selinium reagent

8/2/2019 Selinium Reagent

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Chapter 14 Selenium reagents

Functional group interconversion: alcohols into bromide

syn-Elimination from selenoxides

Allylic selenoxide and selenide

-Selenoaldehydes

Hydrogenolysis of carbon-selenium bonds

Selenium() reagents find use as oxidizing agents


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A multitalented element: selenium reagents offer

numerous possibilities in organic synthesis

Commercially available selenium reagents including:

Potassium selenocyanate, KSeCN

Areneselenols, ArSeH

Diary diselenides, ArSeSeAr

Areneselenyl halides, ArSeX (X = Cl, Br or I)

General features


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Functional group interconversion: alcohols

into bromide


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syn-Elimination from selenoxides

Selenoxide can be obtained by oxidation of corresponding selenide. The

oxidants may be hydrogen peroxide, peroxy acids, sodium periodate and

ozone.

Selenoxides with a -hydrogen can readily undergo thermal

eliminationreaction to generate alkene.

Using this procedure, we can achieve conversion of ketones to enones and

synthesis ofallylic alcohols.

The variants of the procedure is in the preparation of the selenide rather

than in the oxidation-elimination stage.


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Conversion of Carbonyl compounds to ,-unsaturated

Carbonyl compounds by Selenoxide Syn Elimination

Preparation of selenides

From an electrophilic selenium reagent and a carbon nucleophile.

From a nucleophilic selenium reagent and a carbon electrophile

From a simpler selenid


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Example


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Preparation of allylic alcohols and -halogenoalkenes by

selenoxide syn-elimination

Preparation of selenide

From addition of benzeneselenenic acid to alkene.

From addition of arylselenenyl halide to alkene.

For synthesis of allylic alcohols, the overall reaction amounts toan allylic oxidation, with a rearrangement of the double bond.

Selenide from addition of aryselenenyl halide to alkene can also

react with nucleophilic functional groups. For alkenes containing

suitably positioned nucleophilic functional groups may undergo

cyclization.


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Unimolecular syn-Eliminations ( Pyrolytic syn-elimination)


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Allylic selenoxide and selenide

Preparation of allylic selenides

Reaction of an allyl halide with a selenide anion

Alkylation of an allylselenide anion

By a wittig reaction

The allyl selenoxide rearrangement

Reaction with trialkylboranes: synthesis of-hydrogen alkene

Reaction with alkyl-lithium reagents: selenium-lithium

exchange


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Hydrogenolysis of carbon-selenium bonds

Reagents for hydrogenolysis of carbon-selenium bonds

Raney nickel

Lithium in ethylamine

Triphenyltin() hydride, Ph3SnH: expensive and air sensitive

Nickel boride, produced in situ by reaction of nickel chloride andsodium borohydride.

Synthetic applications:

Reductive alkylation of aldehydes and ketones

Formation of reduced heterocycles

Oxidation of alkenes to ketones


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Selenium() reagents find use as oxidizing agents

Selenium dioxide as oxidizing reagent

Ketones containing an -methyllene are oxidized to diketones

Elimination of 1,2,3-Selenadiazole

Oxidation using benzeneseleninic acid

Oxidation using benzeneseleninic anhydride


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Summary

Alcohols are converted into alkyl aryl selenides by reaction with aryl

selenocyanates, ArSeCN. These react with bromine in the presence of a

base, giving alkyl bromides: the overall reaction is ROH RBr with

retention of configuration.

Aryl alkyl selenides are preparable either (as above) from electrophilic

selenium reagents and carbon ncleophiles or from nucleophilic selenium

reagents, e.g. ArSe-Na+, and carbon electrophiles. On oxidation they give

selenoxides; if these contain a-hydrogen, they may undergo

spontaneous syn-addition at ambient temp. to give alkenes. Allyl

selenoxides undergo rearrangement to allyl selenenates, which are

hydrolysable to allylic alcohols.

-selenoaldehydes undergo condensation reactions, and a double bond

may then be introduced in the product by oxidation at the selenium atom

followed by elimination.


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Allylic selenides are convertible into allyl-lithium reagents for further

reactio with electrophiles.

Hydrogenolysis of carbon-selenium bonds is achievable using catalytic

methods, dissolving metals, triaryltin hydrides and nickel boride.

1,2,3-Selenadiazole undergo elimination, giving alkynes, either on heating or

treatment with organolithium reagents. Highly reactive cycloalkynes are

preparagble in this way.

Selenium() reagents find use as oxidizing agents, such as selenium()

oxide, benzeneseleninic acid (in combination with hydrogen peroxide) and

benzeneseleninic anhydride.

selinium reagent

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