1 / 30

7.12 Molecules with Multiple Chirality Centers

7.12 Molecules with Multiple Chirality Centers. How many stereoisomers?. maximum number of stereoisomers = 2 n where n = number of structural units capable of stereochemical variation structural units include chirality centers and cis and/or trans double bonds

sue
Télécharger la présentation

7.12 Molecules with Multiple Chirality Centers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 7.12MoleculeswithMultiple Chirality Centers

  2. How many stereoisomers? maximum number of stereoisomers = 2n where n = number of structural units capable of stereochemical variation structural units include chirality centers and cis and/or trans double bonds number is reduced to less than 2n if meso forms are possible

  3. O HOCH2CH—CH—CH—CHCH OH OH OH OH Example 4 chirality centers 16 stereoisomers

  4. CH3 HO H CH3 CH2CH2CO2H H H3C H H OH HO H Cholic acid 11 chirality centers 211 = 2048 stereoisomers one is "natural" cholic acid a second is the enantiomer of natural cholic acid 2046 are diastereomers of cholic acid

  5. How many stereoisomers? maximum number of stereoisomers = 2n where n = number of structural units capable of stereochemical variation structural units include chirality centers and cis and/or trans double bonds number is reduced to less than 2n if meso forms are possible

  6. How many stereoisomers? 3-Penten-2-ol R E E S OH H H HO R Z Z S OH H H HO

  7. 7.13 Chemical Reactions That Produce Diastereomers

  8. + E—Y E Y C C C C Stereochemistry of Addition to Alkenes In order to know understand stereochemistry of product, you need to know two things: (1) stereochemistry of alkene (cis or trans; Z or E) (2) stereochemistry of mechanism (syn or anti)

  9. Bromine Addition to trans-2-Butene anti addition to trans-2-butene gives meso diastereomer S R Br2 S R meso

  10. Bromine Addition to cis-2-Butene anti addition to cis-2-butene gives racemic mixture of chiral diastereomer S R Br2 + S R 50% 50%

  11. Epoxidation of trans-2-Butene syn addition to trans-2-butene gives racemic mixture of chiral diastereomer S R RCO3H + R S 50% 50%

  12. Epoxidation of cis-2-Butene syn addition to cis-2-butene gives meso diastereomer R S RCO3H S R meso

  13. Stereospecific reaction Of two stereoisomers of a particular starting material, each one gives differentstereoisomeric forms of the product Related to mechanism: terms such assyn addition and anti addition refer tostereospecificity

  14. cis-2-butene bromination anti 2R,3R + 2S,3S trans-2-butene bromination anti meso cis-2-butene epoxidation syn meso trans-2-butene epoxidation syn 2R,3R + 2S,3S . Stereospecific reactions

  15. H H CH3 H2 CH3 CH3 Pt CH2 H Stereoselective reaction A single starting material can give two or morestereoisomeric products, but gives one of themin greater amounts than any other H CH3 + H CH3 32% 68%

  16. 7.14 Resolution of Enantiomers Separation of a racemic mixture into its two enantiomeric forms

  17. C(+) C(-) Strategy enantiomers

  18. C(+) C(-) C(+)P(+) C(-)P(+) Strategy enantiomers 2P(+) diastereomers

  19. C(+) C(-) C(+)P(+) C(-)P(+) Strategy enantiomers C(+)P(+) 2P(+) C(-)P(+) diastereomers

  20. C(+) C(-) C(+)P(+) C(-)P(+) Strategy C(+) enantiomers P(+) C(+)P(+) 2P(+) C(-)P(+) P(+) diastereomers C(-)

  21. 7.15Stereoregular Polymers atactic isotactic syndiotactic

  22. Atactic Polypropylene random stereochemistry of methyl groups attached to main chain (stereorandom) properties not very useful for fibers etc. formed by free-radical polymerization

  23. Isotactic Polypropylene stereoregular polymer; all methyl groups onsame side of main chain useful properties prepared by coordination polymerization under Ziegler-Natta conditions

  24. Syndiotactic Polypropylene stereoregular polymer; methyl groups alternate side-to-side on main chain useful properties prepared by coordination polymerization under Ziegler-Natta conditions

  25. 7.16Chirality CentersOther Than Carbon

  26. Silicon b b Silicon, like carbon, forms four bonds in its stable compounds and many chiral silicon compounds have been resolved a a d d Si Si c c

  27. Nitrogen in amines b b Pyramidal geometry at nitrogen can produce a chiral structure, but enantiomers equilibrate too rapidly to be resolved very fast a a : : N N c c

  28. Phosphorus in phosphines b b Pyramidal geometry at phosphorus can produce a chiral structure; pyramidal inversion slower than for amines and compounds of the type shown have been resolved slow a a : : P P c c

  29. Sulfur in sulfoxides b b Pyramidal geometry at sulfur can produce a chiral structure; pyramidal inversion is slow and compounds of the type shown have been resolved slow a a : : + + S S O_ O_

  30. End of Chapter 7

More Related