123.202 lecture 9 - alkenes
DESCRIPTION
123.202 lectures lecture 9 alkenesTRANSCRIPT
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©skycaptaintwo@flickralkenes III
LECTURE NINEN
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PREVIOUSLY...
hydroboration (and hydration)
i. BH3ii. H2O2/ NaOH H
OH
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➏addition of bromine
BrBr
Br Br
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Br Br
Br
Br
racemic mixture of enantiomers
Br Br
Br
Br
a meso compound
stereochemistry
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step 1mechanism
Br:Br Br
H H
bromonium ion
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step 2mechanism
Br
H HBr
Br
Br HHSN2
antiattack
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but...
attackseither end
Br
H HBr
Br
Br HH
SN2
Br
BrHH
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Br
Br HH Br
BrHH
BrH
Br H
HBr
H Br
mirrorplane
(S,S) (R,R)
enantiomers
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step 1mechanism
bromonium ion
Br:Br Br
HH
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Br
HH
BrBr
Br H
HSN2
step 2mechanism
antiattack
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but...
attackseither end
Br
HH
Br
Br
Br H
H
SN2
Br
BrH
H
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meso
Br
Br H
H
(S,R)
Br
BrH
H
(R,S)
Br Br
HH
Br Br
H H
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Br Br
HH
mirrorplane
meso
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©Heidi & Matt@flickr
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HBr Br
Br
Br Br
HH
the mechanismnot
of all stereoisomersmixture
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epoxides➐diol formation from alkenes:
addition
mCPBA
mCPBA
O
H H
O
HH
Cl
O
OO
H
mCPBA
stereospecific
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©Oberazzi@flickr
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mechanismconcerted
OO
OH
R
OO
OH
R
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diol formationO
HH
H3O O
HH
H
OH
H
HO
H O
H
HH
HO
H OH
H
antinormallyaddition
SN2
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but...
O
H HOH
H
HO
H
HHO
OH
HHO
H
OH
mixturecan give SN1
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cation
stability
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O
HH
O
H OH
H
HO
H OH
H
OH
OHH
antiexclusively
addition
base-mediated
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example
O OH O
O
HO N
S
epothilone A
O
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example
O OH O
O
HO N
S
epothilone A
O
O OH O
O
HO N
S
epothilone C
OO
CF3
85%(5:1)
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stereospecific
KMnO4➑diol formation from alkenes:
addition
H H
HH
KMnO4
KMnO4
HO OH
HO OH
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KMnO4➑diol formation from alkenes:
H H
HH
KMnO4
KMnO4
HO OH
HO OHmeso
stereospecificaddition
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KMnO4➑diol formation from alkenes:
H H
HH
KMnO4
KMnO4
HO OH
HO OHmeso
stereospecificaddition
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stereospecific
KMnO4➑diol formation from alkenes:
addition
H H
HH
KMnO4
KMnO4
HO OH
HO OH
racemate
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©Ninja M.@flickr
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mechanismconcerted
stereospecific additionsyn
OMn
O
O O
OMn
O
O O
HH
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mechanism
in situhydrolysis
H2O HO OHO
MnO
O O
HHH H
MnO2
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©A Hermida@flickr
HNOH
OH
O
O OH
O
H
CO2H
OMe
H
dynemicin A
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N
OTBSOTBS
OsO4(cat.), NMO
98%
N
OTBSOTBS
OH
OH
example