Norbornene epoxidation

These torsional interactions are relatively unimportant in radical epoxidations where the diastereoselectivities for norbornene epoxidation are considerably lower, but strongly dependent on the reagent system employed (vide supra). 

Norbornene epoxide (1) is isomerized by this strongly basic reagent cleanly if in only moderate yield to nortricyclanol (2). Acid catalysts act on (1) to give mixtures of products. 

Puzzling, however, is the formation of substantial quantities of exo-norbornene epoxide. Control experiments suggest that a perepoxide (15) is not involved.33"... 

In order to characterize the nature of the oxygenating species formed in the reactions of H2O2, PhIO, MCPBA, and dioxygen plus aldehyde, we studied stereoselectivity in c/5-stilbene epoxidation, regioselectivity in (+)-limonene and norbornene epoxidations, and intermolecular competitive reactions between cyclohexene... 

This thermally unstable compound reacts with triphenylphosphine to form the oxide, and with alkenes to form epoxides. Yields of up to 50% epoxide have been obtained using the alkenes norbornene and cyclohexene. Otsuka et al. have recently isolated the bromo analog compound PtBr(OOCOPh)-(PPh3)2 in a pure state and have shown that it will oxidize norbornene to ejco-norbornene epoxide in 40% yield. 

A study of the regiochemistry of norbornene epoxidation with iodosylxylene and perfluoroiodosylbenzene, catalyzed by tetraarylporphyrinatoiron(III) complexes, has shown that significant amounts of endo-2,3-epoxynorbornane are formed besides the exo-isomer [61]. This pattern has been interpreted by initial electron transfer from norbornene to the oxoiron(IV) porphyrin cation radical, followed by collapse of the radical formed to either one of the two possible carbocation intermediates, which are precursors of the two isomeric epoxides ... 

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, ... 

The stereoselectivity of epoxidation with peroxycarboxylic acids has been well studied. Addition of oxygen occurs preferentially from the less hindered side of the molecule. Norbornene, for example, gives a 96 4 exo endo ratio.76 In molecules where two potential modes of approach are not very different, a mixture of products is formed. 

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 problem of diastereoselectivity in additions to cyclic radicals arising from the opening of bi- or tricyclic epoxides, e. g. cycloheptene oxide or norbornene oxide, has been addressed only recently [32], In the former case, reasonable selectivities can be obtained with titanocene dichloride (trans cis = 76 24), but excellent selectivities are observed with bis(tert-butyl)titanocene dichloride (transxis = 94 6), as shown in Scheme 12.17. 

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. 

A series of bridged dinuclear [(bpy)2Ru(p-L)Ru(bpy)2] " and [(bpy)2Ru(/i-L)Os(bpy)2] complexes have been prepared in which the bridge coordination domains are phen ligands and the spacer is constructed by the stereoselective coupling of units through norbornene and epoxide functionalities. " " ... 

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... 

The desymmetrization works also well with higher substituted meio-epoxides such as ewdo-norbornene oxide (130) , cis-5,6- and 4,7-difunctionalized cyclooctene oxides 132 and 134, giving the alcohols 131, 133 and 135, respectively but for the diastereomer 136, the rearrangement to form the allylic alcohol 138 beside 137 cannot be completely suppressed (equation 29 best results are given). ... 

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... 

From the encouraging results obtained in the reactions of a series of gold(III) oxo complexes with olefins [56], Cinellu et al. tried to achieve the supposed oxametalla-cyclic intermediate, which had never been isolated before [25]. In the reaction of 8 and norbornene 56, if the - atoms were considered to be equivalents of coordinated water, and it was therefore possible to talk about the gold-catalyzed addition of water to an alkene. The metallaoxetane 58 was separated from the gold-alkene complex 57 and characterized by X-ray crystal structure analysis. The subsequent stoichiometric reaction yielded epoxide 59 (Scheme 8.5). 

This subject has recently been reviewed.647 Several additional papers have appeared on the catalytic oxidation of alkenes by 02 in the presence of PdCl(MeCN)2N02(148).64S Terminal alkenes and trans- cyclooctene yield the corresponding ketones, cyclopentene and cyclohexene the corresponding allyl alcohol, and bicyclic alkenes the corresponding epoxide. Heterometallacy-clopentanes such as (152) have been isolated from the reaction of (148) with norbornene (dicy-clopentadiene), and characterized by X-ray crystallography.6486 Glycol monoacetates were obtained from the reaction of (148) with terminal alkenes in acetic acid.649... 

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... 

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