A practical catalytic method to convert alkylbenzenes into the corresponding carboxylic acids under atmospheric dioxygen at ambient temperature using a combined catalytic system consisting of N-hydroxyphthalimide (NHPI) and Co(OAc)2 was developed. For instance, the oxidation of toluene was completed by NHPI combined with Co(OAc)2 under an oxygen atmosphere at room temperature to give benzoic acid in 81% yield. Under these conditions, o- and p-xylenes were selectively converted into the corresponding monocarboxylic acids without the formation of the dicarboxylic acids. ESR measurements showed that Co(II) species assists in the formation of phthalimide-N-oxyl (PINO), which is a key species in this oxidation, from NHPI.

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An innovation of the aerobic oxidation of hydrocarbons through catalytic carbon radical generation under mild conditions was achieved by using N‐hydroxyphthalimide (NHPI) as a key compound. Alkanes were successfully oxidized with O2 or air to valuable oxygen‐containing compounds such as alcohols, ketones, and dicarboxylic acids by the combined catalytic system of NHPI and a transition metal such as Co or Mn. The NHPI‐catalyzed oxidation of alkylbenzenes with dioxygen could be performed even under normal temperature and pressure of dioxygen. Xylenes and methylpyridines were also converted into phthalic acids and pyridinecarboxylic acids, respectively, in good yields. The present oxidation method was extended to the selective transformations of alcohols to carbonyl compounds and of alkynes to ynones. The epoxidation of alkenes using hydroperoxides or H2O2 generated in situ from hydrocarbons or alcohols and O 2 under the influence of the NHPI was demonstrated and seems to be a useful strategy for industrial applications. The NHPI method is applicable to a wide variety of organic syntheses via carbon radical intermediates. The catalytic carboxylation of alkanes was accomplished by the use of CO and O2 in the presence of NHPI. In addition, the reactions of alkanes with NO2 and SO2 catalyzed by NHPI provided efficient methods for the synthesis of nitroalkanes and sulfonic acids, respectively. A catalytic carbon‐carbon bond forming reaction was achieved by allowing carbon radicals generated in situ from alkanes or alcohols to react with alkenes under mild conditions. 1 Introduction 2 Discovery of NHPI as Carbon Radical Producing Catalyst from Alkanes 2.1 Historical Background 2.2 Catalysis of NHPI in Aerobic Oxidation 3 NHPI‐Catalyzed Aerobic Oxidation 3.1 Oxidation of Benzylic Compounds 3.2 Alkane Oxidations with Molecular Oxygen 3.3 Oxidation of Alkylbenzenes 3.4 Practical Oxidation of Methylpyridines 3.5 Preparation of Acetylenic Ketones via Alkyne Oxidation 3.6 Oxidation of Alcohols 3.7 Selective Oxidation of Sulfides to Sulfoxides 3.8 Production of Hydrogen Peroxide by Aerobic Oxidation of Alcohols 3.9 Epoxidation of Alkenes using Molecular Oxygen as Terminal Oxidant 4 Carboxylation of Alkanes with CO and O2 5 Utilization of NOx in Organic Synthesis 5.1 First Catalytic Nitration of Alkanes using NO2 5.2 Reaction of NO with Organic Compounds 6 Sulfoxidation of Alkanes Catalyzed by Vanadium 7 Carbon‐Carbon Bond Forming Reaction via Catalytic Carbon Radicals Generated from Various Organic Compounds Assisted by NHPI 7.1 Oxyalkylation of Alkenes with Alkanes and Dioxygen 7.2 Synthesis of α‐Hydroxy‐γ‐lactones by Addition of α‐Hydroxy Carbon Radicals to Unsaturated Esters 7.3 Hydroxyacylation of Alkenes using 1,3‐Dioxolanes and Dioxygen 8 Conclusions

A novel class of catalysts for alkane oxidation with molecular oxygen was examined. N-Hydroxyphthalimide (NHPI) combined with Co(acac)(n)() (n = 2 or 3) was found to be an efficient catalytic system for the aerobic oxidation of cycloalkanes and alkylbenzenes under mild conditions. Cycloalkanes were successfully oxidized with molecular oxygen in the presence of a catalytic amount of NHPI and Co(acac)(2) in acetic acid at 100 degrees C to give the corresponding cycloalkanones and dicarboxylic acids. Alkylbenzenes were also oxidized with dioxygen using this catalytic system. For example, toluene was converted into benzoic acid in excellent yield under these conditions. Ethyl- and butylbenzenes were selectively oxidized at their alpha-positions to form the corresponding ketones, acetophenone, and 1-phenyl-1-butanone, respectively, in good yields. A key intermediate in this oxidation is believed to be the phthalimide N-oxyl radical generated from NHPI and molecular oxygen using a Co(II) species. The isotope effect (k(H)/k(D)) in the oxidation of ethylbenzene and ethylbenzene-d(10) with dioxygen using NHPI/Co(acac)(2) was 3.8.

Highly efficient catalytic oxidation of alcohols with molecular oxygen by N-hydroxyphthalimide (NHPI) combined with a Co species was developed. The oxidation of 2-octanol in the presence of catalytic amounts of NHPI and Co(OAc)2 under atmospheric dioxygen in AcOEt at 70 degrees C gave 2-octanone in 93% yield. The oxidation was significantly enhanced by adding a small amount of benzoic acid to proceed smoothly even at room temperature. Primary alcohols were oxidized by NHPI in the absence of any metal catalyst to form the corresponding carboxylic acids in good yields. In the oxidation of terminal vic-diols such as 1,2-butanediol, carbon-carbon bond cleavage was induced to give one carbon less carboxylic acids such as propionic acid, while internal vic-diols were selectively oxidized to 1,2-diketones.

Direct conversion of cyclohexane into adipic acid was achieved by the use of the radical catalyst, N-hydroxyphthalimide (NHPI), in the presence of a small amount of a transition metal. For instance, cyclohexane could be converted into adipic acid in 73% selectivity at 73% conversion under atmospheric oxygen (1 atm) in the presence of NHPI (10 mol %) and Mn(acac) 2 (1 mol %) at 100 °C for 20 h. ESR measurements show that the formation of phthalimide N-oxyl generated from NHPI with O 2 was assisted by Co(II) species. Thus, the oxidation of cyclohexane to adipic acid was found to be enhanced by the addition of a small amount of Co(OAc) 2 to the NHPI/Mn(acac) 2 system. The NHPI-catalyzed oxidation of cyclohexane provides an attractive direct method which has long been desired in the chemical industry for the manufacturing of adipic acid. The present oxidation seems to be the first practical environmentally friendly process, in which nitric acid is not used as the oxidant, for the production of adipic acid from cyclohexane.

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A facile method for the synthesis of organophosphonates from alkenes and dialkyl phosphites was developed by the use of Mn(II) under air. Thus, the reaction of 1-octene with diethyl phosphite in the presence of Mn(OAc)2 (5 mol %) under air at 90 degrees C led to diethyl octylphosphonate (78%) and diethyl (2-hexyl)decylphosphonate (6%). Internal alkenes such as cis-2-octene gave a regioisomeric mixture of the corresponding hydrophosphorylation products in 84% yields.