©skycaptaintwo@flickralkenes III

LECTURE NINEN

PREVIOUSLY...

hydroboration (and hydration)

i. BH3ii. H2O2/ NaOH H

OH

➏addition of bromine

BrBr

Br Br

Br Br

Br

Br

racemic mixture of enantiomers

Br Br

Br

Br

a meso compound

stereochemistry

step 1mechanism

Br:Br Br

H H

bromonium ion

step 2mechanism

Br

H HBr

Br

Br HHSN2

antiattack

but...

attackseither end

Br

H HBr

Br

Br HH

SN2

Br

BrHH

Br

Br HH Br

BrHH

BrH

Br H

HBr

H Br

mirrorplane

(S,S) (R,R)

enantiomers

step 1mechanism

bromonium ion

Br:Br Br

HH

Br

HH

BrBr

Br H

HSN2

step 2mechanism

antiattack

but...

attackseither end

Br

HH

Br

Br

Br H

H

SN2

Br

BrH

H

meso

Br

Br H

H

(S,R)

Br

BrH

H

(R,S)

Br Br

HH

Br Br

H H

Br Br

HH

mirrorplane

meso

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HBr Br

Br

Br Br

HH

the mechanismnot

of all stereoisomersmixture

epoxides➐diol formation from alkenes:

addition

mCPBA

mCPBA

O

H H

O

HH

Cl

O

OO

H

mCPBA

stereospecific

©Oberazzi@flickr

mechanismconcerted

OO

OH

R

OO

OH

R

diol formationO

HH

H3O O

HH

H

OH

H

HO

H O

H

HH

HO

H OH

H

antinormallyaddition

SN2

but...

O

H HOH

H

HO

H

HHO

OH

HHO

H

OH

mixturecan give SN1

cation

stability

O

HH

O

H OH

H

HO

H OH

H

OH

OHH

antiexclusively

addition

base-mediated

example

O OH O

O

HO N

S

epothilone A

O

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)

stereospecific

KMnO4➑diol formation from alkenes:

addition

H H

HH

KMnO4

KMnO4

HO OH

HO OH

KMnO4➑diol formation from alkenes:

H H

HH

KMnO4

KMnO4

HO OH

HO OHmeso

stereospecificaddition

KMnO4➑diol formation from alkenes:

H H

HH

KMnO4

KMnO4

HO OH

HO OHmeso

stereospecificaddition

stereospecific

KMnO4➑diol formation from alkenes:

addition

H H

HH

KMnO4

KMnO4

HO OH

HO OH

racemate

©Ninja M.@flickr

mechanismconcerted

stereospecific additionsyn

OMn

O

O O

OMn

O

O O

HH

mechanism

in situhydrolysis

H2O HO OHO

MnO

O O

HHH H

MnO2

©A Hermida@flickr

HNOH

OH

O

O OH

O

H

CO2H

OMe

H

dynemicin A

N

OTBSOTBS

OsO4(cat.), NMO

98%

N

OTBSOTBS

OH

OH

example