Oxidation of cyclohexene catalyzed

Suresh, Lee and coworkers demonstrated oxidation of cyclohexene catalyzed by Mn or Cu complexes using H202 in aqueous phase in a microreador (width = 200 pm and depth = 50 pm) [36], Water-soluble ionic liquid [bmim]BF4 was added (0.5%, v/v) to improve the solubility of cydohexene in the readionbuffer. With the use of a reduced Schiff base-Cu complex, 2-hydroxycyclohexanone was obtained as the maj or produd with 2 5 min residence time, whereas the bulk scale reaction gave 2-cyclohexenol as the major reaction produd. 

Synergistic elfects have been discussed for the oxidation of cyclohexene catalyzed by Cr-Mn complexes immobilized on silica. 

James and Ochiai [472,473] studied the oxidation of a cyclooctene rhodium(I) complex and found infrared evidence for cyclooctene hydroperoxide as an intermediate. These authors postulated oxygen transfer directly to the olefin within the co-ordination sphere, equation (278), and suggested that such a scheme may account for the formation of 2-cyclohexene-l-one during oxidation of cyclohexene catalyzed by Rh(I) complexes. 

Table 12.1. Enantioselective Copper-Catalyzed Allylic Oxidation of Cyclohexene...

In 1989, Isobe and coworkers reported on an organometallic polyoxometalate cluster [(Rhcp )4V60i9] (cp = /j -CsMcs) that catalyzes the oxidation of cyclohexene with TBHP as oxidant to give mainly ally lie oxidation products (l-ferf-butylperoxycyclohex-2-ene 42% and cyclohex-2-en-l-one 21%) and only little epoxide (15%) (equation 62). The yield of 1 -ferf-butylperoxy cyclohex-2-ene increased with decreasing molar ratio of cyclohexene to TBHP, while the yield of cyclohex-2-en-l-one has a maximum at the ratio of 0.2. 

Table III. Oxidation of Cyclohexene by H2O2 Catalyzed by TMSP...

Fig. 25. ATR spectra recorded during epoxidation of cyclohexene catalyzed by a Ti-Si aerogel with TBHP as the oxidant under the influence of forced modulation of the cyclohexene concentration (a) time-resolved spectra (reference recorded before modulation) (b) difference spectra obtained by subtracting one (arbitrarily chosen) spectrum (c) phase-resolved (demodulated) spectra. The data set for the spectra in (a)-(c) is the same (SO).

Cyclohexene oxidation in the presence of the molybdenum complex, [C5Hr)Mo(CO)3]2, gave two major products at low conversion VI and VII nearly 1 1 mole ratio, Table V. The ketone, VIII, was formed in very low yield in contrast to oxidations using the iron complex. This reaction is far more selective than the oxidation of cyclohexene in the presence of Mo02(acac)2 reported by Gould and Rado (24). When a cyclohexene solution of V was exposed to [CsHsMk COJs] at 70°C, VI and VII were formed in approximately equimolar amounts (Table VI). These data show that the molybdenum complex efficiently catalyzes the epoxidation of cyclohexene by V before the allylic hydroperoxide decomposes substantially. Reaction 16 represents the predominant course of cyclohexene oxidation in the presence of cyclopentadienyltricarbonyl molybdenum dimer. 

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... 

The results confirm the previously reported low reactivity of cyclohexene when the catalyst is TS-1 and indicate that Ti-beta is active for the oxidation of cyclohexene and other bulky olefins. However, for cyclohexene and the linear olefins, the major reaction products formed in the presence of Ti-beta are glycols and glycol ethers, whereas in the presence of TS-I, epoxides are predominantly formed. Also in this case, the epoxides initially formed in the presence of Ti-beta undergo secondary reactions catalyzed by the acidic centers associated with the aluminum in the material, as previously seen for allyl alcohol and for the epoxidation of 1-butene on aluminum-containing TS-1 (Bellussi et al., 1991a). A different product composition was observed for cyclododecene,... 

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... 

Gold-catalyzed oxidation of styrene was firstly reported by Choudhary and coworkers for Au NPs supported on metal oxides in the presence of an excess amount of radical initiator, t-butyl hydroperoxide (TBHP), to afford styrene oxide, while benzaldehyde and benzoic acid were formed in the presence of supports without Au NPs [199]. Subsequently, Hutchings and coworkers demonstrated the selective oxidation of cyclohexene over Au/C with a catalytic amount of TBHP to yield cyclohexene oxide with a selectivity of 50% and cyclohexenone (26%) as a by-product [2]. Product selectivity was significantly changed by solvents. Cyclohexene oxide was obtained as a major product with a selectivity of 50% in 1,2,3,5-tetramethylbenzene while cyclohexenone and cyclohexenol were formed with selectivities of 35 and 25%, respectively, in toluene. A promoting effect of Bi addition to Au was also reported for the epoxidation of cyclooctene under solvent-free conditions. 

It has recently been reported495 that the complex CsH5V(CO)4 (CSHS = cy-clopentadienyl) is an efficient catalyst for the stereoselective oxidation of cyclohexene to ris-l,2-epoxycyclohexane-3-ol in good yield (65% at 10% conversion). This high stereoselectivity is reminiscent of the highly selective vanadium-catalyzed epoxidations of allylic alcohols with alkyl hydroperoxides discussed earlier. The mechanism of reaction,... 

As mentioned above, catalytic oxidation of olefins via coordination catalysis with an intermediate such as LnM (olefin) 02 seemed an attractive possibility, and Collman s group (45) tentatively invoked such catalysis in the 02-oxidation of cyclohexene to mainly 2-cyclo-hexene-1-one promoted by IrI(CO)(PPh3)2, a complex known to form a dioxygen adduct. Soon afterwards (4, 46, 47) such oxidations involving d8 systems generally were shown to exhibit the characteristics of a radical chain process, initiated by decomposition of hydroperoxides via a Haber-Weiss mechanism, for example Reactions 10 and 11. Such oxidations catalyzed by transition-metal salts such as... 

Tabushi and Koga reported the use of manganese porphyrins to catalyze the 02-oxidation of cyclohexene to cyclohexanol and cyclohexene-ol in the presence of borohydride these workers suggest that an equilibrium such as depicted in Reaction 32 is involved in non free-radical pathways (112). 

Thus, the isotope effect for the allylic oxidation of cyclohexene by cytochrome P 450 is about 5 and is the same for the reconstituted, NADPH-dependent and the peroxide-dependent paths. This similarity suggests that although product ratios may change from one oxygen donor to another, the mechanism of oxygen transfer may be invariant. Efforts to develop a clearer understanding of the relationships between the 02-dependent and peroxide-dependent pathways for oxygen transfer catalyzed by cytochrome P 450 are currently underway. 

Photooxidation of cycloalkenes. The Fe( I,-catalyzed photooxidation of cycloal-kenes in pyridine can lead to three types of products, depending on the substitution pattern of the cycloalkenes. The three general reactions are shown in equations (l)-(lll) for the oxidation of cyclohexenes. 

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