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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

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7.12 Molecules with Multiple Chirality Centers

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  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

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