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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.