Cyclohexyl hydroperoxide

Inspired by Gif or GoAgg type chemistry [77], iron carboxylates were investigated for the oxidation of cyclohexane, recently. For example, Schmid and coworkers showed that a hexanuclear iron /t-nitrobenzoate [Fe603(0H) (p-N02C6H4C00)n(dmf)4] with an unprecedented [Fe6 03(p3-0)(p2-0H)] " core is the most active catalyst [86]. In the oxidation of cyclohexane with only 0.3 mol% of the hexanuclear iron complex, total yields up to 30% of the corresponding alcohol and ketone were achieved with 50% H2O2 (5.5-8 equiv.) as terminal oxidant. The ratio of the obtained products was between 1 1 and 1 1.5 and suggests a Haber-Weiss radical chain mechanism [87, 88] or a cyclohexyl hydroperoxide as primary oxidation product. 

Benzoyl hydroperoxide was used for the conversion of divinyl sulphide into divinyl sulphoxide by Levin as early as 1930. In 1954 Bateman and Hargrave reported that saturated sulphides may be oxidized to sulphoxides by means of cyclohexyl or t-butyl hydroperoxide. These authors found that in both polar and non-polar solvents oxygen transfer occurred to give quantitative yields of sulphoxides over a wide range of experimental conditions according to equation 7. It was also reported that a quantitative yield of sulphoxides was obtained from the reaction of unsaturated sulphides with t-butyl and cyclohexyl hydroperoxides in methanol. With t-butyl hydroperoxide in benzene the sulphoxide yield was in no case stoichiometric, varying from 90 to 5% under the condition chosen. 

Results of the cyclohexane oxidation tests are shown in Table 41.4. Mono-oxygenated products are cyclohexanone, cyclohexanol and cyclohexyl hydroperoxide. Cu and Cr were very active, but subsequent tests showed considerable leaching for both metals, whereas Co-Si-TUD-1 did not show ai r leaching. Tests with different Co loadings indicate that the lowest Co concentration has the best conversion and ketone selectivity. Isolated cobalt species are most efficient for the conversion of cyclohexane, as agglomeration of Co reduces... 

Cyclohexyl radicals react with cyclohexyl hydroperoxide to yield Cyclohexane and the cyclohexyl peroxy radical ... 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

The detailed mechanism for these Co AlPO-18- and Mn ALPO-18-cata-lyzed oxidations are unknown, but as previously pointed out vide supra) and by analogy to other metal-mediated oxidations a free-radical chain auto-oxidation (a type IIaRH reaction) is anticipated [63], This speculation is supported by several experimental observations that include (1) an induction period for product formation in the oxidation of n-hexane in CoAlPO-36, (2) the reduction of the induction period by the addition of free-radical initiators, (3) the ability to inhibit the reaction with addition of free-radical scavengers, and (4) the direct observation of cyclohexyl hydroperoxide in the oxidation of cyclohexane [62],... 

Vanoppen et al. [88] have reported the gas-phase oxidation of zeolite-ad-sorbed cyclohexane to form cyclohexanone. The reaction rate was observed to increase in the order NaY < BaY < SrY < CaY. This was attributed to a Frei-type thermal oxidation process. The possibility that a free-radical chain process initiated by the intrazeolite formation of a peroxy radical, however, could not be completely excluded. On the other hand, liquid-phase auto-oxidation of cyclohexane, although still exhibiting the same rate effect (i.e., NaY < BaY < SrY < CaY), has been attributed to a homolytic peroxide decomposition mechanism [89]. Evidence for the homolytic peroxide decomposition mechanism was provided in part by the observation that the addition of cyclohexyl hydroperoxide dramatically enhanced the intrazeolite oxidation. In addition, decomposition of cyclohexyl hydroperoxide followed the same reactivity pattern (i.e., NaY < BaY... 

The goal here was to find new solid catalysts for cyclohexyl hydroperoxide (chhp) decomposition in cyclohexanol and cyclohexanone. The requirement list had foreseen a study on silica-supported metals of groups 4 and 5 and the need for a heterogeneous catalyst without metal Bxiviation. 

Table 3.8 Comparison of group 4 metals in cyclohexyl hydroperoxide (chhp) deperoxidation.

Figure 3.29 Si-0-Ta(0Me)4 catalyst recycling in the cyclohexyl hydroperoxide deperoxidation reaction.

