Epoxidation, alkenes epoxides

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

Three membered rings that contain oxygen are called epoxides At one time epox ides were named as oxides of alkenes Ethylene oxide and propylene oxide for exam pie are the common names of two industrially important epoxides... 

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

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

Relative Rates of Epoxidation of Some Representative Alkenes with Peroxyacetic Acid... 

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... 

The mechanism of alkene epoxidation is believed to be a concerted process mvolv mg a single bimolecular elementary step as shown m Figure 6 14... 

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

Epoxidation of alkenes is a stereospecific syn addition Which stereoisomer of 2 butene reacts with peroxyacetic acid to give meso 2 3 epoxybu tane Which one gives a racemic mixture of (2/ 3/ ) and (25 35) 2 3 epoxybutane ... 

Epoxidation of alkenes (Sec tion 6 18) IS another example of a cycloaddition... 

Ethylene glycol and propy lene glycol are prepared industrially from the corre spending alkenes by way of their epoxides Someapplica tions were given in the box in Section 6 21... 

A complementary method one that gives anti hydroxylation of alkenes by way of the hydrolysis of epoxides will be described m Section 16 13... 

Epoxidation of alkenes by reaction with peroxy acids... 

Epoxidation of alkenes was discussed m Section 6 18 and is represented by the general equation... 

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... 

The formation of vicinal halohydrms from alkenes was described m Section 6 17 Halo hydrins are readily converted to epoxides on treatment with base... 

Overall the stereospecificity of this method is the same as that observed m per oxy acid oxidation of alkenes Substituents that are cis to each other m the alkene remain CIS m the epoxide This is because formation of the bromohydrm involves anti addition and the ensuing intramolecular nucleophilic substitution reaction takes place with mver Sion of configuration at the carbon that bears the halide leaving group... 

Epoxidation of an alkene followed by lithium aluminum hydride reduction of the result mg epoxide gives the same alcohol that would be obtained by acid catalyzed hydration (Section 610) of the alkene... 

A method for achieving net anti hydroxylation of alkenes combines two stereo specific processes epoxidation of the double bond and hydrolysis of the derived epoxide... 

Many naturally occurring substances are epoxides You have seen two examples of such compounds already m disparlure the sex attractant of the gypsy moth (Section 6 18) and m the carcinogenic epoxydiol formed from benzo[a]pyrene (Section 118) In most cases epoxides are biosynthesized by the enzyme catalyzed transfer of one of the oxy gen atoms of an O2 molecule to an alkene Because only one of the atoms of O2 is trans ferred to the substrate the enzymes that catalyze such transfers are classified as monooxy genases A biological reducing agent usually the coenzyme NADH (Section 15 11) is required as well... 

Alkene Peroxy acid Epoxide Carboxylic acid... 

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

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

The alkene is allowed to react at low temperatures with a mixture of aqueous hydrogen peroxide, base, and a co-solvent to give a low conversion of the alkene (29). These conditions permit reaction of the water-insoluble alkene and minimise the subsequent ionic reactions of the epoxide product. Phase-transfer techniques have been employed (30). A variation of this scheme using a peroxycarbimic acid has been reported (31). 

In general, peroxomonosulfates have fewer uses in organic chemistry than peroxodisulfates. However, the triple salt is used for oxidizing ketones (qv) to dioxiranes (7) (71,72), which in turn are useful oxidants in organic chemistry. Acetone in water is oxidized by triple salt to dimethyldioxirane, which in turn oxidizes alkenes to epoxides, polycycHc aromatic hydrocarbons to oxides and diones, amines to nitro compounds, sulfides to sulfoxides, phosphines to phosphine oxides, and alkanes to alcohols or carbonyl compounds. 

---