An Unconventional Cobalt‐Catalyzed Aerobic Oxidation of Tertiary Nitrogen Compounds to N‐Oxides.

Abstract

The propensity of cobalt complexes to bind with molecular oxygen[1] and the use of such dioxygen–cobalt complexes as catalysts for various oxidation reactions have been subjects of intensive research in recent years.[2] The oxygenation of organic compounds catalyzed by such dioxygen–cobalt complexes and with aldehyde as a sacrificial agent[3] has led to several important methodologies for the epoxidation of olefins,[4] oxidation of sulfides to sulfones,[5] and oxidation of imines to oxaziridines.[6] However, the oxygenation of organic compounds, in which molecular oxygen serves as the oxidant and cobalt complexes as catalysts under ambient conditions without the sacrificial agent, although a desirable goal, has not been very successful. To the best of our knowledge there are only two relevant literature reports describing the oxygenation of trialkylphosphanes[7] and olefins, and cyclic ethers.[8] The cobalt Schiff base complexes 1,[4b] 2,[9] 3,[10] and 4[11] are well known to bind with molecular oxygen[4b,9,10] and show very high reactivity in oxygenation reactions with molecular oxygen as the oxidant along with an aldehyde as a sacrificial agent, but there are no reports on the oxygenation of organic compounds with these complexes as catalysts and molecular oxygen as the sole oxidant. Oxidation of tertiary nitrogen compounds to N-oxides is an important synthetic transformation. In the search for environmentally friendly methods for this transformation the use of hydrogen peroxide with the catalysts methyltrioxorhenium(vii),[12] manganese porphyrin,[13] flavin,[14] TS-1,[15] molecular sieves,[16] and tungstate-exchanged Mg/Al-layered double hydroxide[17] has been reported. In continuation of our studies on oxidation with molecular oxygen as the primary oxidant[18] we report here on the first successful oxidation of tertiary nitrogen compounds 5 to N-oxides 6 catalyzed by cobalt(ii) Schiff base complexes (Scheme 1). A wide variety of tertiary nitrogen compounds were oxidized to give their corresponding N-oxides in nearly quantitative yields by simply bubbling molecular oxygen into a solution of the tertiary nitrogen compound in 1,2dichloroethane at room temperature in the presence of 5-5 molecular sieves and with the cobalt Schiff base complex 1 as the catalyst. These results are summarized in Table 1. Pyridines containing electron-donating groups (Table 1, entries 2, 3, and 6) were found to react faster than pyridines bearing electron-withdrawing groups (Table 1, entries 4 and