Next to TBHP also ferf-pentyl hydroperoxide, cumyl hydroperoxide and cyclohexyl hydroperoxide could be employed as oxidant and 2-hydroxycyclobutanone and 2-hydro xycyclododecanone were prepared by this method as well. In 1985, Vedejs and Larsen reported on a preparative method for the a-hydroxylation of camphor and a variety of other ketones utilizing overstoichiometric amounts of oxodiperoxomolybdenum(pyridine)(hexamethylphosphoric triamide) as source of oxygen (equation 67). Yields of products ranged from 34-81% and in some cases also the a-diketone is formed as by-product (0-26%). 

Cyclohexene, primary ozonide, 720 Cyclohexyl hydroperoxide, flame ionization detection, 689 Cyclopentadiene... 

The cobalt-catalyzed oxidation of cyclohexane takes place through cyclohexyl hydroperoxide with the cobalt catalyst acting primarily in the decomposition of the hydroperoxide to yield the products 870 877... 

Decomposition of the hydroperoxides would be considered in terms of the following general scheme for example, for cyclohexyl hydroperoxide ... 

Adipic acid is a most important petrochemical product which is mostly used for the synthesis of nylon 6.6 from its condensation with hexamethylenediamine. Cyclohexane is transformed to adipic acid in two steps (a) oxidation of cyclohexane to a cyclohexanol-cyclohexanone mixture (ol-one) via the formation of cyclohexyl hydroperoxide followed by (b) oxidation of the ol-one mixture to adipic acid by nitric acid (equation 239). 

The oxidation of cyclohexane to cyclohexanone and cyclohexanol is an important industrial procedure used in the synthesis of adipic acid. Srinivas and Mukhopadhyay (1994) reported the oxidation of cyclohexane in sc C02, yielding cyclohexanone and cyclohexanol as the major reaction products. At the high temperatures employed in this study (>137°C), cyclohexyl hydroperoxide (c-C6HnOOH), which is produced by the mechanism outlined in Scheme 4.11, decomposes to cyclohexanone and cyclohexanol. 

Scheme 4. Generation of cyclohexanone from cyclohexyl-hydroperoxide via a 1,2-H atom shift mechanism.

Generally, the issue of whether a truly solid Cr catalyst has been created for the aforementioned reactions is unresolved. This point is illustrated most clearly by all the work that has been devoted, in vain, to Cr molecular sieves (55-57). Particularly the silicates Cr-silicalite-1 and Cr-sihcahte-2 and the aluminophosphate Cr-AlPO-5 have been investigated. These materials have been employed, among others, for alcohol oxidation with t-BuOOH, for allylic (aut)oxidation of olefins, for the autoxidation of ethylbenzene and cyclohexane, and even for the catalytic decomposition of cyclohexyl hydroperoxide to give mainly cyclohexanone ... 

One-electron Fe redox catalysts may also be immobilized by incorporation into aluminophosphate frameworks. Dugal el al. (143) reported the oxidation of cyclohexane to give adipic acid with air as the oxidant in the presence of Fe-AlPO-31. This molecular sieve has narrow pores, with a 0.54-nm diameter. Cyclohexane is easily adsorbed in the micropores, but desorption of initial products such as cyclohexyl hydroperoxide or cyclohexanone is slow. Consequently, subsequent radical reactions occur until the cyclohexyl ring is broken to form linear products that are sufficiently mobile to diffuse out of the molecular sieve ... 

Cyclohexane is oxidized to a ketone/alcohol (KA) mixture at 125 to 165 °C and 8 to 15 bar. The reaction is conducted in the liquid phase with air and Mn-or Co-salts as catalysts. Cyclohexyl hydroperoxide, the primary product of this... 

In recent years increasing use has been made of an alternative procedure involving the oxidation of hydrocarbon substrates in polar solvents, usually acetic acid, in the presence of relatively large amounts of metal catalysts, usually the metal acetate. These reactions are characterized by high rates of oxidation, high conversions, and more complete oxidation of the substrate. For example, the classic autoxidation of cyclohexane is carried out to rather low conversions and affords mainly cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone. Autoxidation of cyclohexane in acetic acid, in the presence of substantial amounts of cobalt acetate catalyst, results in the selective formation of adipic acid at high conversions (see Section II.B.3.c). 

Today the main route for cyclohexanone manufacturing is liquid-phase oxidation of cyclohexane. The synthesis involves the formation of cyclohexyl-hydroperoxide, further converted to cyclohexanone, cyclohexanol and byproducts, as illustrated by the following scheme ... 

The first step, oxidation of cyclohexane to cyclohexanol and cyclohexanone, follows the general mechanism outlined by reactions 8.13 to 8.17. Trace quantities of cyclohexyl hydroperoxide 8.9 can initiate the radical chain, where the radicals 8.10 and 8.11 take part in the propagation steps. 

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