Epoxidation of cyclohexenes

Fig. 7. The effect of preparation on the pore size distribution (a), titanium dispersion (b), and the activity for epoxidation of cyclohexene (c) of titania—siUca containing 10 wt % titania and calcined in air at 673 K. Sample A, low-temperature aerogel Sample B, high-temperature aerogel Sample C, aerogel.

TABLE 33 Epoxidation of Cyclohexene with Hydrogen Peroxide at Oxodiperoxohexa-methylphosphortriamidomolybdenum(VI) in the Presence of Alkanesulfonates (50°C, 40 mmol cyclohexene in 30 g ter/-butanol, 0.4 mmol Mo05 HMPT, 50 mmol H,02 as 30 wt % aqueous solution)... 

A heterogeneous olefin epoxidation catalyst containing both V and Ti in the active site was prepared by sequential non-hydrolytic grafting. The silica was exposed first to VO(OiPr)3 vapor followed by Ti(0 Pr)4 vapor. Formation of propene is evidence for the creation of Ti-O-V linkages on the surface. Upon metathesis of the 2-propoxide ligands with BuOOH, the catalyst becomes active for the gas phase epoxidation of cyclohexene. The kinetics of epoxidation are biphasic, indicating the presence of two reactive sites whose activity differs by approximately one order of magnitude. 

The stoichiometry and kinetics of gas phase epoxidation of cyclohexene by silica-supported Ti(0 Pr)4 upon treatment with /< r/-butyl hydro peroxide were the... 

The vanadium(IV) complex of salen in zeolite was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butyl hydroperoxide as oxidant.88 Well-characterized vanadyl bis-bipyridine complexes encapsulated in Y zeolite were used as oxidation catalysts.101 Ligation of manganese ions in zeolites with 1,4,7-triazacyclononanes gives rise to a binu-clear complex stabilized by the zeolites but allows oxidation with excellent selectivity (Scheme 7.4). 

For the Ti(OiPr)4/silica system, the advantage of MCM-41 (a mesoporous silica) over an amorphous silica is not evident either in terms of activity or selectivity for the epoxidation of cyclohexene with H202 in tert-butyl-alcohol.148 Nevertheless, deactivation of the catalysts seems slower, although the selectivity of the recovered catalysts is also lower (allylic oxidation epoxidation = 1 1). Treatment of these solids with tartaric acid improves the properties of the Ti/silica system, but not of the Ti/MCM-41 system, although NMR,149 EXAFS,150 and IR151 data suggest that the same titanium species are present on both supports. 

Sajus et al. [243,244] synthesized the peroxo complex of molybdenum(VI) and studied its reaction with a series of olefins. This peroxo complex M0O5 was proved to react with olefins with epoxide formation. The selectivity of the reaction increases with a decrease in the complex concentration. It was found to be as much as 95% at epoxidation of cyclohexene by M0O3 in a concentration 0.06 mol L-1 at 288 K in dichloroethylene [244], The rate of the reaction was found to be... 

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

It is instructive to compare the properties of metal peroxo and alkyl (or hydro) peroxo groups for the case of Ti because experimental structures of both types are known [117, 119-121] and Ti compounds are catalysts for such important processes as Sharpless epoxidation [22] and epoxidation over Ti-silicalites [122], where alkyl and hydro peroxo intermediates, respectively, are assumed to act as oxygen donors. Actually, the known Ti(t 2-02) complexes are not active in epoxidation. [121-124] However, there is evidence [123] that (TPP)Ti(02) (TPP = tetraphenylporphyrin) becomes active in epoxidation of cyclohexene when transformed to the cis-hydroxo(alkyl peroxo) complex (TPP)Ti(OH)(OOR) although the latter has never been isolated. 

Table 1. Copper complexes used as catalysts for epoxidation of cyclohexene by iodosylbenzene in acetonitrile.

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

Table 11.6 Epoxidation of cyclohexene with H2O2 using selected species .

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

Table 14.1 Comparison between catalytic performances of Ti-based silsesquioxanes and MCM-supported Ti centers in the catalytic epoxidation of cyclohexene [61-63],... 

The rate of epoxidation of cyclohexene with perbenzoic acid decreases with increasing solvent polarity. The epoxidation by poly(peracrylic acid) shows the opposite trend. A polar solvent causes the polar polymer to swell to a greater extent and the reaction rate is increased due to a higher local concentration of cyclohexene [Takagi, 1975]. 

Ru(0)Cl(ding)2 (dmg=dimethylglyoximato mono-anion). This is prepared from K[Ru" Clj(dmg)2] and PhIO IR (v(Ru=0) 835 cm ), electronic spectra and cyclic voltammetric data were recorded. The magnetic moment is 3.9 B.M. It may be involved as an intermediate in the catalytic epoxidation of cyclohexene by RuClj(dmg)2/PhIO/water [640],... 

The approach of DMDO to -2-butene is spiro in nature with nearly equally developing C—O bond distances (Figure 18). The 0—0 bond in the TS is elongated to 1.879 A. The calculated gas-phase enthalpy of activation (A77 = 13.5 kcalmoD ) is higher than the experimental A77 = 7.4 kcalmoD for the DMDO epoxidation of cyclohexene in acetone solvent while the calculated entropy of activation (—39.7 calmol K ) is in better agreement with experiment (—35.5 calmoD K ). 

TABLE 5. Calculated [B3LYP/6-311+G(d,p)] activation parameters (kcal mol and eu) for the epoxidation of cyclohexene and isobutene with dimethyldioxirane (DMDO), peroxybenzoic add (PBA), m-chloroperoxybenzoic add (m-CPBA) and peroxyformic acid (PFA). Solvent corrections were performed with the COSMO model. The numbers in bold are experimental values -Numbers in parentheses are at the B3LYP/6-311- -G(3df,2p)//B3LYP/6-311- -G(d,p) level of theory... 

DMDO epoxidation of cyclohexene (Table 5) is rednced by 4.1 kcalmoD when a single water molecule is hydrogen-bonded to the distal oxygen of DMDO (a bimolecular process relative to a prereaction clnster of DMDO, H2O) and by 6.3 kcalmoD with two complexed water molecules [B3LYP/6-311+G(d,p)]. The H-bonded DMDO-CH3OH prereaction cluster has a stabilization energy of —6.9 kcalmoD. The calculated barriers for the DMDO epoxidation of -2-butene with and without water catalysis are 11.0 and... 

These composite data strongly suggest that the presence of adventitious water or other hydrogen donors can markedly affect the observed rate of epoxidation. For example, Murray and Gu have reported AH = 5.0 kcalmol" for the DMDO epoxidation of cyclohexene in CDCI3 and 7.4 kcalmol" in acetone as solvent . Curci and coworkers also reported an a value of 9.3 kcalmol" for the DMDO epoxidation of isobutylene in acetone . These barriers are significantly lower than the 13-18 kcalmoD gas-phase barriers reported " at the B3LYP level of theory (Tables 3 and 4). Activation barriers of 12.6,... 

FIGURE 19. B3LYP/6-31 H-G(d,p)-optimized transition stmctures for the epoxidation of cyclohexene with DMDO in the presence of one (a) and two (h) water molecules. The transition structure for the epoxidation of -2-butene (c) is optimized at the same level of theory in the presence of one water molecule. The classical barriers are estimated using total electronic energies of the transition stmctures, cyclohexene (—234.71316 au), ii-2-butene (—157.27453 au), DMDO with one water molecule (—344.81523 au) and DMDO with two water molecules (—421.28672 au)... 

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 . ... 

SCHEME 73. Epoxidation of cyclohexene utilizing immobilized vanadium catalysts... 

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