Cyclohexanol chain oxidation

Catalysis by transition metals in liquid-phase oxidation has been thor- oughly investigated. The roles of other ions have not been sufficiently studied. This paper is concerned with catalysis by hydrogen ions and some anions, in the chain oxidation of secondary alcohols such as cyclohexanol and 2-propanol. Secondary alcohols, because of their polarity, are convenient for studying ionic homolytic reactions and their role in chain oxidation. 

Negative Catalysis by JfC03 Ions in Chain Oxidation of Cyclohexanol... 

The inhibiting effect of NaHC03 on chain oxidation was established by studying the effect of ions on the oxidation of cyclohexanol. The latter was oxidized at 75°C. with AIBN as initiator (R = 6.9 X 107 mole liter"1 sec."1). To dissolve NaHC03, 9% of water was added to cyclohexanol. The rate of oxidation was measured volumetrically. 

Adipic acid may be obtained from cyclohexanol by oxidation with hot nitric acid. Cyclohexanol is prepared from phenol by hydrogenation. Thus the entire series of reactions represents (a) the change of an aromatic six-carbon compound to an alicyclic one (6) the opening of a six-carbon ring to an open chain compound (c) the cyclization of an open chain compound. The object of the present experiment is to illustrate this last step through the preparation of cyclopentanone from adipic acid. 

Let us compare the ratio of radicals in oxidized 2-propanol and cyclohexanol at different temperatures when oxidation occurs with long chains and chain initiation and termination do not influence the stationary state concentration of radicals. The values of the rate constants of the reactions of peroxyl radicals (kp) with alcohol and decomposition of the alkylhydroxy-peroxyl radical (k ) are taken from Table 7.4 and Table 7.5. 

A different situation in the oxidation of these two alcohols is seen. The hydroperoxyl radical is the main chain propagating species in oxidized 2-propanol the portion of alkylhydroxy-peroxyl radicals in this reaction is less than 2.5%. In oxidized cyclohexanol, on the contrary, the stationary state concentrations of both radicals are close and both of them take important part in chain propagation. 

In the absence of an initiator, alcohols are oxidized with self-acceleration [7-9]. As in the oxidation of hydrocarbons, the increase in the reaction rate is due to the formation of peroxides initiating the chains. The kinetics of radical formation from peroxides was studied for the oxidation of isopropyl alcohol [58] and cyclohexanol [59,60]. 

The very active unstable tin(III) ion is supposed to play an important role in this chain mechanism of tin(II) oxidation. Cyclohexane, introduced in the system Sn(II) + dioxygen, is oxidized to cyclohexanol as the result of the coupled oxidation of tin and RH. Hydroxyl radicals, which are very strong hydrogen atom acceptors, attack cyclohexane (RH) with the formation of cyclohexyl radicals that participate in the following transformations ... 

Another situation is observed when salts or transition metal complexes are added to an alcohol (primary or secondary) or alkylamine subjected to oxidation in this case, a prolonged retardation of the initiated oxidation occurs, owing to repeated chain termination. This was discovered for the first time in the study of cyclohexanol oxidation in the presence of copper salt [49]. Copper and manganese ions also exert an inhibiting effect on the initiated oxidation of 1,2-cyclohexadiene [12], aliphatic amines [19], and 1,2-disubstituted ethenes [13]. This is accounted for, first, by the dual redox nature of the peroxyl radicals H02, >C(0H)02 and >C(NHR)02 , and, second, for the ability of ions and complexes of transition metals to accept and release an electron when they are in an higher- and lower-valence state. 

Varieties of primary and secondary alcohols are selectively oxidized to aldehyde or carbonyl compounds in moderate to excellent yields as summarized in Table 3. As can be seen, /(-substituted benzyl alcohols (e.g., -Cl, -CH3, -OCH3, and -NO2) yielded > 90% of product conversion in 3-4 h of reaction time with TOP in the range of 84-155 h (entries 2-5, Table 3), Heterocyclic alcohols with sulfur- and nitrogen-containing compoimds are found to show the best catalytic yield with TOP of 1517 and 902 h for (pyrindin-2-yl)methanol and (thiophene-2-yl) methanol, respectively (entries 9 and 10, Table 3). Some of aliphatic primary alcohols (long chain alcohols) and secondary alcohols (cyclohexanol, its methyl substituted derivatives and norboman-2-ol) are also selectively oxidized by the membrane catalyst (entries 11-14 and 15-17, Table 3) with TOP values in the window of 8-... 

The 4-exo cyclization of open-chain substrates 63 proceeds in trans-fashion with moderate to excellent selectivity (trans/cis = 77/23 > 99/1 )90. The trans selectivity is dependent on the substitution pattern of R2, R3 and R4. The reactions giving trans-l-aza-2-silacyclobutanes 65 have been applied to the stereoselective syntheses of syn-amino alcohols 66 via the Tamao oxidation as exemplified by the reaction of 63d (R = HMe2Si), affording 66d (76% overall yield in 4 steps, syn/anti = > 99/1) via 65d (trans/cis = > 99/1) in equation 2690. In the case of 3-iV-disilylamino-l-cyclohexene 63f (R = HMe2Si), however, cis-l-aza-2-silacyclobutane 65f is formed exclusively, that is converted to ds-2-amino-l-cyclohexanol (66f) (equation 27)90. 

