Perbenzoic acids 4- olefins

The most common method of epoxidation is the reaction of olefins with per-acids. For over twenty years, perbenzoic acid and monoperphthalic acid have been the most frequently used reagents. Recently, m-chloroperbenzoic acid has proved to be an equally efficient reagent which is commercially available (Aldrich Chemicals). The general electrophilic addition mechanism of the peracid-olefin reaction is currently believed to involve either an intra-molecularly bonded spiro species (1) or a 1,3-dipolar adduct of a carbonyl oxide, cf. (2). The electrophilic addition reaction is sensitive to steric effects. 

Several steroid olefins, especially A -steroids, do not give exclusive a-epoxidation. The a-oxirane usually predominates in the epoxidation mixture, its proportion varying from 50% to 90% or greater when either perbenzoic acid or monoperphthalic acid is employed. The claims that the ratio of a- to p-epoxide is high in compounds containing a keto group d or a j5-substituent at may be misleading since epoxidation of 17a,20 20,... 

Hirst and Woolvin169 preferentially methylated positions 3, 4 and 6 by the use of glucal, an unsaturated sugar derivative having a 1,2-olefinic bond, introduced by the reduction of triacetyl-a-D-glucopyranosyl bromide with zinc dust and acetic acid. By the action of an ethereal solution of perbenzoic acid on an aqueous solution of 3,4,6-trimethyl-glucal a sirupy product was obtained. This was shown to contain... 

The evidence for the mechanism is that it is a second order reaction. Further, since the reaction proceeds readily in nonpolar solvents, hence the formation of ions from perbenzoic acid, as postulated earlier, is inhibited. The final evidence is that the reaction is stereospecific and proceeds with the retention of cis or trans configurations present in the initial olefine. 

Another reaction in which epoxy ethers have been suggested lo be intermediates is the oxidation of enol ethera with perbenzoic acid. This topic need only be mentioned briefly here, since olefin oxidation with perbenzoie acid has been taken up in greater detail in section III.1..I. 

Perbenzoic acid is used for the conversion of olefinic compounds into epoxides. 

The number of olefinic linkages in a given compound can be established with accuracy by quantitative titration with perbenzoic acid. A solution of the substance and excess perbenzoic acid in chloroform is allowed to stand for several hours at a low temperature and the amount of unreacted perbenzoic acid in solution is determined a blank experiment is run simultaneously. 

Perbenzoic acid is used for the conversion of olefinic compounds into epoxides. y= + Ph-CO-02H ------------ Qr + Ph COjH... 

Perbenzoic acid is an important reagent for the preparation of epoxides from olefinic compounds (method 126). When the epoxides are unstable in aqueous solution, glycols ate formed directly. The over-all reaction results in trans addition of hydrcscy groups to the double bond for crotonic and isocrotonic acids. ... 

Olefinic compounds are conveniently converted to epoxy compounds by treatment with an organic peracid, commonly perbenzoic acid or peracetic acid in chloroform solution at 0-5°. The preparation of perbenzoic acid has been described. Performic and monoperphthalic acids have also been successfully employed. The reaction has been reviewed. ... 

Other Addition Reactions.—A quantitative study of the epoxidation of 3-substituted cholest-5-enes with peroxy-acid shows that both the rate and the epimer ratio vary according to the C(3)-substituent. " The epoxidation clearly has some electrophilic character. o-Sulpho-perbenzoic acid, which may be used in aqueous-organic solvents, converted cholesterol efficiently into the a-epoxide (89%). The A -olefinic bond in cholestan-5,16-dien-3 -ol is sufficiently reactive, perhaps as a consequence of ring strain, to permit selective 16a, 17a-epoxidation. " ... 

Uses. The reagent is used in the same way as perbenzoic acid, mainly for the conversion of olefins into epoxides. However, as compared to perbenzoic acid in chloroform, perphthalic acid in ether has the advantage that the course of a reaction can be followed by noting the size of the precipitate of phthalic acid formed, or by filtering the solution and seeing if any more phthalic acid separates. One can note also if a solution stored in a refrigerator in a stoppered flask deteriorates on standing. 

