Epoxidation of cyclohexene, with TBHP

Influence ofsilynation on epoxidation of cyclohexene with TBHP over Ti-SBA-15... 

Table 11.7 Epoxidation of cyclohexene with TBHP (tert-butyl hydroperoxide) using selected species [31] .

Zrrconium(IV) and hafnium(IV) complexes have also been employed as catalysts for the epoxidation of olefins. The general trend is that with TBHP as oxidant, lower yields of the epoxides are obtained compared to titanium(IV) catalyst and therefore these catalysts will not be discussed iu detail. For example, zirconium(IV) alkoxide catalyzes the epoxidation of cyclohexene with TBHP yielding less than 10% of cyclohexene oxide but 60% of (fert-butylperoxo)cyclohexene °. The zirconium and hafnium alkoxides iu combiuatiou with dicyclohexyltartramide and TBHP have been reported by Yamaguchi and coworkers to catalyze the asymmetric epoxidation of homoallylic alcohols . The most active one was the zirconium catalyst (equation 43), giving the corresponding epoxides in yields of 4-38% and enantiomeric excesses of <5-77%. This catalyst showed the same sense of asymmetric induction as titanium. Also, polymer-attached zirconocene and hafnocene chlorides (polymer-Cp2MCl2, polymer-CpMCls M = Zr, Hf) have been developed and investigated for their catalytic activity in the epoxidation of cyclohexene with TBHP as oxidant, which turned out to be lower than that of the immobilized titanocene chlorides . ... 

Figure 8. Effect of the degree of trimethylsilylation on Ti-MCM-41 on the initial reaction rate for catalytic epoxidation of cyclohexene with TBHP.

Figure 12. Initial reaction rate and selectivity to epoxide during epoxidation of cyclohexene with TBHP on silylated Ti-MCM-41 catalysts.

In another approach, benzimidazole-fimctionalized dendrons were used as supports for molybdenum species [59]. Metal loading was carried out by treatment with Mo(CO)e or Mo02(acac)2 and the dendritic complexes were used as catalysts for the epoxidation of cyclohexene with TBHP. Reactions were shown to be heterogeneously catalyzed and recyclability of the catalysts was demonstrated. 

Figure 5. Epoxidation of cyclohexene with TBHP using CPI-DAT.Mo and homogeneous Mo.

The first indication of selective substitution reactions came from experiments with 1 and 2 as catalysts for alkene epoxidation. NMR experiments have shown that both compounds catalyze the epoxidation of cyclohexene with t-butyl hydroperoxide (TBHP). The catalytic activity is comparable to that of the model compound Hex7Si70i2Ti(0 Pr). " The measured turn over numbers indicate that all four Ti centers are involved in the catalytic process. The catalysts could be recovered quantitatively, a proof of core-functionalization and for the core stability during many catalytic cycles. A more detailed catalytic study has recently been performed with the cubic titanasiloxane [(2,6- Pr2C6H3) (Me3Si)NSi]40i2[Ti0 Bu]4 (12). This compound was prepared by the reaction of 9 with t-butanol and catalyzes the epoxidation of cyclohexene with TBHP. The titanium butylperoxo intermediate could be isolated after a stoichiometric reaction with TBHP. This intermediate then reacted with cyclohexene to produce cyclohexene oxide. A schematic representation of the catalytic process is given in Figure 28.4. 

Figure 6.36 Epoxidation of cyclohexene with tBHP catalysed by Mo(VI) immobilised on polystyrene resin functionalised with a 2-aminomethylpyridine ligand.

Table 4.2 Catalytic epoxidation of cyclohexene with anhydrous TBHP (tert-butylhydroperoxide).

Like V(V), Nb(V) as well as Ta(V) alkoxides do catalyze the epoxidation of alkenes with TBHP as catalyst, but reaction times are long and yields are low due to side reactions (e.g. formation of (ferf-butylperoxo)cyclohexene as major product from cyclohexene °). Grubbs and coworkers and Sala-Pala and coworkers could show that free and polymer-supported Cp2NbCl2 in the presence of hydrogen peroxide shows low or no catalytic activity for the epoxidation of alkenes (with cyclohexene only 36% epoxide selectivity). 

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. 

