Peroxy acid

Epoxides are very easy to prepare via the reaction of an alkene with a peroxy acid This process is known as epoxidation... 

A commonly used peroxy acid is peroxyacetic acid (CH3CO2OH) Peroxyacetic acid is normally used m acetic acid as the solvent but epoxidation reactions tolerate a variety of solvents and are often earned out m dichloromethane or chloroform... 

Peroxy acid and alkene Transition state for oxygen transfer from the OH group of the peroxy acid to the alkene Acetic acid and epoxide ... 

Epoxidation of alkenes with peroxy acids is a syn addition to the double bond Substituents that are cis to each other in the alkene remain cis in the epoxide substituents that are trans in the alkene remain trans m the epoxide... 

As shown m Table 6 4 electron releasing alkyl groups on the double bond increase the rate of epoxidation This suggests that the peroxy acid acts as an electrophilic reagent toward the alkene... 

Epoxidation (Section 6 18) Peroxy acids transfer oxygen to the double bond of alkenes to yield epoxides The reaction IS a stereospecific syn addition... 

In this example addition to the double bond of an alkene converted an achiral mol ecule to a chiral one The general term for a structural feature the alteration of which introduces a chirality center m a molecule is prochiral A chirality center is introduced when the double bond of propene reacts with a peroxy acid The double bond is a prochi ral structural unit and we speak of the top and bottom faces of the double bond as prochiral faces Because attack at one prochiral face gives the enantiomer of the com pound formed by attack at the other face we classify the relationship between the two faces as enantiotopic... 

When (R) 3 buten 2 ol is treated with a peroxy acid two stereoisomenc epoxides are formed in a 60 40 ratio The minor stereoisomer has the structure shown... 

Epoxidation of alkenes by reaction with peroxy acids... 

The following section describes the preparation of epoxides by the base promoted ring closure of vicinal halohydrms Because vicinal halohydrms are customarily prepared from alkenes (Section 6 17) both methods—epoxidation using peroxy acids and ring closure of halohydrms—are based on alkenes as the starting materials for preparing epoxides... 

Peroxy acids usually in dichloromethane as the solvent are also reliable reagents for converting sulfides to sulfoxides... 

One equivalent of a peroxy acid or of hydrogen peroxide converts sulfides to sulf oxides two equivalents gives the corresponding sulfone... 

Alkene Peroxy acid Epoxide Carboxylic acid... 

Section 16 16 Oxidation of sulfides yields sulfoxides then sulfones Sodium metaper lodate IS specific for the oxidation of sulfides to sulfoxides and no fur ther Hydrogen peroxide or peroxy acids can yield sulfoxides (1 mole of oxidant per mole of sulfide) or sulfone (2 moles of oxidant per mole of sulfide)... 

Peroxy acids have been seen before as reagents for the epoxidation of alkenes (Section 6 18)... 

The reaction of ketones with peroxy acids is both novel and synthetically useful An oxygen from the peroxy acid is inserted between the carbonyl group and one of the attached car bons of the ketone to give an ester Reactions of this type were first described by Adolf von Baeyer and Victor Vilhger m 1899 and are known as Baeyer—Villiger oxidations... 

Step 1 The peroxy acid adds to the carbonyl group of the ketone This step is a nucleophilic addition analogous to gem diol and hemiacetal formation... 

The product (6 hexanohde) is a cyclic ester or lactone (Section 19 15) Like the Baeyer-Vilhger oxidation an oxygen atom is inserted between the carbonyl group and a carbon attached to it But peroxy acids are not involved m any way the oxidation of cyclohexanone is catalyzed by an enzyme called cyclohexanone monooxygenase with the aid of certain coenzymes... 

Section 17 16 The oxidation of ketones with peroxy acids is called the Baeyer-Vilhger oxidation and is a useful method for preparing esters... 

The diminished rr electron density m the double bond makes a p unsaturated aide hydes and ketones less reactive than alkenes toward electrophilic addition Electrophilic reagents—bromine and peroxy acids for example—react more slowly with the carbon-carbon double bond of a p unsaturated carbonyl compounds than with simple alkenes... 

Its p/Ca IS 8 2 versus 4 7 for acetic acid Why are peroxy acids weaker than car boxylic acids ... 

Baeyer-Villiger oxidation (Section 17 16) Oxidation of an aldehyde or more commonly a ketone with a peroxy acid The product of Baeyer-Vilhger oxidation of a ketone is an ester... 

Epoxidation (Section 6 18) Conversion of an alkene to an epoxide by treatment with a peroxy acid... 

Conversion of Aromatic Rings to Nonaromatic Cyclic Structures. On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically ate converted to o- and -quinoid stmctures and oxinane derivatives of quinols. Because of thein relatively high reactivity, these stmctures often appear as transient intermediates rather than as end products. Further reactions of the intermediates lead to the formation of catechol, hydroquinone, and mono- and dicarboxyhc acids. 

Oxygen donors like peroxy acids, ozone, and pyridine IV-oxides cause carbon-carbon cleavage, perhaps by formation of a perepoxide (43 Scheme 30) (81JCS(P1)1871). Other oxidants have also been reported to react with oxiranes (64HC( 19-1)228). 

Recently (79MI50500) Sharpless and coworkers have shown that r-butyl hydroperoxide (TBHP) epoxidations, catalyzed by molybdenum or vanadium compounds, offer advantages over peroxy acids with regard to safety, cost and, sometimes, selectivity, e.g. Scheme 73, although this is not always the case (Scheme 74). The oxidation of propene by 1-phenylethyl hydroperoxide is an important industrial route to methyloxirane (propylene oxide) (79MI5501). 

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