Epoxidations of electron-rich olefins

In this example the solvent - a fluorinated alcohol - forms higher order aggregates and activates for the epoxidation of electron rich olefins. HFIP accelerates this oxidation reaction up to 100,000-fold (relative to that in 1,4-dioxane as solvent). Which hydrogen bond network involving olefin, and fluorinated alcohol gives rise to such spectacular accelerations ... 

Scheme 13.33 Epoxidation of electron-rich olefins catalysed by peptide catalysts 58 and 59.

Not only oxidants with nucleophilic character but also intermediates of electrophilic nature can be used in the enantioselective epoxidation of a,p-unsaturated carbonyl compounds. Among the possible candidates, dioxirane reagents have been successfully used for this purpose. Contrary to the usual nucleophilic oxidants, dioxiranes add to double bonds in a concerted manner. These dioxiranes could be easily prepared in situ by reaction of oxone (2KHS05-KHS0 K3S0 ) with chiral ketones. Chiral ketones derived from quinic acid such as compounds 61 [67], which have been successfully used in the enantioselective epoxidation of electron rich olefins, have been also applied to the epoxidation of electron poor olefins such as chalcone 7 or a,P-unsaturated esters 59 (Scheme 4.10) to give the corresponding... 

The oxidation of organic substances by cyclic peroxides has been intensively studied over the last decades , from both the synthetic and mechanistic points of view. The earliest mechanistic studies have been carried out with cyclic peroxides such as phthaloyl peroxide , and more recently with a-methylene S-peroxy lactones and 1,2-dioxetanes . During the last 20 years, the dioxiranes (remarkable three-membered-ring cyclic peroxides) have acquired invaluable importance as powerful and mild oxidants, especially the epoxidation of electron-rich as well as electron-poor alkenes, heteroatom oxidation and CH insertions into alkanes (cf. the chapter by Adam and Zhao in this volume). The broad scope and general applicability of dioxiranes has rendered them as indispensable oxidizing agents in synthetic chemistry this is amply manifested by their intensive use, most prominently in the oxyfunctionalization of olefinic substrates. 

Deprotonated peroxide coordinates to metals in low- and intermediate-oxidation states tend to bind side-on and display nucleophilic properties. For example, iron(III) porphyrin-peroxo complexes are nucleophilic they did not transfer an oxygen atom to electron-rich substrates (such as electron-rich olefins), but brought about epoxidation of electron-poor olefins or oxidative deformylation of aldehydes.130 Dinuclear... 

In sharp contrast to the oxidation reactions of electron-rich olefins just described, attempts to carry out nucleophilic epoxidation reactions of a,p-unsaturated carbonyl compounds have enjoyed only limited success (Scheme 8.7) [19]. The most successful attempts have been with chalcones, using standard basic peroxidation conditions with additives such as a quinine-derived phase-transfer catalyst first... 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

The reaction of superoxide ion with carbon tetrachloride is important for olefin epoxidations. This reaction includes the formation of the trichloromethyl peroxide radical Oj" + CCI4 —> Cl + CI3COO. The trichloromethyl peroxide radicals formed oxidize electron-rich olefins. The latter gives the corresponding epoxides. This peroxide radical is a stronger oxidizing agent than the superoxide ion itself (Yamamoto et al. 1986). 

The bismuth ylides, Ph3Bi=CHCOR, do not react with simple ketones and electron-rich olefins probably because of their relatively low electrophilic character. However, Ph3Bi=CHCOR reacts with a-keto esters [46, 67, 68], benzils [46, 67-69], orf/to-quinones [46, 67, 68], and acenaphthenequinone [70] to give epoxides, (9-arovl enolates, 3-hydroxytropones, and 3-hydroxyphenalenones, respectively, accompanied by the formation of Ph3Bi (Scheme 11). In particular, transposition and ring expansion reactions are of interest from a mechanistic point of view, since these reaction modes are unprecedented in ylide chemistry. 

The 2,4,6-triphenylpyrylium terafluoroborate (TPT)-sensitized electron transfer of aryl-substituted epoxides such as 250 leads to ring opening via selective C—O bond cleavage, while subsequent [3 + 2]-cycloaddition of the resultant carbonyl ylide with electron-rich olefins 251 leads to the synthesis of substituted THF derivatives 252 and 253 (Scheme 8.69) [109]. 

TT-cation radical complexes.In the reactions, oxoiron(IV) porphyrin rr-cation radicals of electron-rich porphyrins reacted fast with ROOH (i.e., catalase and peroxidase type of chemistry one-electron oxidation of ROOH) (Scheme 2, pathway A). On the other hand, oxoiron(IV) porphyrin rr-cation radicals of electron-deficient porphyrins reacted fast with olefins to yield epoxide products (i.e., cytochrome P450 type of chemistry oxygen atom transfer) (Scheme 2, pathway B). These results demonstrated that electron-deficient iron porphyrin complexes are better catalysts in hydrocarbon oxygenations by hydroperoxides, since these complexes can avoid the facile decomposition of oxoiron intermediates by ROOH (Scheme 2, pathway A). Indeed, highly electron-deficient iron(III) porphyrin complexes efficiently catalyze alkane hydroxylations by H2O2 in aprotic solvents. 

Second Round of Nucleophilic Addition. Electrophilic epoxidations of 3-benzenesulfonyl-cyclohexa-2,4-dienol, and the protected version, result in stereospecific cis and trans epoxides from the unconjugated, more electron-rich olefin (eq 3). ... 

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