Epoxidation of stilbenes

FIGURE 6.14 Epoxidations of stilbenes over Mn- and Ti-containing zeolites.8889... 

Epoxidation.1 This combination is known to oxidatively cleave double bonds but to effect epoxidation when catalyzed by a metalloporphyrin. Epoxidation of alkenes can also be effected by catalysis with a simple amine. The choice of the amine depends on the olefin. N,N-Dimethylethylenediamine is the most efficient ligand for epoxidation of a 1-alkene (68% yield). Pyridine is the best ligand for epoxidation of stilbene (93%), and imidazole is preferred for epoxidation of QH5CH=CHCH, (71% yield). 

Salicylaldimine complexes of iron(III) catalyze epoxidation of stilbene by hypochlorite. ... 

The hydroxylation of n-hexane and epoxidation of stilbens were conducted at 300K in the suspended solutions with PhIO for each powdered sample such as FePc(t-Bu)4/NaY, FePc(t-Bu)4+NaY, FePc/NaY and FePc+NaY. The products in solution were filtered and analyzed by FID gc using a capillary column(PEG-25M). 

Many transition-metal complexes have been widely studied in their application as catalysts in alkene epoxidation. Nickel is unique in the respect that its simple soluble salts such as Ni(N03)2 6H20 are completely ineffective in the catalytic epoxidation of alkenes, whereas soluble manganese, iron, cobalt, or copper salts in acetonitrile catalyze the epoxidation of stilbene or substituted alkenes with iodosylbenzene as oxidant. However, the Ni(II) complexes of tetraaza macrocycles as well as other chelating ligands dramatically enhance the reactivity of epoxidation of olefins (90, 91). 

Caps and coworkers studied the solvent effect in the epoxidation of stilbene by varying solvents and the supports [200], In methylcyclohexane (MCH), the activated radical species proposed were MCH peroxy radicals, which were formed by the radical transfer from TBHP and reaction with molecular oxygen. Except for MCH, the solvent effect is not fully understood however, the choice of solvent and supports that can trap or stabilize the radical species affected the catalytic performance of Au. 

Typical Procedure for Epoxidation of Stilbenes under Yang Conditions with CH3CN-H20 Solvent System [53] (p. 405)... 

An obvious way to immobilize MTO would be by coordinative binding on a PVP resin. However, in our experience, peroxidized MTO is not efficiently retained on PVP. Two alternative approaches have been proposed. First, MTO was supported on Nb2Os, and its activity was tested in the epoxidation of stilbenes and of styrenes (382). However, competitive metathesis and pronounced hydrolysis of the epoxides to the diols were observed. A second approach was suggested by Neumann and Wang (383). In a strategy similar to that for H5PV2M010O40 (370), they adsorbed MTO in a surface layer of... 

The active oxidant was proposed to be a Ru(V)=0 species and access of benzene towards the Ru=0 bond is facilitated by the flat structure of the salicyldiimine ligand (see Fig. 8). This catalytic system was also applied to the epoxidation of stilbene, C-H bond activation of cyclohexane or cyclohexene and the oxidation of tetrahydrofuran to y-butyrolactone [37]. We conclude however, that a suitable and catalytic system for the selective oxidation of benzene to phenol has not yet been forthcoming. 

On the basis of theoretical studies by Bach and co-workers,17 it was found that the nucleophilic 71-bond of the alkene attacks the 0-0 cr-bond in an Sn2 fashion with displacement of a neutral carboxylic acid. There are, however, some mechanistic anomalies. For example, a protonated peracid should be a much more effective oxygen transfer agent over its neutral counterpart, but experiments have shown only modest rate enhancements for acid catalysed epoxidation. Early attempts to effect acid catalysis in alkene epoxidation where relatively weak acids such as benzoic acid were employed proved unsuccessful.18 The picture is further complicated by contradictory data concerning the influence of addition of acids on epoxidation rates.19 Trichloroacetic acid catalyses the rate of epoxidation of stilbene with perbenzoic acid, but retards the rate of a double bond containing an ester constituent such as ethyl crotonate.20 Recent work has shown that a seven-fold increase in the rate of epoxidation of Z-cyclooctene with m-chloroperbenzoic acid is observed upon addition of the catalyst trifluoroacetic acid.21 Kinetic and theoretical studies suggest that the rate increase is due to complexation of the peroxy acid with the undissociated acid catalyst (HA) rather than protonation of the peroxy acid. Ab initio calculations have shown that the free energy of ethylene with peroxy-formic acid is lowered by about 3 kcal mol-1 upon complexation with the catalyst.21... 

