Cyclohexene autoxidation reaction

Tables I and II include data for the co-oxidations of styrene and butadiene in chlorobenzene and ferf-butylbenzene solutions, as well as with no added solvent. These solvents were chosen because the rate of oxidation of cyclohexene varies significantly in them at the the same rate of initiation (6). There is a variation in the over-all rate of oxidation under these solvent conditions, but there appears to be no significant difference in the measured ra and rb (Table II). If the solvent does affect the propagation reaction in autoxidation reactions, it affects the competing steps to the same degree.

When a slow steady-state autoxidation of a suitable hydrocarbon is disturbed by adding either a small amount of inhibitor or initiatory a new stationary state is established in a short time. The change in velocity during the non-steady state can be followed with sensitive manometric apparatus. With the aid of integrated equations describing the nonsteady state the individual rate constants of the autoxidation reaction can be derived from the results. Scope and limitations of this method are discussed. Results obtained for cumene, cyclohexene, and Tetralin agree with literature data. 

Metalloporphyrins catalyze the autoxidation of olefins, and with cyclohexene at least, the reaction to ketone, alcohol, and epoxide products goes via a hydroperoxide intermediate (129,130). Porphyrins of Fe(II) and Co(II), the known 02 carriers, can be used, but those of Co(III) seem most effective and no induction periods are observed then (130). ESR data suggest an intermediate cation radical of cyclohexene formed via interaction of the olefin with the Co(III) porphyrin this then implies possible catalysis via olefin activation rather than 02 activation. A Mn(II) porphyrin has been shown to complex with tetracyanoethylene with charge transfer to the substrate (131), and we have shown that a Ru(II) porphyrin complexes with ethylene (8). Metalloporphyrins remain as attractive catalysts via such substrate activation, and epoxidation of squalene with no concomitant allylic oxidation has been noted and is thought to proceed via such a mechanism (130). Phthalocyanine complexes also have been used to catalyze autoxidation reactions (69). 

We have demonstrated recently that epoxidation and hydroxyl-ation can be achieved with simple iron-porphine catalysts with iodosylbenzene as the oxidant (24). Cyclohexene can be oxidized with iodosylbenzene in the presence of catalytic amounts of Fe(III)TPP-Cl to give cyclohexene oxide and cyclohexenol in 55% and 15% yields, respectively. Likewise, cyclohexane is converted to cyclohexanol under these conditions. Significantly, the alcohols were not oxidized rapidly to ketones under these conditions, a selectivity shared with the enzymic hydroxylations. The distribution of products observed here, particularly the preponderance of epoxide and the lack of ketones, is distinctly different from that observed in an autoxidation reaction or in typical reactions of reagents such as chromates or permanganates (15). 

Cyclohexenone, the major product of cyclohexene autoxidation was reduced to cyclohexanol and cyclohexenol in a ratio of 1 1.4 under the oxidation condition without oxygen, while cyclohexenol was not reduced appreciably. Thus, the reduction of cyclohexenone during the oxidation can account for only a part of the cyclohexanol formation in the TPPMn-NaBH4-02 reaction, and a much more important source of cyclohexanol seems to be cyclohexene oxide this is leased on the following observations (see Figure 11) ... 

In 1939, Criegee made an important contribution by showing that the primary product of cyclohexene autoxidation is the allylic hydroperoxide [2]. The classical textbook on radical pathway for the thermal autoxidation of a general hydrocarbon RH is summarized in reactions (1.1 -1.5) [3 - 5] ... 

Air oxidation of JQ in cyclohexene gives remarkably high yields (>95% ( )) of a ca. 10 1 mixture of 15 and 16 ( ). Obtaining the same oxidation products in similar ratio in the autoxidation reaction as in the self-decomposition of 9 suggests that 9 is the precursor of 15 and 16, although it is not clear why 9 is not intercepted by oxygen under these conditions. " In acetonitrile, although 15 and 16 are still found in low yield, the principal product is the diazenium salt 11, with unknown counterion. Much remains to be learned" about this unusual autoxidation reaction (30). 

The presence of ascorbic acid as a co-substrate enhanced the rate of the Ru(EDTA)-catalyzed autoxidation in the order cyclohexane < cyclohexanol < cyclohexene (148). The reactions were always first-order in [H2A]. It was concluded that these reactions occur via a Ru(EDTA)(H2A)(S)(02) adduct, in which ascorbic acid promotes the cleavage of the 02 unit and, as a consequence, O-transfer to the substrate. While the model seems to be consistent with the experimental observations, it leaves open some very intriguing questions. According to earlier results from the same laboratory (24,25), the Ru(EDTA) catalyzed autoxidation of ascorbic acid occurs at a comparable or even a faster rate than the reactions listed in Table III. It follows, that the interference from this side reaction should not be neglected in the detailed kinetic model, in particular because ascorbic acid may be completely consumed before the oxidation of the other substrate takes place. 

