Cyclohexene autoxidation

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] ... 

The mechanistic details of the subsequent chemistry are unclear, but it is apparent that the catalytic decomposition of hydroperoxide follows a chain process that resembles a Haber-Weiss pathway. It has also been found that complexes show a greater activity for cyclohexene autoxidation that do complexes, but no rationale has been provided to explain this observation. 

The Pacman catalyst selectively oxidized a broad range of organic substrates including sulfides to the corresponding sulfoxides and olefins to epoxides and ketones. However, cyclohexene gave a typical autoxidation product distribution yielding the allylic oxidation products 2-cyclohexene-l-ol (12%) and 2-cyclohexene-1-one (73%) and the epoxide with 15% yield [115]. 

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. 

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.

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. 

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. 

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. 

Epoxides can also be formed from the oxidation of alkenes by molecular oxygen via in situ generation of hydroperoxides by autoxidation.251,252 An interesting example is the direct stereoselective oxidation of cyclohexene by 02 to syn-l,2-epoxycyclohexan-3-ol catalyzed by CpV(CO)4 with a 65% yield and 99% stereoselectivity (equation 78).253... 

In comparison with metal porphyrins, the corresponding metal phthalocyanines are much more stable against oxidative decomposition. Murahashi et al. reported that chlorinated Fe(II) phthalocyanine is particularly well suited for aerobic allylic oxidation employing acetic aldehyde as a cofactor (Scheme 3.27) [118]. Under these conditions, cyclohexene la is converted to a mixture of 2a and 3a in 70% overall yield and the epoxide 4a as byproduct (30%). Acetic aldehyde is proposed to autoxidize by... 

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-... 

An oxygen activation mechanism is favored by Ochiai for the autoxidation of cyclohexene catalyzed by copper phthalocyanine.197a,b Kamiya also proposed that the CuPc—02 complex initiates the autoxidation of a-methylstyrene by an addition mechanism196 ... 

Two research groups496 497 have recently studied the autoxidation of cyclohexene at 60° to 65°C in the presence of a mixture of a low-valent Group VIII metal complex, e.g., RhCl(Ph3P)3 or (Ph3P)2Pt02, and an epoxidation catalyst (molybdenum complexes). Cyclohexen-l-ol and cyclohexene oxide are formed in roughly equimolar amounts. The results could be explained by a scheme involving two successive catalytic processes ... 

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. 

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). 

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. 

What are the main products of metal-catalyzed autoxidation of methyl cyclohex-2-ene Why is cyclohexene more susceptible to autoxidation than cyclohexane ... 

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... 

Small but significant effects of solvent polarity were found in the autoxidation of a variety of alkenes and aralkyl hydrocarbons [216-220] (styrene [216, 218, 219], ethyl methyl ketone [217], cyclohexene [218], cumene [218, 219], tetralin [219], etc.). An extensive study on solvent effects in the azobisisobutyronitrile (AIBN)-initiated oxidation of tetralin in a great variety of solvents and binary solvent mixtures was made by Kamiya et al. [220],... 

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. 

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