Norbomene, epoxidation

An example of this type of forward reaction has already been given in Seheme 2.4. Further examples are provided by (i) the use of norbomene epoxide with W-based catalysts to enhance the ROMP of norbomene derivatives (Devine 1982) (ii) the use of oxygen with norbomene in Ru-based systems to generate such an epoxide (Ivin 1981b) (iii) the use of cyclopropane derivatives with Re207/Al203 (Anisimov 1991) see Ch. 2. 

Ketones are the principle products in the case of terminal alkenes. In contrast, with cyclic olefins, such as cyclopentene, cycloheptene and norbomene, epoxides were formed as the major products in the oxygen atom transfer reactions catalyzed by 45 [121]. This observation is significant, since it preludes the development of catalytic systems for direct epoxidation of olefins using both oxygen atoms of molecular O2, which is contrast to the traditional methods converting only one oxygen atom of O2 to generate the desired epoxide. 

Stereochemical aspects of epoxidation of substituted norbomenes and accompanying intramolecular transformations 98UK299. 

Gunter and co-workers have applied this methodology to the condensation of norbomene derivative 90 (block A, Scheme 32) with epoxide 94 (block B, Scheme 33) <98S593>. Thus, using the Crossley porphyrin-a-dione 47, condensation with 1,2,4,5-benzenetetramine tetrahydrochloride and then the product with a strained dione, gave 90 as block A. 

The results of the olefin oxidation catalyzed by 19, 57, and 59-62 are summarized in Tables VI-VIII. Table VI shows that linear terminal olefins are selectively oxidized to 2-ketones, whereas cyclic olefins (cyclohexene and norbomene) are selectively oxidized to epoxides. Cyclopentene shows exceptional behavior, it is oxidized exclusively to cyclopentanone without any production of epoxypentane. This exception would be brought about by the more restrained and planar pen-tene ring, compared with other larger cyclic nonplanar olefins in Table VI, but the exact reason is not yet known. Linear inner olefin, 2-octene, is oxidized to both 2- and 3-octanones. 2-Methyl-2-butene is oxidized to 3-methyl-2-butanone, while ethyl vinyl ether is oxidized to acetaldehyde and ethyl alcohol. These products were identified by NMR, but could not be quantitatively determined because of the existence of overlapping small peaks in the GC chart. The last reaction corresponds to oxidative hydrolysis of ethyl vinyl ether. Those olefins having bulky (a-methylstyrene, j8-methylstyrene, and allylbenzene) or electon-withdrawing substituents (1-bromo-l-propene, 1-chloro-l-pro-pene, fumalonitrile, acrylonitrile, and methylacrylate) are not oxidized. 

Week et al. [65] further reported the Co salen complex supported on norbomene polymers (23, 24) with stable phenylene-acetylene linker (Figure 8). The polymer-supported salen catalysts were investigated for HKR of the racemic terminal epoxides that showed outstanding catalytic activities and comparable selectivities to the original catalysts reported by Jacobsen. However, the polymeric catalyst was recycled only once after its precipitation with diethylether as the catalyst became less soluble and less reactive in subsequent catalytic... 

Some typical epoxidations are listed in Table 3.1. The first Ru-catalysed epoxida-tion was reported in 1983 by James et al., in which RuBrlPPh XOEPl/PhlO/CHjClj was used to epoxidise styrene, norbomene and c/x-stilbene in low yields trans-stilbene was not oxidised [588]. It was later noted that tranx-RulOl lTMPl/Oj/C H aerobically oxidised cyclic alkenes, and a catalytic cycle involving Ru 0(TMP) was proposed, in which there is disproportionation of Ru(0)(TMP) to Ru(TMP) and fran -Ru(0)2(TMP), the latter epoxidising the alkene and the former being oxidised back to the latter by (Fig. 1.26) [46, 583]. Stilbene, tranx-styrene and norbomene were efficiently epoxidised by trani-RulOl lTMPl/lCl pyNOl/CgH [589], as was epoxidation of exo-norbomenes catalysed by trani-RulOl lTMPl/Oj/ CgHg [590]. 

As stoich. [Ru(0)(bpy)(tmtacn)]VCH3CN it functioned as a competent (sic) epoxidant for alkenes, though the products were often contaminated with by-products (e.g. fran -stilbene gave fran -stilbene oxide and benzaldehyde cw-stilbene gave cis- and trans- epoxides). Kinetics of the epoxidation of norbomene and styrene were reported, with activation parameters measured and discussed [682]. Kinetics of its non-stereospecific, stoicheiometric epoxidation of aromatic alkenes in CH3CN were studied, and the rates compared with those of oxidations effected by other Ru(IV) 0x0 complexes with N-donors, e. g. [Ru(0)(tmeda)(tpy)] ", trans-[Ru(0)(Cl3bpy)(tpy)] " and [Ru(0)Cl(bpy)(ppz )] + [676]. 