The majority of studies on oxidation reactions in scC02 have involved catalyzed processes promoted by molecular oxygen, in which the role of the catalyst is to generate free radicals that will react with the chemical oxidant, leading to a product distribution that is typical of an unselechve chain process. Among these can be mentioned the oxidation of cyclohexane to cyclohexanol and cyclohexanone (Scheme 2.2) as an intermediate step in the production of the adipic acid that is a key component in the production of Nylon 6,6 polyamide [52-54],... 

Iron(III) weso-tetraphenylporphyrin chloride [Fe(TPP)Cl] will induce the autoxidation of cyclohexene at atmospheric pressure and room temperature via a free radical chain process.210 The iron-bridged dimer [Fe(TPP)]2 0 is apparently the catalytic species since it is formed rapidly from Fe(TPP)Cl after the 2-3 hr induction period. In a separate study, cyclohexene hydroperoxide was found to be catalytically decomposed by Fe(TPP)Cl to cyclohexanol, cyclohexanone, and cyclohexene oxide in yields comparable to those obtained in the direct autoxidation of cyclohexene. However, [Fe(TPP)] 20 is not formed in the hydroperoxide reaction. Furthermore, the catalytic decomposition of the hydroperoxide by Fe(TPP)Cl did not initiate the autoxidation of cyclohexene since the autoxidation still had a 2-3 hr induction period. Inhibitors such as 4-tert-butylcatechol quenched the autoxidation but had no effect on the decom-... 

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. 

The 0x0 species abstracts a hydrogen from cyclohexane to form Fe" -OH and c clohexyl radical which rebounds at rates as high as lO /sec to form cyclohexanol and Fe porphyrin is regenerated". Even though free radicals are involved, the oxidation is not a chain reaction and it does not involve alkyl peroxide radicals as in the commercial processes described earlier. 

Oxidation of cyclohexane with Oj can be described by a series of elementary reactions similar to the ones for n-butane. In this case, however, the hydrocarbon contains no primary hydrogens and three times as many secondary hydrogens as the acyclic alkane. Chain lengths are, therefore, longer and reaction products less numerous. Cyclohexyl hydroperoxide and cyclohexanol are the major propagation products while cyclohexanone and cyclohexanol are formed in termination ... 

Water-soluble, cyclic alcohols having secondary hydroxyl groups may be successfully oxidized to the corresponding ketones in neutral or weakly acidic solutions. In alkaline solution, further oxidation readily takes place, with rupture of the chain in excess alkali, carbon dioxide is produced. Secondary alcohols that are insoluble in water can readily be oxidized in heptane thus, cyclohexanol and its derivatives can be rapidly converted, in high yield, to the corresponding ketones. ... 

Analogous results were obtained on oxidation of benzyl alcohol, isopropanol, and cyclohexanol with CBT [68JCS(CC)1305 69JCS(C)1474], Methylphenylcarbinol and diphenylcarbinol under these conditions form acetophenone and benzophenone, respectively. Oxidation of alcohols in organic solvents is regarded as a radical chain process in which chlorine is the chain carrier [69JCS(C)1474] (Scheme 81). 

TBA-I catalyzed the oxidation of cyclohexane with 1 atm molecular oxygen at 365 K. The main products wa e cyclohexanol and cyclohexanone and an induction period was observed. The selectivities changed little with time. A small amount of dicyclohexyl, which is formed by the reaction of two cyclohexyl radicals, was observed. Neither acids nor oxoesters were observed. The induction period and the formation of dicyclohexyl suggest that the reaction involves a radical-chain autoxidation mechanism. The... 

Oxidation of cyclohexanol with a mixture of 02 and 03 at 80—100° C proceeds by a chain mechanism [70], The rate of free radical formation is 1000 times lower than that of ozone consumption and the activation energy for chain initiation by ozone is 11 kcal mole-1. The cyclohexanone formed is oxidized by ozone without formation of free radicals. 

Hydroxyperoxy radicals can induce both oxidation and reduction. If the inhibitor is present in two states, oxidized and reduced, and each state reacts with hydroxyperoxy radicals only, terminating the chains, then negative catalysis will take place, each inhibitor molecule terminating chains an infinite number of times. This is the case on addition of CuS04 to cyclohexanol [83]. Cupric ions in a concentration of 10-smolel I virtually stop the initiated oxidation of cyclohexanol. The mechanism of the retarding action of cupric ions is... 

Similar results were obtained when transition metal stearates were added to cyclohexanol (Table 5). The dioxymine complexes of Co, Cu and Fe retard oxidation of 2-propanol [274] by termination of chains. The rate of termination obeys the equation... 

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