Hydroperoxide cyclohexenyl

It has been reported that molecnlar oxygen plays an important role in the allylic oxidation of olefins with TBHP (25, 26). Rothenberg and coworkers (25) proposed the formation of an alcoxy radical via one-electron transfer to hydroperoxide, Equation 4, as the initiation step of the allylic oxidation of cyclohexene in the presence of molecnlar oxygen. Then, the alcoxy radical abstracts an allylic hydrogen from the cyclohexene molecnle. Equation 5. The allylic radical (8) formed reacts with molecular oxygen to yield 2-cyclohexenyl hydroperoxide... 

To seek further evidence for a bromonium ion-mediated dioxabicyclization and to investigate the regioselectivity of ring closure, we studied reactions with 3,4-dibromocyclohexyl hydroperoxides411. We developed a synthesis of 3-cyclohexenyl hydroperoxide based on oxidation of the corresponding N-tosylhydrazine by the procedure of Caglioti et al.42). Anisole was the starting material and the full reaction sequence is shown in Eq. 29. 

Chlorophenyl)-l,l-dimethylethyl hydroperoxide, 3018 2-Cyclohexenyl hydroperoxide, 2435 1,1-Dichloroethyl hydroperoxide, 0794 3,5-Dimethyl-3-hexyl hydroperoxide, 3075 Ethyl hydroperoxide, 0925... 

When cyclohexene is oxidized with oxygen on a Co-zeolite, the major product is cyclohexenyl hydroperoxide together with minor amounts of 1,2-epoxycyclohexane and 2,3-epoxy-l-cyclohexanol [45]. However a combination of Co-zeolite with V0(acac)2 and Ho(CO)g (6/1/1) increases the conversion strongly and the epoxides dominate in the product mixture. 

Mo containing Y zeolites were also tested for cyclohexene oxidation with oxygen as oxidant and t-butyl hydroperoxide as initiator [86]. In this case the selectivity for cyclohexene oxide was maximum 50%, 2-cyclohexene-l-ol and 2-cyclohexene-l-one being the main side products. The proposed reaction scheme involves a free radical chain mechanism with intermediate formation of cyclohexenyl hydroperoxide. Coordination of the hydroperoxide to Mo + in the zeolite and oxygen transfer from the resulting complex to cyclohexene is believed to be the major step for formation of cyclohexene oxide under these conditions. 

Materials. Chemically pure solvents and reagent grade ceric ammonium nitrate were used as received. Cumene hydroperoxide was purified via the sodium salt. Lucidol tert-butyl hydroperoxide was purified by low temperature crystallization. Tetralin hydroperoxide, cyclohexenyl hydroperoxide, and 2-phenylbutyl-2-hydroperoxide were prepared by hydrocarbon oxidation and purified by the usual means. 1,1-Diphenyl-ethyl hydroperoxide and triphenylmethyl hydroperoxide were prepared from the alcohols by the acid-catalyzed reaction with hydrogen peroxide (10). 

However, the ratios of the unsaturated materials to the saturated materials and of the ketones to the alcohols (Table I) indicate that the yields of unsaturated materials are higher than those of saturated products, especially cyclohexenol. [The excess alcohol might come from 2ROO - 2RO- + 02 however, its importance in the gas phase is unknown.] It was suggested from the above that some cyclohexenol and possibly cyclo-hexenone may be formed from cyclohexenyl hydroperoxide which is produced from chain reactions initiated by the cyclohexyl peroxy radical and cyclohexenyl peroxy radical as shown below. 

We studied the oxidation of cyclohexene at 70°C in the presence of cyclopentadienylcarbonyl complexes of several transition metals. As with the acetylacetonates, the metal center was the determining factor in the product distribution. The decomposition of cyclohexenyl hydroperoxide by the metal complexes in cyclohexene gave insight into the nature of the reaction. With iron and molybdenum complexes the product profile from hydroperoxide decomposition paralleled that observed in olefin oxidation. When vanadium complexes were used, this was not the case. Variance in product distribution between the cyclopentadienylcarbonyl metal-promoted oxidations as a function of the metal center were more pronounced than with the acetylacetonates. Results are summarized in Table V. 

Table VI. Reactions of a Cyclohexene Solution of Cyclohexenyl Hydroperoxide in the Presence of Cyclopentadienylmetalcarbonyl...

When we added a cyclohexene solution of cyclohexenyl hydroperoxide, V, to catalytic quantities of [C5H5V(CO)4] at room temperature, an exothermic reaction occurred with rapid liberation of oxygen (Table VI). The major product was 2-cyclohexene-l-ol, VII. Smaller amounts of VI were formed, and only traces (< 0.2% ) of the epoxy alcohol, IX, were detected. This result was consistent with the observation of Gould and Rado concerning the decomposition of V in the presence of VO(acac)2 at 70°C. Decomposition of V in the presence of [(CsH VC ] followed the same course as with [C5H5V(CO)4] however, reaction occurred much more slowly at room temperature. 

From these results it seems that the epoxy alcohol, IX, does not arise via an intramolecular vanadium-catalyzed rearrangement of cyclohexenyl hydroperoxide, V. An alternative pathway is an intermolecular epoxida-tion reaction between the allylic hydroperoxide, V, and the allylic alcohol which is formed during the oxidation (Reaction 17). We found that IX was produced from reaction of V with VII in the presence of [C5H5V-... 

