Oxidation of stilbene

Fig. 12.3. Transition structure for oxidation of stilbene by (DHQD)2PHAL-0s04 catalyst. Reproduced from J. Am. Chem. Soc., 121, 10186 (1999), by permission of the American Chemical Society.

Stereoselectivity is also observed in the oxidation of stilbens with the zeolite-entrapped t-butyl substituted FePc catalyst. With the cis-stilben as substrate the ratio of trans to cis product (trans-stilben is produced via the epoxidation of cis-stilben)is remarkably higher for FePc(t-Bu)4/NaY, compared with those for FePc(t-Bu)4+NaY and even for FePc/NaY and FePc+NaY samples under the similar conditions, as shown in Table 2. It was found that the stereoselectivities toward trans-products (trans-stilben + trans-stilben oxide) kept remained(48-53% sel) for the different conversions of cis-stilben. The... 

The reaction depicted in Figure 3 can also be applied to the copper(II) oxidative-cleavage of lignin. In this case, copper(II) is reduced to copper(I) as the lignin is oxidized. Interestingly, copper(I) can be reoxidized to copper(II) by air (23). This autoxidation of copper(I) could explain why air was necessary for the copper(II) oxidation of stilbenes whereas, when nitrobenzene was used, identical experimental results were obtained under either an air or nitrogen atmosphere (Table II). 

In the second approach, a chiral nitrogen-containing compound has most often been used as the ligand to achieve enantioselectivity. Thus, oxidation of ( )-stilbene (22 equation 9) with a stoichiometric quantity of osmium tetroxide in toluene at room temperature, in the presence of dihydroquinine acetate (23), yielded r/ireo-hydrobenzoins (24) after reductive hydrolysis, with an enantiomeric excess of 83.2% in favor of the (15,25)-(-)-isomer performing the reaction at -78 C increased the eiuuitiomeric excess to 89.7%. 

With diamine (25), 1-heptene afforded (/ )- ,2-heptanediol as the major ixoduct (86% ee) in 75% yield by this procedure but, curiously, oxidation of ( )-stilbene proceeded with lower optical yield (34% ee). Particularly efficient enantioface differentiation was achieved in the reaction of ( )-l-phe-nylpropene with a stoichiometric amount of osmium tetroxide in the presence of 1 mol equiv. of (-)-(27) when essentially optical pure (>99% ee) (15,2S)-l-phenylpropane-l,2-diol was obtained in 73% yield. This procedure is effective for mono-, ( )-di- and tri-substituted alkenes, with enantioface selection being as shown in Scheme 2 but, notably, the oxidation of (Z)-alkenes does not give satisfactory optical yields. 

Kojima and co-workers also examined the photo-oxidation of stilbenes in zeolite Y [150]. The presence of O2 in the stilbene-NaY sample led to a red-shifting of the tail end of the diffuse reflectance spectrum and was assigned to the charge-transfer complex with O2. Upon excitation at 313/366 nm, both cis- and /ra .v-stilbene undergo photo-oxygenation to form benzaldehyde. Phenanthrene, the expected product from the ]2 -f 4] cycloaddition reaction of excited m-stilbene, was also formed. The details of the mechanism leading to benzaldehyde and phenanthrene... 

SCHEME 67. Hydrogen peroxide oxidation of stilbenes catalyzed by HRP... 

Photorearrangements of aryl esters in zeolites (6.40) give different product distributions.210 Photosensitized oxidation of stilbene with oxygen (6.41) leads to different products in solution and in a zeolite.211... 

It is well known that BLM oxidizes not only double-stranded DNA but also various olefinic substrates. Hitherto reported are oxidation of stilbene, styrene, chalcone, cinnamic acid, cyclohexene, norbornene, and indene using Fe(II), Fe(III), Cu(II), Mn(III), or Zn(III) complexes of BLM in the presence of O2, CgHsIO, or KHSO2 as oxygen donor [7, 8]. We reported that stilbene can be epoxidized with Fe(III)-H202 complex system of PYMLs [59]. Although the formation of chiral iron chelate was reasonably proposed as shown in Fig. 2, there has been no report on asymmetric induction in the oxidation mediated by BLM-metal complex systems. We found that cis-P-methylstyrene is affected by the chiral environment of the iron center of BLM and PYML. 

Fig. 1 Photocatalytic oxidation of stilbene by self-assembled Mn-porphyrins. Reproduced from ref. 47 with permission from The Royal Society of Chemistry,...

Taking advantage of OAGDM-induced purple benzene [37], the oxidation of stilbene to benzoic acid is enhanced by approximately a factor of 10 [35]. The rate of esterification of benzyl bromide with potassium acetate depends on the OAGDM added and on the solvent present [35]. In ethanol, the reaction proceeded homogeneously even without OAGDM nevertheless, an additional acceleration effect was observed in the presence of OAGDM. This indicates that the enhance-... 

A dinuclear ruthenium(II)-NHC complex (structure 80, Fig. 28) has been reported to efficiently catalyze the oxidation of stilbenes and other disubstituted olefins conjugated to an aromatic system to the corresponding... 

Further, itwas shown [89] that electrolysis of benzoic acid in acetonitrile with triethylamine and cis- and trans-stilbene gives only meso-hydrobenzoin dibenzoate irrespective of the stilbene configuration. The authors attempted an explanation of this unexpected result on the assumption that stepwise oxidation and inversion of an intermediate unstable product occur. An important part is played here by adsorption and oxidation of stilbene itself at the electrode, although no evidence was obtained for its possible isomerization under the experimental conditions. 

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