FeCl(Tp,Bu)], prepared from the reaction of Tpefficient catalytic activity with high selectivity and reaction rate for epoxidation by using 02 with isobutyralde-hyde as co-reductant.84... 

Radical epoxidation. In the presence of this radical cation, iodosylbcnzcnc can effect epoxidation of stilbene, but only in low yield because of decomposition of the catalyst. Selenium dioxide (Se02) is a suitable oxidant for this catalyzed epoxidation, but suffers from low solubility in CH2CI2 at 0°. The most useful... 

More recently 58 was synthesised in our laboratory via a three step reaction scheme by the epoxidation of stilbenes (271), followed by deprotection (272) and acid catalyzed cyclization (273) as shown in Fig. 

The product of the reaction with c/s-stilbene was a trichloroacetate ester which yielded the wcso-glycol upon reduction with lithium aluminium hydride the /raHs-olefin yielded the racemic glycol after the same treatment . Such reaction products are consistent with ring-opening of the epoxide by strong acid, and it was shown that (-)-rraHs-stilbene oxide reacted rapidly (relative to the rate of epoxidation) with trichloroacetic acid in benzene the overall observations are indicative of acid-catalysed epoxidation of stilbene but may admit other interpretations, one of which involves thermodynamic intervention of trichloroacetic acid dimer in a solvating capacity rather than in a truly kinetic function. 

Table 8 compares substituent effects in the epoxidation of stilbenes with those in the oxidation of nitrosobenzenes. 

Several other groups have reported effective dioxirane systems employing Oxone as the terminal oxidant. For example, Armstrong et al. have developed a spirocyclic iV-carbethoxy-azabicyclo[3.2.1]octanone precatalyst, which affords up to 91.5% ee in the epoxidation of stilbenes (eq 102). Shing et al. have developed an arabinose-derived ketone and employed this in the enantioselective synthesis of the Taxol side chain however, enantioselectivities for the epoxidation were only up to 68% (eq 103). Bortolini et al. have also described the epoxidation of alkenes with the stoichiometric keto bile acid-Oxone system, a range of ee values were observed over several substrate types but up to 98% was observed for the epoxidation of tran -stilbene, although the yield was only 50% (eq 104). ... 

C) is critical in achieving superior catalytic performance (TOP of 42 based on conversion of stilbene, the best result to date in the aerobic epoxidation of stilbene). High stereoselectivity can be achieved in this reaction, although the use of an initiator (TBHP) was required. The proposed mechanism and experimental data support the notion that mostly Oj from the air was used during the process [170]. 

Epoxidation of stilbenes and other alkenes occupies a great deal of attention. Asymmetric epoxidation is an industrially important method for synthesizing epoxides from readily available olefins. In particular, the use of coordination complexes of transition metals as catalysts is of abiding importance, as it proffers an effective possibility for the synthesis of enantiomericaHy pure compounds ((19, 20] references therein). The manganese(111) complex with a diamide ligand was found to catalyze both the epoxidation of (Z)- and ( )-sti]benes with high conversion and the oxidation of benzyl alcohol to benzaldehyde (Figure 2.3) [21]. 

Several cyclodextrin ketones with a ketone attached to the secondary face of the cyclodextrin in the form of a 2,3-0-( 1,3-acetone) group (79), and some selected cyclodextrin ketones having the ketone at the primary face, were investigated for their catalysis of epoxidation of stilbenes and styrene by oxone in 1 1 acetonitrile/H20. It was found that secondary face ketones were better catalysts giving a cat uncat over 10, and more stereoselective giving up to 76% ee in (5)-styrene oxide. ... 

Lignier, R, Mangematin, S., Morfin, R, et al (2008). Solvent and Oxidant Effects on the Au/Ti02-Catalyzed Aerobic Epoxidation of Stilbene, Catal. Today, 138, pp. 50-54. 

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