The catalytic activity of the trimethylsilylated Ti-MCM-41 samples for epoxidation of cyclohexene with tercbutylhydioperoxide (TBHP) is reported in figures 7a and 7b. 

Previous studies indicated that the structure of the alkyl hydroperoxide in molybdenum catalyzed epoxidations has only a minor effect on the rate and selectivity [10]. Hence, we were initially surprised to observe that PHP failed to give the expected epoxidation of cyclohexene (1) and limonene (2) in the presence of a molybdenum catalyst (Tablel). Epoxidation of limonene with TBHP as oxidant, in contrast, gave the epoxide of the more highly substituted double bond in 84% selectivity, consistent with nucleophilic attack of the olefin on the alkylperoxomolybdenum(Vl) [3,5]. We tentatively concluded that this low reactivity of PHP is a result of steric hindrance in the putative alkylperoxomolybdenum(VI) intermediate. This prompted us to carry out a systematic investigation [8] of steric effects of the alkyl substituents in the alkyl hydroperoxide on the rate of molybdenum catalyzed epoxidations. 

Polyimide particulates carrying a functional group have been prepared by non-aqueous suspension polycondensation. Molybdenum(VI) complex has been supported on a functional polyimide bead and used as a catalyst in the liquid-phase epoxidation of cyclohexene with tert-butylhydroperoxide (TBHP), as oxygen source. The polyimide-supported Mo catalyst was highly active and selective, and has been recycled 10 times vrith no detectable loss of Mo from the support. 

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).

The catalytic activity of the silylated and the non-silylated Ti-MCM-41 materials was tested in epoxidation of cyclohexene using tercbutylhydroperoxide (TBHP) as oxidant. In a typical catalytic run 56 mmol of olefin were mixed with 14 mmol of TBHP (olefin/TBHP ratio = 4) at the reaction temperature, 60°C. Under these reactions conditions the water content was 2 wt.%. Then, 30 mg of catalyst (0.5wt% catalyst) were added to the reaction medium. This instant was taken as time zero of reaction, and aliquots of the reaction media were withdrawn at different reaction times and subsequently analyzed by Gas Chromatography using a 5 % phenylsilicone column (HP-5) of 25 meters length. 

This cubic silicon-titanium g-oxo complex was also shown to be a model system for insoluble titanosilicates and related catalysts which have been used for epoxidation reactions of olefins. The cubic silicon-titanium g-oxo complex, when immobilized on a silica matrix by dissolving it in tetraethoxysilane and further treatment with acetic anhydride followed by heating up to 60 °C for 20 h, resulted in the formation of a Si02—Ti02 mixed oxide (63). The resulting solid was separated by filtration, and the filtrate formed a gel in about 1 week. This gel showed an enhanced catalytic activity (epoxidation yield of cyclohexene was 72%) as a solid catalysf for epoxidation of cyclohexene in the presence of TBHP in the liquid phase. 

Suspension polycondensation of pyromellitic dianhydride and 3,5-diamino-1,2,4-triazole yielded triazole-containing polyimide beads that were used as a support for Mo02(acac)2 [60]. The resulting catalyst showed high activity and selectivity in the epoxidation of cyclohexene and cycloctene as well as in the epoxidation of noncyclic alkenes such as styrene, 1-octene, and 1-decene with TBHP. The catalyst could be recycled 10 times and activity decreased significantly in the case of 1-octene epoxidation whereas activity remained high in the epoxidation of cyclic alkenes. 

Molybdenum-catalyzed epoxidations of cyclohexene under pseudo first-order reaction conditions showed the highest rate, of the investigated tertiary alkyl hydroperoxides (Figure 1), with TBHP. 

Similar results were observed with vanadium catalyzed epoxidation of cyclohexene (1) and limonene (2) with TBHP and PHP (Table 2). Epoxidations with TBHP gave reasonable conversions and... 

The catalytic tests were done on calcined and silylated Ti-MCM-48 materials. The reaction of epoxidation of cyclohexene was tested using tert-butylhydroperoxide (TBHP) as oxidant with olefin/TBHP ratio = 4 at a reaction temperature of 60 C. The reaction was monitored by Gas Chromatography using a 5 % phenylsilicone column (HP-5) of 25 meters length. 

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