1. 9.5 Oxymercuration-Demercuration
Because rearrangements often produce a mixture of products, the synthetic utility of Markovnikov hydration reactions is somewhat limited
Oxymercuration-demercuration is an alternative process that can yeild Markovnikov products more cleanly
Copyright 2012 John Wiley & Sons, Inc.

2. 9.5 Oxymercuration-Demercuration
Oxymercuration begins with mercuric acetate
How would you classify the mercuric cation?
As a nucleophile or an electrophile?
As a Lewis acid or Lewis base?
How might an alkene react with the mercuric cation?
Copyright 2012 John Wiley & Sons, Inc.

3. 9.5 Oxymercuration-Demercuration
Similar to how we saw the alkene attack a proton previously, it can also attack the mercuric cation
Resonance stabilizes the mercurinium ion and the carbocation. Draw a reasonable resonance
hybrid
Copyright 2012 John Wiley & Sons, Inc.

4. 9.5 Oxymercuration-Demercuration
The mercurinium ion is also a good electrophile, and it can easily be attacked by a nucleophile, even a weak nucleophile such as water
NaBH4 is generally used to replace the –HgOAc group with a –H group via a free radical mechanism
Copyright 2012 John Wiley & Sons, Inc.

5. 9.6 Hydroboration-Oxidation
To achieve anti-Markovnikov hydration, Hydroboration-Oxidation is often used
Note that the process occurs in two steps
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6. 9.6 Hydroboration-Oxidation
Hydroboration-Oxidation reactions achieve syn addition
Anti addition is NOT observed
To answer WHY, we must investigate the mechanism
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7. 9.6 Hydroboration-Oxidation
Let’s examine how this new set of reagents might react
The BH3 molecule is similar to a carbocation but not as reactive, because it does not carry a formal charge
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8. 9.6 Hydroboration-Oxidation
Because of their broken octet, BH3 molecules undergo intermolecular resonance to help fulfill their octets
The hybrid that results from the resonance (diborane) involves a new type of bonding called banana bonds
Copyright 2012 John Wiley & Sons, Inc.

9. 9.6 Hydroboration-Oxidation
In the hydroboration reaction, BH3•THF is used. BH3•THF is formed when borane is stabilized with THF (tetrahydrofuran)
What general role do you think BH3•THF is likely to play in a reaction?
Copyright 2012 John Wiley & Sons, Inc.

10. 9.6 Hydroboration-Oxidation
Let’s examine the first step of the Hydroboration mechanism on the next slide
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11. 9.6 Hydroboration-Oxidation
What evidence is there for a concerted addition of the B-H bond across the C=C double bond?
Use sterics and electronics to explain the regioselectivity of the reaction
Practice with conceptual checkpoint 9.17
Copyright 2012 John Wiley & Sons, Inc.

12. 9.6 Hydroboration-Oxidation
Copyright 2012 John Wiley & Sons, Inc.

13. 9.6 Hydroboration-Oxidation
Start Here
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14. 9.6 Hydroboration-Oxidation
When ONE chirality center is formed, a racemic mixture results
WHY? What is the geometry of the alkene as the borane attacks?
The squiggle bond above shows two products, a 50/50 mixture of the R and the S enantiomer
Copyright 2012 John Wiley & Sons, Inc.

15. 9.6 Hydroboration-Oxidation
When TWO chirality centers are formed, a racemic mixture results
Why aren’t the other stereoisomers formed?
Practice with SkillBuilder 9.4
Copyright 2012 John Wiley & Sons, Inc.

16. 9.6 Hydroboration-Oxidation
Predict the major product(s) for the reactions below
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17. 9.7 Catalytic Hydrogenation
The addition of H2 across a C=C double bond
If a chirality center is formed, syn addition is observed
Draw the other stereoisomers that are NOT produced
Copyright 2012 John Wiley & Sons, Inc.

18. 9.7 Catalytic Hydrogenation
Analyze the energy diagram below
Why is a catalyst necessary?
Does the catalyst affect the spontaneity of the process?
Typical catalysts include Pt, Pd, and Ni
Copyright 2012 John Wiley & Sons, Inc.

19. 9.7 Catalytic Hydrogenation
The metal catalyst is believed to both adsorb the H atoms and coordinate the alkene
The H atoms add to the same side of the alkene pi system
Copyright 2012 John Wiley & Sons, Inc.

20. 9.7 Catalytic Hydrogenation
Draw product(s) for the reaction below. Pay close attention to stereochemistry
How many chirality centers are there in the alkene reactant above?
How does the term, mesocompound, describe the product(s) of the reaction?
Practice with SkillBuilder 9.5
Copyright 2012 John Wiley & Sons, Inc.

21. 9.7 Catalytic Hydrogenation
If catalysis takes place on the surface of a solid surrounded by solution, the catalyst is heterogeneous. WHY?
Homogeneous catalysts also exist
What advantage might a homogeneous catalyst have?
Copyright 2012 John Wiley & Sons, Inc.