The reagent is prepared by gradual addition of p-nitrobenzoyl chloride to a stirred suspension of sodium peroxide in tetrahydrofurane at —20 to —5 in the presence of a catalytic amount of frozen water. This peracid, a crystalline solid completely stable at room temperature, is 7-20 times as reactive as perbenzoic acid in the epoxidation of olefins. 

Reaction with olefins. The reaction of perbenzoic acid with an olefin usually proceeds smoothly at a low temperature (0-25°) and affords an epoxide in high yield. The olefin is dissolved at room temperature in a solution of perbenzoic acid in chloroform or benzene, with use of more solvent if required, and the solution is lei stand at room temperature or below for several hours. lodimetric titration can be used to follow the course of a reaction or, by using an excess of reagent, to determine the total uptake of reagent and hence the number of double bonds present. 

Mark and Rechnitz [3] systematized a vast amount of experimental material that can be used directly in KGCM. Some data are presented here that show the wide differences in organic compounds with regard to their kinetic characteristics. Table 2.1 [14] gives the relative rates of reaction of olefins with perbenzoic acid and Table 2.2 summarizes the rates of the etherification reaction of carboxylic acids with diphenyldiazomethane [15]. The tabulated data are indicative of large differences in organic compounds as far as their reactivity is concerned. The rates of reaction of some isomers differ so widely that one can, for example, analyse secondary and tertiary alkyl bromides in the presence of primary alkyl bromides in a reaction with silver nitrate [16]. It is possible to differentiate between CIS and trans isomers of 1,3-dienes by their reaction with dienophils (e.g., chloromethylene anhydride) because the cis isomer reacts much more slowly than the trans isomer [17]. 

RELATIVE RATES OF REACTION OF OLEFINS WITH PERBENZOIC ACID... 

Carbonyldi-l, 2,4-triazole (2,61) can be used in place of benzoyl isocyanate. In this case the epoxidation reagent is believed to have structure (2). This reagent epoxidizes olefins much more rapidly than perbenzoic acid. The hydro-... 

Boeseken et a/. first studied the kinetics of such reactions the first report dealt with the reaction of perbenzoic acid with derivatives of styrene in chloroform solution and despite the ease of decomposition of the reagent, styrene was proved to react less rapidly than /3-methyl- or /3-ethyl-styrene. In the second paper, the rates of reaction of a number of olefins with peracetic acid in acetic acid were shown to increase with substitution of the olefinic bond, viz. 

Many mechanisms have been proposed for this reaction, such as the epoxidation via initial attack of a hydroxyl cation, 1,3-Dipolar Cycloaddition of a hydroxycarbonyl oxide to an olefinic double bond, and the commonly accepted planar butterfly transition state, " by which the n HOMO orbital of the olefin approaches the terminal oxygen of perbenzoic acid and interacts with the a LUMO of the 0-0 bond at 180". The planar butterfly transition state is further extended by Sharpless to a spiro-trunsition state, which has been consolidated by many other investigators. An illustrative mechanism from mCPBA epoxidation is provided here. 

The reaction has been modified to avoid the application of perbenzoic acid by treating the solutions of olefins with air or oxygen in the presence of benzaldehyde so that perbenzoic... 

An oxy group, when acting as an acceptor (e.g., RO ), is soft. Therefore, it is easily understood why peroxycarboxylic acids are thermodynamically unstable and reactive molecules. Moreover, the stronger (harder) the carboxylic acid, the more reactive its corresponding peracid is since there exists a wider softness gap between OH and RCOO . Thus, trifluoroperacetic acid is a more potent oxidant than peracetic acid, and triazole-1-peroxycarboxylic acid (2) is about two hundred times more reactive than perbenzoic acid toward olefins. 

Weisenborn, F. L., and D. Taub The Reaction of Perbenzoic Acid with certain Olefins. J. Amer. Chem. Soc. 74,1329 (1952). 

Authors of works made also rather essential supervision connected to rather smaller or greater oxidability of double bonds, caused by influence of the neighboring substitutions, (oxymethylene groups, aldehyde and ketone groups) which is testified by publications of different years [7-9]. From peracids for oxidation of olefins apply peracetic acid [7, 10], pertri-fluoroacetic acid [11], m-chloroperbenzoic acid [10, 12] and perbenzoic acid [4, 13]. 

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