Epoxidation of norbornenes

Epoxidation of norbornene was found some time prior to this work not to stop at the monoepoxide step. Instead, this intermediate goes on to rearrange to the bicyclic aldehyde, 31, ... 

Figure 3.3 Effect of neighbouring functional groups on the epoxidation of norbornene derivatives.

Traylor et al. reached different conclusions in studies of alkene epoxidation. In homogeneous solutions in which iodosylbenzene was solubilized in dichloro-methane/alcohol/water, very rapid epoxidation (300 turnovers/sec) was found to be independent of substrates or concentration of alkane (124). No dependence on alkene concentrations was reported by Dicken et al. when they employed /7-cyanodimethylaniline 7V-oxide (725). Meanwhile, Traylor et al. found an accumulation of 7V-alkyliron porphyrin complexes (35) during the course of epoxidation of norbornene (126), and showed that it is the only intermediate accumulated in either the homogeneous system or the heterogeneous system 35 was found to form with a bimolecular rate constant of 450 M sec and decomposes at a rate of 0.07 sec. Epoxide formation occurs with a rate constant of at least 10 Af" sec in these conditions. Thus 35 is not an intermediate in the production of epoxide however, it could account for at least a small percentage of the products, since 35 was found to catalyze alkene epoxidation by utilizing PhIO (Scheme XIX). 

Traylor, T.G., T. Nakano, A.R. Mikztal, and B.E. Dunlap (1987). Transient formation of N-alkyl-hemins during hemin-catalyzed epoxidation of norbornene. Evidence concerning the mechanisms of epoxidation. J. Am. Chem. Soc. 109,3625-3632. 

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 norbornene with r-BuOOH or Phi as terminal oxidant, catalysis driven by formation of r-BuOO radicals employing a Co redox process. 

Anhydrous Fe CH catalyzes the stereospecific epoxidation of norbornene, the demethylation of N. N-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 products indicates that the reactive oxygen in the Fe CH/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). 

During the epoxidation of norbornene an N-alkylhemin catalyst is obtained when the initial porphyrin interacts with the alkene [62]. After the oxidant has been consumed, it reverts to the original catalyst. 

The highly distorted Ru(DPP)(0)2 (X-Ray data revealed that its precursor, Ru(DPP)(CO)(py), also exhibits both saddle and ruffle distortion)catalyzes in benzene-MeCN (9 1) some epoxidation of norbornene, styrene, cyclohexene, cyclooctene and cA-stilbene under 1 atm O2 with turnovers of 8 - 40 in 4 h before deactivation of catalyst the numbers are comparable with those for Ru(TMP)(0)2 and greater than for Ru(0EP) 0)2 and Ru(TPP)(0)2. There were also co-oxidation products benzaldehyde (7 turnovers) and trace phenylacetylaldehyde were formed from styrene, cyclohex-2-en-l-ol (20 turnovers) and cyclohex-2-en-l-one (11 turnovers) from cyclohexene, and trace benzaldehyde and trans-stilbene... 

The more hindered alkoxide Ti(OiPr)4 was used as the precursor complex with surface silanols of an amorphous silica support this reaction is reported to lead to the same environment of Ti as in TS-1, but only when the reaction is carried in cyclohexanol as the solvent. Epoxidation of octene, cyclohexenol, and norbornene with FI202 in phenylethanol leads to 95-98% epoxide selectivity.147... 

The epoxidation of alkenes using iodosylbenzene, with tetra-n-butylammonium bromide and a manganese or cobalt polytungstate as co-catalysts [24], appears to have little advantage as a synthetic procedure over other methods. n-Hexene produces the oxirane (58%), when catalysed by the manganese salt, whereas norbornene is more readily converted (96%) into the oxirane with the cobalt salt. 

The kinetics of the reaction of [Ru (0)2(poi with various alkenes have been investigated either in CH2Cl2/MeOH or CH2Cl2/Hpz. In the absence of MeOH or Hpz, clean kinetics are not always observed, presumably because dimerization or disproportionation of the intermediate Ru =0 species occurs at comparable rates as epoxidation. The reactions have the following rate law rate = k[Ru (0)2(por)][alkenel. The rate constants for the oxidation of styrene are in the range 6 x 10 —4.8 x 10 s. The oxidation of norbornene occurs at... 

