123.202 lectures lecture 9 alkenes

1. LECTURE NINE
alkenes III ©skycaptaintwo@flickr

2. PREVIOUSLY...
i. BH3
ii. H2O2/
NaOH H
OH
hydroboration (and hydration)

3. addition of bromine
Br Br Br
Br

4. Br
Br Br
Br
stereochemistry racemic mixture
of enantiomers
Br
Br Br
Br
a meso
compound

5. mechanism
step 1
Br
Br
:Br
H H
bromonium ion

6. mechanism
step 2
Br Br
SN2 H
Br
H H H
Br
anti
attack

7. but...
Br
H
H
Br Br
SN2
Br
H H Br
H
H
Br
attacks
either end

8. mirror
plane
Br Br
H H
H H
Br Br
enantiomers
Br H H Br
H Br Br H
(S,S) (R,R)

9. mechanism
step 1
Br
Br
:Br
H
H
bromonium ion

10. mechanism
step 2
Br Br
SN2 H
Br
H Br H
H
anti
attack

11. but...
Br
H
Br Br H
SN2
Br
H Br H
H
H
Br
attacks
either end

12. Br H Br
H
H
Br Br H
meso
Br Br Br Br
H H
H H
(R,S) (S,R)

13. meso mirror
plane
Br Br
H H

14. ©Heidi & Matt@flickr

15. the
not mechanism
Br
Br Br
H
mixture Br Br
of all stereoisomers
H H

16. from alkenes:
diol formation epoxides
O
mCPBA
H H
O
mCPBA
O H
O
H
H
O
stereospecific
Cl
mCPBA addition

17. ©Oberazzi@flickr

18. concerted mechanism
R R
O O
H O H O
O O

19. 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

20. but... H
H
O HO
O
H H
H
can give SN1
mixture
H
HO OH
HO O H
H
H

21. cation
stability

22. O O
H
H OH H
H OH
H H
O
base-mediated
HO
exclusively an H
ti
addition H
OH

23. O
S
HO N
O
O OH O
epothilone A
example

24. 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

25. from alkenes:
diol formation KMnO4
HO OH
KMnO4
H H
HO OH
KMnO4
H
H
stereospecific addition

26. from alkenes:
diol formation KMnO4
HO OH
KMnO4
H H
meso
HO OH
KMnO4
H
H
stereospecific addition

27. from alkenes:
diol formation KMnO4
HO OH
KMnO4
H H
meso
HO OH
KMnO4
H
H
stereospecific addition

28. from alkenes:
diol formation KMnO4
HO OH
KMnO4
H H
HO OH
KMnO4
H
H
stereospecific additie n
racemat o

29. ©Ninja M.@flickr

30. concerted mechanism
O O O O
Mn Mn
O O O O
H H
syn addition
stereospecific

31. mechanism
O O
Mn H2O HO OH
O O MnO2
H H
H H
hydrolysis
in situ

32. H CO2H
OH O HN
O
OMe
H
OH O OH
dynemicin A
©A Hermida@flickr

33. example
OH
N OsO4(cat.), N
NMO
OH
98%
OTBS OTBS
OTBS OTBS