NHPI-catalyzed aerobic oxidation

Here we show a novel methodology for the functionalizations of hydrocarbons, including oxygenation, nitration, sulfoxidation, epoxidation, carboxylation, and oxyalkylation through generation of the catalytic carbon radical. In particular, the NHPI-catalyzed aerobic oxidations of alkanes, which are very important in industry worldwide, are described in detail. 

NHPI-CatalYzed Aerobic Oxidation 1191 NHPl Species (25 uniol)... 

Representative results for the NHPI-catalyzed aerobic oxidation of various alkylbenzenes in the presence of Co(OAc)2 in acetic acid under ambient conditions are listed in Table 6.1. Both p- and o-xylenes are selectively oxidized to p- and o-toluic adds without the formation of dicarboxylic acids. o-Ethyltoluene undergoes selective oxidation to form a mixture of the corresponding alcohol and ketone in which the ethyl moiety was selectively functionalized. It is of interest to examine the effect of substituents on the aromatic ring in the oxidation of substituted toluenes. p-Methoxytoluene is more rapidly oxidized than the toluene itself, while... 

NHPI-Catalyzed Aerobic Oxidation 195 Table 6.1 Aerobic oxidation of various alkylbenzenes at room temperature . ... 

Various substituted phenols were selectively synthesized by a one-pot reaction through the NHPI-catalyzed aerobic oxidation of l,l -diarylethanes to hydroperoxides followed by treatment with dilute sulfuric acid [75]. For example, the oxidation of l-(4-methoxyphenyl)-l-phenylethane was performed under O2 (1 atm) in the presence of AIBN (3 mol%) and NHPI (10mol%) in MeCN (3mL) at 75 °C for 15 h, and treatment with sulfuric add afforded 4-methoxyphenol and acetophenone in 61% yield (97% selectivity) at 63% conversion (Eq. (6.17)). In this reaction, the degradation of hydroperoxides was selectively induced to give more electron-rich phenols in high selectivity (Scheme 6.6). 

It is interesting to develop a novel route to the CL precursor, PDHA, which was hitherto prepared by hydrogen peroxide oxidation of cyclohexanone (3) followed by treatment with ammonia [126,130]. Because ofthe ease of transformation of PDHA to a 1 1 mixture of CL and 3 under the influence of an appropriate catalyst such as lithium halides, the CL production via PDHA is considered to be a superior candidate for a next-generation waste-free process for CL. The NHPI-catalyzed aerobic oxidation of KA oil was applied to the synthesis of PDHA without formation of any ammonium sulfate waste. The strategy is outlined in Scheme 6.10. The NHPI-catalyzed oxidation of KA oil (a mixture of 3 and 2) with O2 produces 1,1 -dihydroxydicyclohexyl peroxide, which seems to exist in equilibrium with cyclohexanone and H2O2 (path 1). Subsequent treatment of the resulting reaction mixture... 

The relatively low BDE value (88.1 kcal/mol) of the O-H bond for NHPI suggested that peracids and dioxiranes could undergo induced homolysis of NHPI under mild conditions, generating the PINO radical [Eqs. (6.5) and (6.6)], which plays a key role in the aerobic oxidations catalyzed by NHPI. 

These results suggest the possibility of using the aerobic oxidation of aldehydes, catalyzed by NHPI, for the epoxidations of alkenes by peracids generated "in situ" under mild conditions [Eq. (6.11)]. 

TABLE 6.3 Epoxidation of olefins by aerobic oxidation of acetaldehyde, catalyzed by NHPI... 

Scheme 16.5 Aerobic oxidation of adamantane catalyzed by NHPI.

Scheme 16.6 Aerobic oxidation of cyclohexane to adipic acid catalyzed by NHPI.

The aerobic oxidations of 1 must be carried out in an appropriate solvent such as acetic acid or acetonitrile because of the lower solubility of NHPI in nonpolar solvents such as hydrocarbons. It is noteworthy that the NHPI-catalyzed reaction of 1 could proceed without any solvent by the use of a lipophilic NHPI derivative. Of a series of 4-alkyloxycarbonyl N-hydroxyphthalimides examined as lipophilic NHPI catalysts, 4-lauryloxycarbonyl N-hydroxyphthalimide was found to be an efficient catalyst for the aerobic oxidation of 1 under solvent-free conditions (Figure 6.1) [30]. 

The autoxidation of isobutane is now mainly carried out to obtain terf-butyl hydroperoxide [36]. Halogenated metalloporphyrin complexes are reported to be efficient catalysts for the aerobic oxidation of isobutane [18,37]. It was found that the oxidation of isobutane by air (lOatm) catalyzed by NHPI and Co(OAc)2 in benzoni-trile at 100 °C produced tert-butyl alcohol in high yield (81%) along with acetone (14%) (Eq. (6.3)) [38]. 2-Methylbutane was converted into the carbonacetic acid, rather than the alcohols, as prindpal products. These cleaved products seem to be formed via P-sdssion of an alkoxy radical derived from the decomposition of a hydroperoxide by Co ions. The extent of the P Scission is known to depend on the stability of the radicals released from the alkoxy radicals [39]. It is thought that the 3-scission of a terf-butoxy radical to acetone and a methyl radical occurs with more difficulty than that of a 2-methylbutoxy radical to acetone and an ethyl radical. As a result, isobutane produces terf-butyl alcohol as the principal product, while 2-methylbutane affords mainly acetone and acetic acid. 

Scheme 6.3 A plausible reaction path for the aerobic oxidation of toluene catalyzed by NHPI combined with Co(ll).

Scheme 2.13 Aerobic oxidation of (a) cycloalkanes and (b) cycloalkenes, catalyzed by NHPI/FeBTC.

In this multi-authored book selected authors in the field of oxidation provide the reader with an up to date of a number of important fields of modern oxidation methodology. Chapter 1 summarizes recent advances on the use of green oxidants such as H2O2 and O2 in the osmium-catalyzed dihydroxylation of olefins. Immobilization of osmium is also discussed and with these recent achievements industrial applications seem to be near. Another important transformation of olefins is epoxidation. In Chapter 2 transition metal-catalyzed epoxidations are reviewed and in Chapter 3 recent advances in organocatalytic ketone-catalyzed epoxidations are covered. Catalytic oxidations of alcohols with the use of environmentally benign oxidants have developed tremendously during the last decade and in Chapter 4 this area is reviewed. Aerobic oxidations catalyzed by N-hydroxyphtahmides (NHPI) are reviewed in Chapter 5. In particular oxidation of hydrocarbons via C-H activation are treated but also oxidations of aUcenes and alcohols are covered. 

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