Alkenes peroxy acids

There have been several computational studies of the peroxy acid-alkene reaction. The proposed spiro TS has been supported in these studies for alkenes that do not present insurmountable steric barriers. The spiro TS has been found for ethene (B3LYP/6-31G ), propene and 2-methylpropene (QCISD/6-31G ), and 2,3-dimethylbutene and norbornene (B3LYP/6-311- -G(c , These computational studies also correctly predict the effect of substiments on the and account for these effects in terms of less synchronous bond formation. This is illustrated by the calculated geometries and a(B3LYP/6-31G ) of the TS for ethene, propene, methoxyethene, 1,3-butadiene, and cyanoethene, as shown in Eigure 5.3. Note that the TSs become somewhat unsymmetrical with ERG substituents, as in propene, methoxyethene, and butadiene. The TS for acrylonitrile with an EWG substituent is even more unsymmetrical and has a considerably shorter C(3)- O bond, which reflects the electronic influence of the cyano group. In this asynchronous TS, the nucleophilic character of the peroxidic oxygen toward the (3-carbon is important. Note also that the is increased considerably by the EWG. 

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 ... 

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... 

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... 

Alkene Peroxy acid Epoxide Carboxylic acid... 

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

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... 

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

Peroxy acid oxidation of alkenes (Sections 6.18 and 16.9) Peroxy acids transfer oxygen to alkenes to yield epoxides. Stereospecific syn addition is observed. 

We will now explore each aspect of this two-step process. In the first step, a peroxyacid (RCO3H), sometimes called a per-acid, reacts with the alkene. Compare the structure of a carboxylic acid with the structure of a peroxy acid ... 

Treatment of 51 with an excess of sodium benzoate in DMF resulted in substitution and elimination, to yield the cyclohexene derivative (228, 36%). The yield was low, but 228 was later shown to be a useful compound for synthesis of carba-oligosaccharides. <9-Deacylation of228 and successive benzylidenation and acetylation gave the alkene 229, which was oxidized with a peroxy acid to give a single epoxide (230) in 60% yield. Treatment of 230 with sodium azide and ammonium chloride in aqueous 2-methoxyeth-anol gave the azide (231,55%) as the major product this was converted into a hydroxyvalidamine derivative in the usual manner. On the other hand, an elimination reaction of the methanesulfonate of 231 with DBU in toluene gave the protected precursor (232, 87%) of 203. 

DL-Valiolamine (205) was synthesized from the exo-alkene (247) derived from 51 with silver fluoride in pyridine. Compound 247 was treated with a peroxy acid, to give a single spiro epoxide (248, 89%) which was cleaved by way of anchimeric reaction in the presence of acetate ion to give, after acetylation, the tetraacetate 249. The bromo group was directly displaced with azide ion, the product was hydrogenated, and the amine acety-lated, to give the penta-A, 0-acetyl derivative (250,50%). On the other hand. 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents and the reactivity of the peroxy acids is increased by EWG substituents.72 These structure-reactivity relationships demonstrate that the peroxyacid acts as an electrophile in the reaction. Decreased reactivity is exhibited by double bonds that are conjugated with strongly electron-attracting substituents, and more reactive peroxyacids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds.73 Electron-poor alkenes can also be epoxidized by alkaline solutions of... 

Scheme 12.11. Synthesis of Epoxides from Alkenes Using Peroxy Acids...

Alkenes can also be epoxidized directly with a variety of organic peroxy acids or related reagents such as peroxy carboximidic acid, RC(NH)OOH, which is readily available through an in situ reaction of... 

The most widely used method for synthesizing epoxides is the reaction of an alkene with an organic peroxy acid (peracid). 

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