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123.202 Lecture 9 - alkenes
 

123.202 Lecture 9 - alkenes

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123.202 lectures

123.202 lectures
lecture 9
alkenes

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    123.202 Lecture 9 - alkenes 123.202 Lecture 9 - alkenes Presentation Transcript

    • LECTURE NINE alkenes III ©skycaptaintwo@flickr
    • PREVIOUSLY... i. BH3 ii. H2O2/ NaOH H OH hydroboration (and hydration)
    • addition of bromine Br Br Br Br
    • Br Br Br Br stereochemistry racemic mixture of enantiomers Br Br Br Br a meso compound
    • mechanism step 1 Br Br :Br H H bromonium ion
    • mechanism step 2 Br Br SN2 H Br H H H Br anti attack
    • but... Br H H Br Br SN2 Br H H Br H H Br attacks either end
    • mirror plane Br Br H H H H Br Br enantiomers Br H H Br H Br Br H (S,S) (R,R)
    • mechanism step 1 Br Br :Br H H bromonium ion
    • mechanism step 2 Br Br SN2 H Br H Br H H anti attack
    • but... Br H Br Br H SN2 Br H Br H H H Br attacks either end
    • Br H Br H H Br Br H meso Br Br Br Br H H H H (R,S) (S,R)
    • meso mirror plane Br Br H H
    • ©Heidi & Matt@flickr
    • the not mechanism Br Br Br H mixture Br Br of all stereoisomers H H
    • from alkenes: diol formation epoxides O mCPBA H H O mCPBA O H O H H O stereospecific Cl mCPBA addition
    • ©Oberazzi@flickr
    • concerted mechanism R R O O H O H O O O
    • diol formation H O H H3O O O H H H H H normally anti addition SN2 HO HO H H H H O H OH H
    • but... H H O HO O H H H can give SN1 mixture H HO OH HO O H H H
    • cation stability
    • O O H H OH H H OH H H O base-mediated HO exclusively an H ti addition H OH
    • O S HO N O O OH O epothilone A example
    • S HO N O O OH O epothilone C CF3 example O O 85% (5:1) O S HO N O O OH O epothilone A
    • from alkenes: diol formation KMnO4 HO OH KMnO4 H H HO OH KMnO4 H H stereospecific addition
    • from alkenes: diol formation KMnO4 HO OH KMnO4 H H meso HO OH KMnO4 H H stereospecific addition
    • from alkenes: diol formation KMnO4 HO OH KMnO4 H H meso HO OH KMnO4 H H stereospecific addition
    • from alkenes: diol formation KMnO4 HO OH KMnO4 H H HO OH KMnO4 H H stereospecific additie n racemat o
    • ©Ninja M.@flickr
    • concerted mechanism O O O O Mn Mn O O O O H H syn addition stereospecific
    • mechanism O O Mn H2O HO OH O O MnO2 H H H H hydrolysis in situ
    • H CO2H OH O HN O OMe H OH O OH dynemicin A ©A Hermida@flickr
    • example OH N OsO4(cat.), N NMO OH 98% OTBS OTBS OTBS OTBS