Download Chapter 14 Selenium reagents

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(Ⅳ) reagents find use as oxidizing agents
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 General features
• 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)
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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 of allylic 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 to
an 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(Ⅳ) hydride, Ph3SnH: expensive and air sensitive
– Nickel boride, produced in situ by reaction of nickel chloride and
sodium borohydride.
•
Synthetic applications:
– Reductive alkylation of aldehydes and ketones
– Formation of reduced heterocycles
– Oxidation of alkenes to ketones
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Selenium(Ⅳ) 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(Ⅳ) reagents find use as oxidizing agents, such as selenium(Ⅳ)
oxide, benzeneseleninic acid (in combination with hydrogen peroxide) and
benzeneseleninic anhydride.
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