Present developments. One might think that an established reaction such as aerobic oxidation (or autoxidation) is not the subject of further research and improvement, but this is definitely not the case and both new homogeneous and heterogeneous catalysts are in development. In the introduction we already mentioned the drawbacks of oxidation of cyclohexene to adipic acid and several researchers address this challenge. Also a highly developed reaction such as the oxidation of paraxylene is subject to further improvements. 

The observed half life at 100°C. of 23 hours for a dilute solution of hydroperoxide in benzene indicates that significant decomposition may occur in the autoxidation of butene, depending on reaction conditions. No reliable evaluation can be made because of the known complications introduced on hydroperoxide decomposition by the effect of the solvent, the hydroperoxide concentration (2), the presence of oxygen (12), and the possibility of a strong acceleration in rate in the presence of oxidizing olefin, observed in at least one system (8). However, using the data reported by Bateman for a benzene solvent at 100 °C. in the presence of air (2), l-butene-3-hydroperoxide decomposes 13 times faster than cyclohexene hydroperoxide, a product which may be formed in extremely high yield by the oxidation of cyclohexene. 

Autoxidation without Discharge. To compare our results with normal autoxidation, the reaction was carried out using a reaction mixture similar to Run 4 without silent discharge. Low conversion of cyclohexene (0.051% ) was observed at 60°C., indicating that the discharge oxidation was hardly affected by the normal autoxidation process under the present reaction conditions. The major product was 3-cyclohexenylhydroperoxide, and minor products were 3-cyclohexenol, 3-cyclohexenone, cyclohexene oxide, and trace amounts of residue saturated materials such as cyclo-hexanol and cyclohexanone were not detected. The conversion of cyclohexene was raised to 0.15% when the reaction temperature was elevated to 140°C. however, the kinds of product were not changed. 

Alcohols may be oxidized in a similar way. However, these reactions strongly resemble those reported for Cr molecular sieves, and a small concentration of Cr in solution may well account for most of the observations of catalysis. Binary molybdenum-chromium oxides supported on alumina have been used in the autoxidation of cyclohexene with 02 and r-BuOOH as an initiator (62). This is a complex reaction in which uncatalyzed and Cr-catalyzed oxidation combine to yield 2-cyclohexen-l-one, 2-cyclohexen-l-ol, and 2-cyclohexenyl hydroperoxide the Mo compound can use the hydroperoxide formed in situ as an oxidant for the epoxidation of cyclohexene. Although much lower oxygen consumption was observed for the reaction filtrate than for the suspension, it is unclear how the Cr is held by the oxide. 

Due to the possibility of chain initiation by direct reaction of a metal-dioxygen complex with substrate, many of these complexes have been examined as autoxi-dation catalysts, particularly for the oxidation of olefins.136 139-141 172-179 Thus, Collman et al.172 reported that dioxygen complexes of Ir(I), Rh(I), and Pt(0) catalyzed the autoxidation of cyclohexene at 25° to 60°C in benzene or methylene chloride. Cyclohexene-3-one is the major product (together with water) and cyclohexene oxide a minor product ... 

More recent investigations have shown that these reactions involve metal-catalyzed decomposition of hydroperoxides via the usual redox cycles. Thus, inhibition, polymerization and product studies in the RhCl(Ph3 P)3-catalyzed autoxidation of cyclohexene,136 ethylbenzene,136 and diphenylmethane137 were compatible only with metal-catalyzed decomposition of the alkyl hydroperoxide and not a direct reaction of the metal-dioxygen complex with substrate. Complexes Rh(III) (acac)3, Rh(III) (2-ethylhexanoate)3, and Co(II) (2-ethylhexanoate)2, gave results that were almost the same as those obtained with RhCl(Ph3P)3. The redox cycle may involve Rh(II) and Rh(III) ... 

Further evidence against initiation by direct oxygen activation in the oxidation of olefins is provided by the following two observations.185 First, no reaction was observed between olefins (e.g., cyclohexene, 1-octene, and styrene) and metal-dioxygen complexes, such as I, II, and V, when they were heated in an inert atmosphere (nitrogen). Second, no catalysis was observed with these metal complexes in the autoxidation of olefins, such as styrene, that cannot form hydroperoxides. 

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-... 

Typical autoxidation catalysts, such as Co, Mn, Cu and Fe, afforded mainly cyclohexen-l-ol and cyclohexen-l-one. Other epoxidation catalysts, such as Mo or V, afforded mainly cyclohexene oxide and cyclohexen-l-ol (see Section III.B.2). It was concluded that the atomic number and the oxidation state of the transition metal are more important than the detailed catalyst structure in determining the course of the reaction. 

Reaction Pathway and Products. In the presence of a catalytic amount of a tetraphenylporphyrin (TPP) Mn(III) complex (34) and sodium borohydride, treatment of cyclohexene with excess oxygen (air) in benzene-ethanol leads effectively to cyclohexanol and cy-clohexenol. The reaction is quite different from the known autoxidation catalyzed by TPP Mn(III) in the absence of NaBH4 (Figure 7). The most significant characteristics of the present TPPMn-NaBH4-02 reaction compared with the autoxidation are ... 

Figure 7. TPP Mn(III)-catalyzed autoxidation of cyclohexene with NaBH4, Reaction 1 without NaBH4, Reaction 2...

