Cyclohexenes epoxidation

The reactivity of T8[OSiMe2H]g is dominated by its capacity to undergo hydrosilylation reactions with a wide variety of vinyl and allyl derivatives (Figure 30) that have subsequently mainly been used as precursors to polymers and nanocomposites by the introduction of reactive terminating functions as shown in Table 19. For example, T8[OSiMe2H]g has been modified with allyglycidyl ether, epoxy-5-hexene, and 1,2-cyclohexene-epoxide to give epoxy-terminated FOSS. These have then been treated with m-phenylenediamine, with polyamic acids or... 

Epoxides such as cyclohexene epoxide are converted by MesSiSiMes 857/12 or TMS 14/NaI, via O-silylated-2-iodocyclohexanol 1777 [26], into cyclohexene [27, 28] (cf also Ref. [33]). Pyridine-N-oxides such as 2-, 3-, or 4-methylpyridine-N-oxides 1778 are reduced by Me3SiI 17/Zn in acetonitrile, probably via 2-iodopyridines 1779, to picolines in 80-92% yield [29] (Scheme 12.8). 

Next, condensation of di-O-isopropylidenevalienamine (359) with cyclohexene epoxide or cyclohexadiene epoxide was carried out in 2-propanol. 

The synthesis of Ti-Si-TUD-1 is analogous to the silica version a portion of the reactant is a titanium alkoxide, such as titanium (IV) n-butoxide. One of the early comparative catalytic tests of TUD-1 versus MCM-41 was for cyclohexene epoxidation. 

The Ti was loaded using two methods direct incorporation into the synthesis mixture, and post-synthesis grafting. In all cases the Ti-loading was 1.5 -1.8 wt%. Selectivity towards the epoxide was always 100%. Table 41.1 summarizes the results comparing Ti-TUD-1 and Ti-MCM-41 for cyclohexene epoxidation (15). For the direct incorporation, Ti-TUD-1 is five times more active than Ti-MCM-41, even though they have equivalent surface area. However, the grafted MCM-41 is also more active than its as-synthesized counterpart. 

Other TUD-l catalysts proven for selective oxidation (16) include Au/Ti-Si-TUD-1 for converting propylene to propylene oxide (96% selectivity at 3.5% conversion see also (17), Ag/Ti-Si-TUD-1 for oxidizing ethylene to ethylene oxide (29% selectivity at 19.8% conversion), and Cr-Si-TUD-1 for cyclohexene to cyclohexene epoxide (94% selectivity at 46% conversion). 

Cyclohexene epoxides are preferentially reduced by an axial approach by the nucleophile.168... 

With cyclohexene epoxides, the ring opening is frans-diaxial. 

Table 1 Catalyst composition and rate constants for cyclohexene epoxidation.

Fig. 34. Ratio of product concentrations [sum of epoxide and secondary products (a) from oct-1 -ene and (b) from cyclohexene] obtained with mesoporous and conventional TS-1 as a function of the contact time. The results show that the mesoporous TS-1 has a similar activity for oct-1 -ene epoxidation as conventional TS-1. However, the mesoporous TS-1 is significantly more active for cyclohexene epoxidation [Reproduced from Schmidt et al. (188) by permission of the Royal Society of Chemistry].

The treatment leads to a significant improvement in alkene conversion in cyclohexene epoxidation in the case of Ti-MCM-41 and Ti-MCM-48 (273). Although epoxide selectivity improved in the former case, there was a decrease in the latter. In the case of hexane oxidation, silylation did not improve the conversion. An enhancement in the number of turnovers and selectivity for the epoxide on silylation was also observed in the cyclohexene epoxidation with TBHP catalyzed by Ti-SBA-15 (Table LII) (274). Ti-SBA-15 was claimed to be thermally more stable than Ti-MCM-41. Ti leaching was absent. 

The mesoporous materials reported above are usually prepared from relatively expensive surfactants. Some of them have poor hydrothermal stability. Furthermore, the MCM-41 host structure has a one-dimensional pore system with consequent pore blockage and diffusion limitations. Shan et al. (52) reported the synthesis of a three-dimensional and randomly connected mesoporous titano-silicate (Ti-TUD-1, mesopore wall thickness = 2.5-4 nm, surface area — 700-1000 m2/g, tunable pore size —4.5-5.7 nm) from triethanolamine (TEA). Ti-TUD-1 showed higher activity (about 5.6 times) for cyclohexene epoxidation than the framework-substituted Ti-MCM-41. Its activity was similar to that of the Ti-grafted MCM-41 (52). 

