Modern Oxidation Methods, Part 2:

Parentheses show the conversion and selectivity in the nitration using NO2 instead of HNO3.

The essence of biosynthesis is the control of selectivity by some noncovalent binding of the substrate to the enzymatic protein domain. This suggests that porous zeolites could be used as host materials for host-guest composites with sequestered organic molecules, analogous to enzyme-substrate complexes in biooxidation, and enhances molecular orientation for chemical reactions (145). It was proposed that the quinones can make NHPI convert to PINO via one-electron transfer and subsequently facilitate hydrocarbon oxidation, in which zeolite is used to promote reaction selectivity (146). The catalytic redox cycle is presented in Scheme 25, (146).

Tong et al., (154) reported a new o-phenanthroline (or analogues)-mediated, metal free catalytic system for the oxidation of hydrocarbons in the presence of NHPI and a cocatalyst. Molecular bromine (Br2), which has shown a particular promotion effect in some oxidation processes (155), was chosen as a cocatalyst. To test the efficiency of their new proposed catalytic system, Tong et al. (154) used ethylbenzene as a model substrate, Scheme 26, (154).

Among all the used mediators, Phen is a most efficient catalyst, and differences in the oxidation mediated by different analogues are probably ascribed to effects of conjugated structures and substituents.

In the beginning of the reaction, Phen is converted to cation radicals through single-electron oxidation of the nitrogen, then the cations radicals promote the generation of PINO radical under the metal-free conditions, via electron and proton transfer between cation radicals and NHPI (Equation 1, Scheme 27). The next step involves the hydrogen atom abstraction from the hydrocarbon by PINO, and the resulting hydrocarbon radical being trapped by dioxygen provides the peroxy radicals, which are eventually converted into products through hydroperoxide (Equation 2, Scheme 27).

The use of NHPI in organic chemistry is limited due to its low solubility in many organic solvents. A new approach to overcome this limitation is to take into account the possibility of the use of heterogeneous systems. Our very recent findings, in connection with the unexpected behavior of the NHPI/ cycloalkenes system, lead to a new way to generate PINO radical, using sodium periodate (156). Sodium periodate is a widely used reagent for the oxidative cleavage of 1,2-diols to carbonyl compounds (157). Unfortunately, the use of this highly selective reagent is also strongly restricted by its insolubility in organic solvents (158).

The concept of supported reagents (162, 163), either adsorbed on or bound to insoluble matrixes, also offered an attractive solutions.

Silica gel supported metaperiodate was shown to oxidize smoothly 1,2- diols and hydroquinones in dichloromethane and non polar organic solvents. Wet silica gel is an effective support for metaperiodate. The method consists in stirring suspensions of silica gel in dichloromethane, with a freshly prepared solution of sodium periodate in water, and then NHPI is added. The PINO radical is generating according with the Equation 17.

After 5 minutes, a cycloalkene/dichloromethane solution is added, at this point, the radical reaction between PINO and cycloalkene taking place. The reaction occurs by abstraction–addition and addition–abstraction competitive mechanisms, the final products being identical for the both mechanisms in thecase of the symmetrical alkenes.

Once the NHPI has been recognized as a valuable catalyst for the aerobic oxidation of a wide range of organic compounds under mild conditions, researchers’ concern was next turned toward of the use of NHPI derivatives as catalysts for the oxidation reactions, since one the major drawback in the use of NHPI, is the self-decomposition of resulted PINO, thus requiring quite large amounts of NHPI (usually 10 mol%). Scheme 29 shows the most important NHPI/PINO derivatives used in organic catalysis.

Beside PINO, others non persistent nitroxyl radicals have continuously attracted the scientific community because they can act as efficient mediators in many various reactions. These unstable (transient) radicals are generated in situ from their parent hydroxylamines, some of them being presented in Scheme 30.

The use of NHS, in which, one carbonyl group in NHPI is replaced by the more strongly electron-withdrawing sulfonyl group, should provide a more effective catalyst activity in the cycloalkane’s autooxidation. Indeed, this hypothesis was proved by the Sheldon et al. works (172, 173). Table 24, (172, 173) shows the comparative results obtained in the autooxidation of cyclododecane in acetic acid at different temperatures, by using both NHPI and NHS. As expected, NHS proved its superior catalytic activity, allowing a lower reaction temperature. In contrast, at this temperature (50°C), NHPI failed to promote the oxidation reaction of cyclododecane.

8. CONCLUSIONS AND OUTLOOK

There are still disputes linked with the precise value of the BDE in NHPI, various research groups reporting different values for the experiments performed in the same conditions. Another point that can be improved is to carry out systematic studies of the NHPI stability under the same conditions as the reaction implying NHPI are usually carried out (especially in acidic media and high temperatures, when the possibility of the decomposition becomes muchmore favorable). Other issue, which need to be addressed, concern the metal catalyst used with NHPI in the reactions. There are reported several metal types used as cocatalyst with NHPI, alone with in different mixture combinations. Deeper investigations of the metal activity correlate with its electronic structure and metal’s ability to be bonded into ligands complex network can afford the obtaining of the new catalytic systems with a better activity. However, the global concerns liked with the pollution issues, persuade the researchers to rethink the opportunity of the use of metal cocatalysts with NHPI. From this point of view, new systematic research should be directed toward identifying new ‘‘metal free’’ cocatalyst to be used with NHPI, and to optimize the conditions for the already reported systems.