The latter observation is interesting, since coupled with the results of the radical inhibition, this suggests that there are two direct oxidation mechanisms as well as the autoxidation that lead to the products of the TPP Mn-NaBH4-02 reaction a direct oxidation via a free-radical intermediate, but not through the autoxidation in which cyclohexenol formation is inhibited by 2,6-di-t-butyl-p-cresol and another direct uninhibited oxidation that leads to cyclohexene oxide. 

Zeolites have been used as (acid) catalysts in hydration/dehydration reactions. A pertinent example is the Asahi process for the hydration of cyclohexene to cyclo-hexanol over a high silica (Si/Al>20), H-ZSM-5 type catalyst [57]. This process has been operated successfully on a 60000 tpa scale since 1990, although many problems still remain [57] mainly due to catalyst deactivation. The hydration of cyclohexanene is a key step in an alternative route to cyclohexanone (and phenol) from benzene (see Fig. 2.19). The conventional route involves hydrogenation to cyclohexane followed by autoxidation to a mixture of cyclohexanol and... 

Autoxidation, the oxidation of organic compounds by air, normally occurs via a radical chain mechanism. For example, cyclohexene undergoes allylic CH abstraction by an initiator, and the resulting cyclohexenyl radical reacts with O2 to give the corresponding hydroperoxy radical that abstracts an H from cyclohexene. In this case the final product is the allylic hydroperoxide. Conversion of ethers to the hydroperoxides is another familiar example. The conversion of cumene to phenol and acetone is a commercial application of the reaction (equation 9). 

Vanadyl salen complexes epoxidized cyclohexene presumably through intermediate hydroperoxides formed by radical chain autoxidation <1997105927, 1998TL5923>. Reactions using vanadium complexes have been carried out in liquid carbon dioxide <19990M4916>. 

Another approach to achieve higher conversions is to start from cyclohexene, which is much more reactive than cyclohexane towards autoxidation [6], and can be prepared by hydrogenation of benzene over a ruthenium catalyst [7]. The higher reactivity of cyclohexene also allows for lower reaction temperatures thus further limiting overoxidation. The 2-cyclohexen-l-one product formed by decomposition of cyclohexenyl hydroperoxide can subsequently be hydrogenated to cyclohexanone. The net reaction stoichiometry is the same as the current process. We now report our results on the use of CrAPO-5, CrS-1 and other transition-metal substituted molecular sieves for the decomposition of cyclohexenyl hydroperoxide. 

The oxidation of cyclohexene using hydrogen peroxide was chosen as a test reaction for the catalytic evaluation of the titanium modified hexagonal NaY sanq)les. Scheme I illustrates some of the typical products of cyclohexene oxidation. The epoxide and the diol which is a hydrolysis product of the epoxide, generally reflect a concerted process. In contrast the allylic alcohol and ketone are often ascribed to an autoxidation or radical process. We anticipated that some homolytic decomposition of the peroxide may be observed with these acidic zeolites. In fact, there was -74% conversion of H2O2 over calcined hexagonal NaY after heating at 55 C for 24 hours. This resulted in only a 1% conversion of... 

Summing up, we may conclude that insufficient evidence exists in the literature on the real participation of N-bound peroxynitrite intermediates, although with the recognition that a recent proposal provides experimental and theoretical support for the intermediacy of such a species in the oxidation of triphenylphosphine and cyclohexene by 02, catalyzed by Naflon-bound six-coordinate (nitro) cobalt porphyrin complexes.99 In this context, the results and mechanistic interpretations on the autoxidation of Fe(CN)5NO]3 (Equation 7.23)58 may be highlighted (Section 7.4.4). In fact, the above commented ambiguity on the possible rate-limiting NO dissociation is absent for this reaction, because k NO is 10-fold slower, and the measured oxidation rate is comparatively very fast. The stoichiometry and the DFT-calculated results on the structure of the N-bound peroxynitrite intermediate are valuable points in the proposed mechanism. It should be remarked that the NO+ -bound product is equivalent to N02 that is, no isomerization to N03 has been possible because of the higher competitive reactivity of [Feln(CN)5N(0)00]3 with the initial reactant, [Fe(CN)5NO]3 (Equation 7.25). 

The major product from the former is cyclohexen-3-one, along with minor amounts of cyclohexene oxide (17). Epoxide formation has also been identified as a minor product from cyclopentene autoxidation. The intermediacy of 3-cyclohexene hydroperoxide was proposed in this report but not verified/ Subsequent work on the autoxidation of cyclohexene using RhCl(PPh3)3 verified this premise/ and a number of review articles have emphasized this conclusion.The involvement of preformed hydroperoxide has been verified by comparing the rate of cyclohexene oxidation both with hydroperoxide present, and also when the cyclohexene is purified free from peroxide. In the former case the reaction is rapid and there is no induction period. Under conditions where the cyclohexene is peroxide free the reaction proceeds more slowly, and there is an induction period of close to three hours (using IrCl(CO)(PPh3)2 as catalyst) as the hydroperoxide intermediate is being performed (18) ... 

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