic acids such as 2,3-pyrazinedicarboxylic (H2pdca), picolinic (Hpic), and dipicolinic (H2dipic) acids are remarkably efficient and versatile co-catalysts in a high diversity of oxidation systems, the present review summarizes the state-of-the-art knowledge in this field. In particular, this contribution focuses on the use of Hpca as the most active co-catalyst and describes its coordination chemistry with regard to the oxidative transformations of various organic substrates, providing an overview of isolated vanadium, iron, and some other transition metal complexes derived from Hpca. The review also summarizes the applications of Hpca-assisted and related systems in the oxidation of various organic substrates by different oxidants, and highlights the main selectivity, kinetic, and mechanistic features of these oxidative transformations. The paper covers the application of catalytic systems wherein Hpca, H2pdca, Hpic, and H2dipic are used either as co-catalysts (additives) or as ligands within a metal complex catalyst. A special emphasis has been made on the oxidation of alkanes as very inert substrates, and the use of simple and highly efficient [VO3]−/Hpca/H2O2 system. The multifaceted roles of Hpca and analogous co-catalysts have been identified and the analysis of main mechanistic pathways and possible intermediates has been performed.