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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 agentsⅣ

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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 o

xidants may be hydrogen peroxide, peroxy acids, sodium periodate and oz

one.

Selenoxides with a -hydrogen can readily undergo thermal eliminationre

action to generate alkene.

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

synthesis of allylic alcohols.

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

an in the oxidation-elimination stage.

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Conversion of Carbonyl compounds to ,-unsaturated Ca

rbonyl 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 se

lenoxide 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 to

an allylic oxidation, with a rearrangement of the double bond.

Selenide from addition of aryselenenyl halide to alkene can also r

eact 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 excha

nge

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Hydrogenolysis of carbon-selenium bonds

• Reagents for hydrogenolysis of carbon-selenium bonds

– Raney nickel

– Lithium in ethylamine

– Triphenyltin(Ⅳ) hydride, Ph3SnH: expensive and air sensitive

– Nickel boride, produced in situ by reaction of nickel chloride and s

odium 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 Ⅳagents

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 sele

nocyanates, 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 se

lenium reagents and carbon ncleophiles or from nucleophilic selenium re

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

elenoxides; 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 alco

hols.

-selenoaldehydes undergo condensation reactions, and a double bond m

ay then be introduced in the product by oxidation at the selenium atom fo

llowed by elimination.

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Allylic selenides are convertible into allyl-lithium reagents for further reacti

o with electrophiles.

Hydrogenolysis of carbon-selenium bonds is achievable using catalytic meth

ods, dissolving metals, triaryltin hydrides and ‘nickel boride’.

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

r treatment with organolithium reagents. Highly reactive cycloalkynes are prepa

ragble in this way.

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

ide, benzeneseleninic acid (in combination with hydrogen peroxide) and benzen

eseleninic anhydride.