As discussed in previous paragraphs, a disadvantage of TS-1 catalyst is its limited pore size, which restricts the number of oxidants that can be employed ° °. By using larger pore molecular sieves this restriction could be overcome. Titanium-substituted MCM-41 ° and titanium-grafted MCM-41 were shown to be particularly active in the epoxidation of bulky olefins like pinene and norbornene with TBHP. In the case of Ti-MCM-41 silicas, the importance of hydrophobicity in the system was demonstrated by the faster reaction rate observed with 2-methyl-l-phenyl-2-propyl hydroperoxide as the oxygen atom... 

In relation to enzymic cytochrome P-450 oxidations, catalysis by iron porphyrins has inspired many recent studies.659 663 The use of C6F5IO as oxidant and Fe(TDCPP)Cl as catalyst has resulted in a major improvement in both the yields and the turnover numbers of the epoxidation of alkenes. 59 The Michaelis-Menten kinetic rate, the higher reactivity of alkyl-substituted alkenes compared to that of aryl-substituted alkenes, and the strong inhibition by norbornene in competitive epoxidations suggested that the mechanism shown in Scheme 13 is heterolytic and presumably involves the reversible formation of a four-mernbered Fev-oxametallacyclobutane intermediate.660 Picket-fence porphyrin (TPiVPP)FeCl-imidazole, 02 and [H2+colloidal Pt supported on polyvinylpyrrolidone)] act as an artificial P-450 system in the epoxidation of alkenes.663... 

Epoxidation of cycloaikenes Cycloalkenes, particularly strained bicyclic alkenes, can be epoxidized by treatment with l equiv. of 1 under N,. In the case of norbornene (2) the reaction involves formation of the metallacycle 3, which decomposes slowly to exo-epoxynorbomene (4). In fact, epoxidation is favoured over ox idation of a vinyl system in the air oxidation of 5 catalyzed with l. 3... 

However the epoxidation of olefins lacking allylic and other reactive C-H bonds with molecular oxygen has recently been achieved on silver catalysts (Table 1). In 1997 the Eastman Chemical Company started the manufacture of 3,4-epoxy-1-butene, the product of mono-epoxidation of butadiene, on a semiworks production scale (entry 1). Remarkably enough the presence of benzylic hydrogen, as in / -methyl styrene (entry 5), drives the oxidation towards combustion, while sterically hindered allylic C-H s, as in norbornene (entry 6), are inert to oxidation. 

The number of metal zeolites and their application to the epoxidation of olefins rose in parallel from the late 1980s. TS-2, Ti,Al-P, Ti-P, Ti-MWW and, rarely, Ti-MOR are catalysts that have been studied in some detail [7-9, 35, 77-84]. TS-2 behaves, according to the few studies published, similarly to TS-1. The greater spaciousness of pores in Ti-Beta zeolites and of external cups in Ti-MWW allows the epoxidation, under mild conditions, of olefins unable to diffuse in TS-1 and TS-2, such as methylcyclohexenes, cyclododecene, norbornene, camphene and methyl oleate [80-83]. Steric constraints still prevail over electronic factors, however, as in medium pore Ti-zeolites, even in the epoxidation of linear olefins (Table 18.9). It is generally believed that active sites and epoxidation mechanisms are not significantly different from those of TS-1. 

Inoue and co-workers have used ruthenium(ll) porphyrin 80 (Figure 16) to mediate the photochemical epoxidation of several alkenes with excellent selectivity over the formation of other oxidation products, including cyclohexene (up to 97% selectivity), norbornene (up to 99.7% selectivity), and cyclooctene (up to 99.9% selectivity) <2003JA5734>. 

In contrast to the PhlO/Mn(TPP)Cl system, epoxidation of alkenes occurs with high retention of configuration. cis-Alkenes are almost exclusively converted to cis-epoxides. Norbornene is transformed into a 67 3 mixture of exo- and e do-norbornene oxide e.g. equation 223)." ... 

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