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Reaction Stereochemistry

Reaction Stereochemistry. A regioselective reaction: preferential formation of one constitutional isomer. A stereoselective reaction: preferential formation of a stereoisomer. Stereoselectivity vs. Sterospecificity. Chiral Acids-Bases Diastereomers.

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Reaction Stereochemistry

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  1. Reaction Stereochemistry

  2. A regioselective reaction: preferential formation of one constitutional isomer A stereoselective reaction: preferential formation of a stereoisomer

  3. Stereoselectivityvs.Sterospecificity Chiral Acids-Bases Diastereomers http://ep.llnl.gov/msds/orgchem/Chem226/stereo1.html

  4. A stereospecific reaction: each stereoisomeric reactant produces a different stereoisomeric product or a different set of products All stereospecific reactions are stereoselective Not all stereoselective reactions are stereospecific

  5. Many reactions convert achiral reactants to chiral products. If all of the components of the starting state (reactants, catalysts, solvents, etc.) are achiral, any chiral products that will be formed are racemic mixtures. "Optically inactive starting materials can't give optically active products." In order for a substance to be optically active, it must be chiral and one enantiomer must be present in greater amounts than the other.

  6. Stereochemistry of Electrophilic Addition Reactions of Alkenes Can you determine the absolute configuration of the product?

  7. 50: 50: Addition reactions that form one asymmetric carbon: How much of each isomer forms?

  8. Many biochemical reactions convertan achiral reactant to a singleenantiomer of a chiral product Reactions in living systems are catalyzed by enzymes, which are enantiomerically homogeneous. The enzyme (catalyst), which is chiral and optically active, is part of the reacting system, so such reactions don't violate the generalization that"Optically inactive starting materials can't give optically active products."

  9. H HO2C C OH C C HO2CCH2 Example HO2C H H2O fumarase CO2H H Fumaric acid (S)-(–)-Malic acid Achiral Single enantiomer Lactic acid: Chem 226 Web pages http://ep.llnl.gov/msds/orgchem/Chem226/stereo1.html

  10. Addition reactions that form an additional asymmetric carbon

  11. Addition reactions that form two asymmetric carbons A radical reaction intermediate

  12. Stereochemistry of Hydrogen Addition (Syn Addition)

  13. H H CH3 H2 CH3 CH3 Pt CH2 H A stereoselective reaction 36%= Pure - Racemic • What is the optical yield for the reaction? • Will the physical properties of the products be the same? No, diastereomers have different physical properties. H CH3 + H CH3 32% 68%

  14. Stereochemistry of Hydroboration–Oxidation 50: 50 Syn addition, that is from the same side: either top or bottom. What is the distribution and relationship of the products? Enantiomers or Diastereomers? Enantiomers

  15. Addition reactions that form a bromonium ion (anti addition)

  16. Stereochemistry Vocabulary pro-R-hydrogen pro-S-hydrogen Enantiotopic hydrogens have the same chemical reactivity and cannot be distinguished by achiral agents, but they are NOT chemically equivalent toward chiral reagents (Most relevant in biochemistry/physiology.)

  17. Diastereotopic hydrogens do not have the same reactivity with achiral reagents

  18. Applying the Principle:Resolution of Enantiomers Separation of a racemic mixture into its two enantiomeric forms.

  19. Resolution of a Racemic Mixture (S)-base (R)-acid(S)-acid (R,S)-salt (S,S)-salt diastereomers enantiomers (R,S)-salt (S,S)-salt HCl HCl (S)-baseH+ (S)-baseH+ + + (S)-acid (R)-acid

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

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

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

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

  24. ResolvingRacemic Mixtures Chiral Acids-Bases Diastereomers http://ep.llnl.gov/msds/orgchem/Chem226/stereo1.html

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