An allenyllithium intermediate was implicated in the reaction of BuLi with an alkynylated cyclohexene epoxide (Table 9.5) [11], It was found that addition of 2equiv. of BuLi to the alkynyloxirane in the presence of 5mol% CuBr-2PPh3 led, after quenching with H20, not to the expected SN2 butylated allene, but instead to the protonolysis product. Likewise, quenching the reaction with Mel or MeSSMe led to the methylated and thiolated allenes, respectively. Furthermore, the putative lithioallene could be trapped by C02 or PhCHO to yield the expected adducts. 

In connection with our own work on the enzyme-catalysed hydrolysis of cyclohexene epoxide with various fungi we made the unexpected observation that the microorganism Corynesporia casssiicola DSM 62475 was able to interconvert the (1R,2R) and (1S,2S) enantiomers of the product, trans cyclohexan-1,2-dioI 25. As the reaction proceeded the (1R,2R) enantiomer was converted to the (1S,2S) enantiomer [20]. If the racemic trans diol 25 was incubated with the growing fungus over 5 days, optically pure (> 99 % e. e.) (1 S,2S) diol 25 could be isolated in 85% yield. Similarly biotransformation of cis (meso) cycIohexan-1,2-diol 26 yielded the (1S,2S) diol 25 in 41 % (unoptimized) yield (Scheme 11). 

In the absence of an organic solvent, this assembly catalyses cyclohexene epoxidation with 30% aqueous H2O2 with high selectivity, especially when... 

Table 12 Influence of the nature of the tethered polyether on cyclohexene epoxidation with H2O2/MTO...

Effect of modification on cyclohexene epoxidation over Ti(Al)-beta and Ti-MCM-41... 

Influences of Bronsted acidity and silanol group on cyclohexene epoxidation... 

In Table 2 and Fig. 2 the results of cyclohexene epoxidation over hydrophilic TM-1 and silylated catalysts (TM-2 and TM-3) are presented. Apparently, the silylation applied to Ti-MCM-41 improves the activity of cyclohexene epoxidation, enhances the yield of epoxide and reduces the formation of l-ol and 1-one. In contrast to the Ti-beta, the selectivity of diol remained almost unchanged. In accordance with the characterization results, the MSTFA silylated catalyst, TM-3, gives lower selectivities to l-ol and 1-one than TM-2 does. This indicates that the more hydrophobic the catalyst is, the less by-products are produced, while the higher selectivity of epoxide is obtained. In addition, we observe that the sum of selectivities to 1 -ol and to 1 -one remains unchanged during reaction for epoxidation over both Ti-beta and Ti-MCM-41. This implies that l-ol is the primary product and can be further oxidized to 1-one. According to these observations, we can further conclude that the hydrophilic nature of catalysts leads to the formation of l-ol. 

Figure 2. Experimental results of cyclohexene epoxidation with H202 over Ti-MCM-41 catalysts ( ) TM-1 ( ) TM-2 (a) TM-3.

Fig. 5.4 Evolution of the activity of a single catalyst in cyclohexene epoxidation 300 fast GC analyses.

Fig. 5.5 Evolution of cyclohexene epoxide yield through the optimisation process.

The desymmetrization of meso-e poxides such as cyclohexene epoxide (55, Scheme 13.27) has been achieved both by enantioselective isomerization, e.g. to allylic alcohols (56, path A, Scheme 13.27) or by enantiotopos-differentiating opening by nucleophiles, affording trans-/ -substituted alcohols and derivatives (57, path B, Scheme 13.27). As indicated in Scheme 13.27, the allylic alcohols 56 can also be prepared from the ring-opening products 57 by subsequent elimination of the nucleophile. 

Oxonium ions as a source of the oxepane ring are also formed by the irradiation of cyclohexene epoxides <95CPBi62i>. In this work the oxonium ion is also trapped in an intramolecular process to form spiro-derivatives which may undergo further rearrangement as illustrated in Scheme (8). 

Because PBI is expensive, other thermostable polymers were explored and tested as catalysts (246). A cross-linked version of a polyimide (PI) support with incorporated triazole rings (12b) gave better results than PBI for the epoxidation of cyclohexene. Moreover, it can be reused in the cyclohexene epoxidation at least 10 times without any loss of activity (247). Even less expensive, but thermooxidatively stable materials include polysiloxane-based resins, which have also been used for incorporation of Ti (see Section II,A). In this case, the synthesis comprises the polymerization of TEOS and an oligomeric dimethyl silanol with the addition of functional trialkoxysilanes such as trimethoxysilyl-2-ethylpyridine instead of Ti(OiPr)4 (248). Preliminary results show that the activity per Mo atom is higher than that of PBI-Mo. Furthermore, the degree of leaching of Mo is very low. Thus, it is expected that the polysiloxane-based systems may soon find wide application in oxidation chemistry. 

---