Thus, depending on the metal complex used, cyclohexene oxidation can occur via one or more of at least three major pathways, as shown in Reaction 20 path A, radical initiated decomposition of cyclohexenyl hydroperoxide path B, metal catalyzed epoxidation of the olefin and path C, metal catalyzed epoxidation of an allylic alcohol. Ugo found that path B becomes more pronounced when molybdenum complexes are used to modify the oxidation of cyclohexene in the presence of group... 

Metal Catalyzed Reactions of a Cyclohexene Solution of Cyclohexenyl Hydroperoxide, V. Solutions of cyclohexenyl hydroperoxide in cyclohexene were prepared by the methods of Gould and Rado (24) and Van Sickle et al. (41). In either case a solution approximately 0.7-0.8M in cyclohexenyl hydroperoxide is obtained (24, 41). Smaller concentrations of VI (—0.01 M), VII (0.09M), and VIII (0.06M) are also present in solution (24, 41). A solution of cyclohexenyl hydroperoxide (8.0 mmoles by iodometric titration) in 10 ml of cyclohexene was rapidly added to 0.20 mmole of the metal complex and heated with stirring under nitrogen at 70°C for 2 hrs. Metal complexes used were [C5H5Fe(CO)2]2, [C5H5Mo(CO)3]2, and [C5H5V(CO)4]. The reaction mixture was then quickly vacuum transferred at 80°C/0.01 mm. Little or no residue remained. Yields in mmoles of the products in solution were obtained by GLPC analysis of the vacuum transferred reaction mixtures. Correction was made for the amount of the product initially present (24, 41). Results are listed in Table VI. 

Reaction of Cyclohexenyl Hydroperoxide With Allylic Alcohols. Cyclohexenyl hydroperoxide, 8.0 mmoles, in 10 ml of cyclohexene (see above) was added to 0.20 mmole of [C5H5V(CO)4] in 0.50 ml of VII and stirred for 3 hrs at 70°C. Then the reaction mixture was vacuum transferred and analyzed as in previous runs. Iodometric titration of the... 

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. 

Co(II) carboxylates were well documented, and for cyclohexene as substrate the cyclohexenyl hydroperoxide is formed in situ by attack of 02 on the allylic radical produced by allylic hydrogen abstraction, Reaction 12 (4, 48). The products, usually those shown in Reaction 13, are formed via the metal-catalyzed decomposition of the hydroperoxide, and any 02 coordination at the metal is incidental there is... 

Selective decomposition of cyclohexenyl hydroperoxide to 2-cyclohexen-l-one catalyzed by chromium substituted molecular sieves... 

Chromium substituted aluminophosphate-5 is an active and recyclable catalyst for the selective decomposition of cyclohexenyl hydroperoxide to 2-cyclohexen-l-one. The product is of potential industrial interest for the synthesis of caprolactam. 

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. 

Oxygen was bubbled through 350 g of cyclohexene containing a catalytic amount of 2,2 -azobis-(2-methylpropionitrile) as initiator at 75 °C for 24 h. After evaporation of unreacted cyclohexene 65 g of a colourless viscous liquid remained. The composition of this liquid was cyclohexenyl hydroperoxide (80%), 2-cyclohexen-l-one (6%), 2-cyclohexen-l-ol (6%), 2,3-epoxycyclohexanol (5%) and cyclohexene (3%) and was used as such. 

The decompositions were carried out in a 50 ml thermostated glass flask equipped with a condenser and magnetic stirrer. Typically a solution of cyclohexenyl hydroperoxide (2 mmol), n-decane (internal standard) and catalyst (0.02 mmol metal) were stirred (1000 rpm) in 10 ml chlorobenzene at 80 °C for 5 h. The cyclohexenyl hydroperoxide conversion was determined by iodometric titration. Typically, 3.0 g of reaction mixture was diluted with 30 ml acetic acid/chloroform (2 1 v/v), 2.5 ml of saturated aqueous KI solution was added and the solution was allowed to stand for 1 h in fhe dark before adding 50 ml of deionized wafer and titration with a 0.1 M sodium thiosulfate solution. The reaction products were analysed by GC (CP Sil 5 CB column) after destroying remaining cyclohexenyl hydroperoxide by the addition of an excess of triphenylphosphine as a solution in 1,2-dichloroethane (24 g / 1). 

The as-synthesized and calcined CrAPO-5 and CrS-1 were characterized by XRD which showed that the samples were pure and had an API and MFI structure respectively. ICP analysis showed that both catalysts contained about 1 % chromium. The results observed in the decomposition of cyclohexenyl hydroperoxide over several redox active moleular sieves are presented in Table 1. CrAPO-5 and CrS-1 displayed rougly equal activity and selectivity in the decomposition of cyclohexenyl hydroperoxide. Blank reactions carried out with Silicalite-1 (S-1) and silicon incorporated Aluminophosphate-5 (SAPO-5) show low conversions confirming that the chromium was responsible for the catalysis. Other transition- metal subsituted molecular sieves showed low conversions. 

Conversion and selectivities of various redox molecular sieves in the decomposition of cyclohexenyl hydroperoxide. 

In both reactions with CrAPO-5 and CrS-1 a substantial amoimt of 2-cyclohexen-l-ol was formed. The formation of 2-cyclohexen-l-ol can be explained by a competing homolytic decomposition of the peroxide catalyzed by a CrVi species. The 2-cyclohexen-l-one, on the other hand, can be formed from the intramolecular heterolytic decomposition of the cyclohexenyl hydroperoxide over CrVi or by an intermolecular oxidation of 2-cyclohexen-l-ol by CrVi followed by a reoxidation of the formed Criv by cyclohexenyl hydroperoxide (see Figure 1). 

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