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123.202 Lecture 9 - alkenes

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123.202 lectures
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alkenes

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123.202 Lecture 9 - alkenes

  1. 1. LECTURE NINE alkenes III ©skycaptaintwo@flickr
  2. 2. PREVIOUSLY... i. BH3 ii. H2O2/ NaOH H OH hydroboration (and hydration)
  3. 3. addition of bromine Br Br Br Br
  4. 4. Br Br Br Br stereochemistry racemic mixture of enantiomers Br Br Br Br a meso compound
  5. 5. mechanism step 1 Br Br :Br H H bromonium ion
  6. 6. mechanism step 2 Br Br SN2 H Br H H H Br anti attack
  7. 7. but... Br H H Br Br SN2 Br H H Br H H Br attacks either end
  8. 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. 9. mechanism step 1 Br Br :Br H H bromonium ion
  10. 10. mechanism step 2 Br Br SN2 H Br H Br H H anti attack
  11. 11. but... Br H Br Br H SN2 Br H Br H H H Br attacks either end
  12. 12. Br H Br H H Br Br H meso Br Br Br Br H H H H (R,S) (S,R)
  13. 13. meso mirror plane Br Br H H
  14. 14. ©Heidi & Matt@flickr
  15. 15. the not mechanism Br Br Br H mixture Br Br of all stereoisomers H H
  16. 16. from alkenes: diol formation epoxides O mCPBA H H O mCPBA O H O H H O stereospecific Cl mCPBA addition
  17. 17. ©Oberazzi@flickr
  18. 18. concerted mechanism R R O O H O H O O O
  19. 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. 20. but... H H O HO O H H H can give SN1 mixture H HO OH HO O H H H
  21. 21. cation stability
  22. 22. O O H H OH H H OH H H O base-mediated HO exclusively an H ti addition H OH
  23. 23. O S HO N O O OH O epothilone A example
  24. 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. 25. from alkenes: diol formation KMnO4 HO OH KMnO4 H H HO OH KMnO4 H H stereospecific addition
  26. 26. from alkenes: diol formation KMnO4 HO OH KMnO4 H H meso HO OH KMnO4 H H stereospecific addition
  27. 27. from alkenes: diol formation KMnO4 HO OH KMnO4 H H meso HO OH KMnO4 H H stereospecific addition
  28. 28. from alkenes: diol formation KMnO4 HO OH KMnO4 H H HO OH KMnO4 H H stereospecific additie n racemat o
  29. 29. ©Ninja M.@flickr
  30. 30. concerted mechanism O O O O Mn Mn O O O O H H syn addition stereospecific
  31. 31. mechanism O O Mn H2O HO OH O O MnO2 H H H H hydrolysis in situ
  32. 32. H CO2H OH O HN O OMe H OH O OH dynemicin A ©A Hermida@flickr
  33. 33. example OH N OsO4(cat.), N NMO OH 98% OTBS OTBS OTBS OTBS

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