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123.202 Lecture 8 - alkenes

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123.202 alkenes

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123.202 Lecture 8 - alkenes

  1. 1. LEC TURE EIGHT alkenes II ©DaveKav@flickr
  2. 2. PREVIOUSLY... hydrogenation H H X H X H started addition (hydration etc)
  3. 3. regiochemistry ©Stéfan@flickr
  4. 4. Markovnikov Addition get richer the rich ©velo_city@flickr
  5. 5. Markovnikov Addition H➀ H➀ H H ➂ H ➁ H H H get richer the rich
  6. 6. ©tintin44@flickr
  7. 7. cation stability H H X H X H primary cation least stable
  8. 8. oxymercuration i. Hg(OAc)2, H2O ii. NaBH4 HO H Markovnikov Addition
  9. 9. mechanism OAc Hg AcO OAc OAc Hg HgOAc OAc step 1
  10. 10. OAc H HgOAc H Hg SN2-like O O H H anti OAc HgOAc HO mechanism step 2
  11. 11. HgOAc H HO NaBH4 HO mechanism step 3
  12. 12. do NOTT NO believe everything you read on...
  13. 13. hydroboration BH3 H BH2 addition of BH3
  14. 14. simplification gross
  15. 15. BH3 is actually H H H B B H H H
  16. 16. or R R X H B Hmplex co H of Lewis Base
  17. 17. ©the_monk@flickr
  18. 18. group 13 δ– δ+ H H B H
  19. 19. organoboranes are unstable H BH2
  20. 20. ©the_monk@flickr
  21. 21. group 13 δ– δ+ H H B H
  22. 22. typical reaction i. BH3 ii. H2O2/ NaOH H OH anti Markovnikov Addition
  23. 23. ©quinn.anya@flickr
  24. 24. group 13 δ– δ+ H H B H
  25. 25. δ+ > δ– alkyl groups are electron donating
  26. 26. step 1 hydroboration δ– H δ+ H δ+ BH2 BH2 δ– match polarities
  27. 27. step 2 oxidation H2O2 H NaOH H BH2 OH 1,2-shift retention
  28. 28. hydroboration δ– δ+ H BH2 H BH2 δ+ δ– H H2O2 NaOH H OH syn Addition H
  29. 29. hydroboration H BH2 H BH2 H H2O2 NaOH H OH syn Addition H
  30. 30. stereospecific syn Addition H OH i. BH3 ii. H2O2 NaOH H OH concerted bond formation
  31. 31. O anti-TB OH H O H colombiasin A ©Artiii@flickr
  32. 32. OMe OMe OMe OMe i. BH3•THF; aq. NaOH, H2O2 ii. PCC TBSO TBSO H H (79% overall) OMe OMe O example

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