RuBr(PPh3)(OEP) is made from Ru(PPh3)3(OEP) and Br. As RuBr(PPh3)(OEP)/ PhlO/CH Clj it epoxidised styrene, norbomene and cw-stilbene to their epoxides, but frawj-stilbene was not oxidised [588]. The reagent RuBr(PPh3)(OEP)/PhlO/CH3Cl2 oxidised cyclohexane and cyclohexene to a mixture of products [812]. 

Ru(C. 35C(0)CH2C(0)C. j5)3] , a substituted (acac) complex, is made from the 1,3-diketone C j COCH COC Fu with RuClj (quoted as RuCl in the paper) in ethanol with KCHCOj). In biphasic solvents [Ru(C jjC(0)CH2C(0)C j3)3]7per-fluorodecaUn-toluene/O (1 atm)/65°C oxidised aldehydes to ketones, disubstituted alkenes (cyclo-octene, norbomene) to epoxides and sulfides to sulfoxides or sul-fones [821, 822],... 

The first Ru-catalysed epoxidation was reported in 1983 by James et al. using RuBr(PPh3)(OEP)/PhlO/CH2Cl2 with styrene, norbomene and aT-stilbene in low yields cf. mech. Ch. 1 [23]. Eater work showed that fran -Ru(0)2(TMP)/02/CgHg catalysed aerobic alkene epoxidation of cyclo-octene, cis- and trans- -methylstyrenes and norbomene (Fig. 1.26) [24],... 

Much exploratory work has been done on model substrates. Epoxidations of cyclo-octene and styrene give relatively clean products, and several studies have also been carried out on styrene, norbomene and 1-octene. 

Since peroxy acids have a relatively low steric requirement, steric hindrance mainly arises in the epoxidation of bridged cycloalkenes.224 Marked difference in exolendo selectivity was observed in the reaction of norbomene and 7,7-dimethyl-norbomene with m-CPBA236 [Eqs. (9.60) and (9.61)] ... 

In competitive epoxidation norbomene reacts at a rate approximately 100 times that of 7,7-dimethylnorbomene. 

Ti-MCM-41 is also an active catalyst with TBHP and olefins norbomene gives the corresponding epoxides with 90% selectivity in the presence of Ti-MCM-41. However, when H202 is the oxidant, conflicting results have been reported for the oxidation of 1-hexene (see Section III.E). 

The reaction of (148) with norbomene produces a stable, well-characterized bright yellow netallacyclic compound (152) which, upon standing at 25 °C in toluene, slowly decomposes to pve exo-epoxynorbornene in almost quantitative yield (equation 199).210 The particular decomposition, which is related in some aspects to the mechanism of alkene epoxidation by d° metal-peroxo... 

Olefins undergo a two-step oxidative process, with the first step leading to an epoxide that, in the presence of excess oxidant, subsequently is cleaved to afford aldehydes or ketones, dependent on the position of the olefinic bond. Oxidative reactions by peroxovanadates tend to be retarded by protic solvents such as water or methanol. For instance, oxidation of norbomene by picolinatooxomonoperoxo-vanadate in acetonitrile affords 22% of the product epoxide in 9 min. After 120 min in methanol solvent, only 1.8% yield was obtained. In dichloromethane, even cyclohexane is oxidized faster than this, giving 4% cyclohexanol and 9% cyclohexanone in 120 min, whereas benzene in acetonitrile yields 56% of phenol [23],... 

Simple alkenes do not normally react with IOB, unless there is catalysis by metal porphyrins or related metal complexes, in which case epoxidation occurs [1,2]. A great deal of work has been done in this field, especially with the relatively simple catalyst Fe(TPP)Cl (TPP is tetraphenylporphyrin) in some instances this approach can be used advantageously in comparison with other well-known methods of epoxidation. The Fe(TPP)Cl catalysed IOB epoxidation of alkenes is stereospecific, with cis substrates being considerably more reactive than trcrns. Several alkenes underwent efficient epoxidation with this system, e.g. cyclooctene (84% of epoxide), norbomene (67% exo-epoxide, accompanied by 3% of the enc/o-isomer) and... 

The sterically crowded ligand (106) forms a very rare square-planar Co complex, as shown by an X-ray crystal structure, and a spin triplet (paramagnetic) ground state was also identified in the solid state. The Co complex of (107) catalyzes the epoxidation of norbomene with t-BuOOH or Phi as terminal oxidant, catalysis driven by formation of t-BuOO radicals employing a Co redox process. 

Anhydrous Fe Cl3 catalyzes the stereospecific epoxidation of norbomene, the demethylation of A, A-dimethylaniline, and the oxidative cleavage of PhCMe(OH)CMe(OH)Ph (and other a-diols) by hydrogen peroxide (Table 11 and Scheme 4). For each class of substrate, the products parallel those that result from their enzymatic oxidation by cytochrome P-450. The close congruence of the prodncts indicates that the reactive oxygen in the Fe Cl3/HOOH model system and in the active form of cytochrome P-450 is essentially the same, with strong electrophilic oxene character (stabilized singlet atomic oxygen). 

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