22. 9.7 Asymmetric Hydrogenation
In 1968, Knowles modified Wilkinson’s catalyst by using a chiral phosphine ligand
A chiral catalyst can produce one desired enantiomer over another. HOW?
Why would someone want to synthesize one enantiomer rather than a racemic mixture?
Copyright 2012 John Wiley & Sons, Inc.

23. 9.7 Asymmetric Hydrogenation
A chiral catalyst allows one enantiomer to be formed more frequently in the reaction mixture
Some chiral catalysts give better enantioselectivity than others. WHY?
Copyright 2012 John Wiley & Sons, Inc.

24. 9.7 Asymmetric Hydrogenation
BINAP is a chiral ligand that gives very pronounced enantioselectivity
For any reaction, stereoselectivity can only be achieved if at least one reagent (reactant or catalyst) is chiral
Copyright 2012 John Wiley & Sons, Inc.

25. 9.7 Asymmetric Hydrogenation
Predict the major product(s) for the reactions below
Copyright 2012 John Wiley & Sons, Inc.

26. 9.8 Halogenation
Halogenation involves adding two halogen atoms across a C=C double bond
Halogenation is a key step in the production of PVC
Copyright 2012 John Wiley & Sons, Inc.

27. 9.8 Halogenation
Halogenation with Cl2 and Br2 is generally effective, but halogenation with I2 is too slow and halogenation with F2 is too violent
Halogenation occurs with anti addition
Given the stereospecificity, is it likely to be a concerted or a multi-step process?
Copyright 2012 John Wiley & Sons, Inc.

28. 9.8 Halogenation Let’s look at the reactivity of Br2. Cl2 is similar
It is nonpolar, but it is polarizable. WHY?
What type of attraction exists between the Nuc:1- and Br2?
Does the Br2 molecule have a good leaving group attached to it?
Copyright 2012 John Wiley & Sons, Inc.

29. 9.8 Halogenation We know alkenes can act as nucleophiles
Imagine an alkene attacking Br2. You might imagine the formation of a carbocation
However, this mechanism DOES NOT match the stereospecificity of the reaction. HOW? WHY?
Copyright 2012 John Wiley & Sons, Inc.

30. 9.8 Halogenation Mechanism continued on next slide
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31. 9.8 Halogenation Only anti addition is observed. WHY?
Prove to yourself that the products are enantiomers rather than identical
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32. 9.8 Halogenation Only anti addition is observed
Can you design a synthesis for ?
Practice with conceptual checkpoint 9.26
Copyright 2012 John Wiley & Sons, Inc.

33. 9.8 Halogenation Predict the major product(s) for the reactions below
Copyright 2012 John Wiley & Sons, Inc.

34. 9.8 Halohydrin Formation
Halohydrins are formed when halogens (Cl2 or Br2) are added to an alkene with WATER as the solvent
The bromonium ion forms from Br2 + alkene, and then it is attacked by water
Why is the bromonium attacked by water rather than a Br1- ion? Is water a better nucleophile?
Copyright 2012 John Wiley & Sons, Inc.

35. 9.8 Halohydrin Formation
A proton transfer completes the mechanism producing a neutral halohydrin product
The net reaction is the addition of –X and –OH across a C=C double bond
Copyright 2012 John Wiley & Sons, Inc.

36. 9.8 Halohydrin Regioselectivity
The –OH group adds to the more substituted carbon
The key step that determines regioselectivity is the attack of water on the bromonium ion
Copyright 2012 John Wiley & Sons, Inc.

37. 9.8 Halohydrin Regioselectivity
When water attacks the bromonium ion, it will attack the side that goes through the lower energy transition state
Water is a small molecule that can easily access the more sterically hindered site
Practice with SkillBuilder 9.6
Copyright 2012 John Wiley & Sons, Inc.

38. 9.8 Halohydrin Regioselectivity
Predict the major product(s) for the reactions below
Copyright 2012 John Wiley & Sons, Inc.

39. Study Guide for sections 9.5-9.8
DAY 25, Terms to know:
Sections Oxymercuration-Demercuration, Hydroboration-Oxidation, hydrogenation, halogenations, halonium ion, bromonium ion, halohydrin
DAY 25, Specific outcomes and skills that may be tested on exam 4:
Sections
Be able to give reasonable reagents and conditions including an alkene that could be used to produce a desired addition product.
Given an alkene and reaction conditions, be able to predict the product of an addition reaction including the proper stereochemistry and regiochemistry.
Be able to explain why borane conditions yield anti-Markovnikov hydration products using sterics and electronics to explain regioselectivity and stereoselectivity.
Be able to give the mechanism for each step of the Hydroboration-Oxidation reaction.
Be able to explain the role of the metal catalyst in hydrogenation reactions.
Be able to predict region- and steroselectivity in hydrogenation reactions.
Be able to predict region- and steroselectivity in halogenation reactions.
Be able to predict region- and steroselectivity in halohydrin reactions.
Be able to draw a reasonable mechanism for each addition reaction mechanism covered in class.
Klein, Organic Chemistry 2e

40. Practice Problems for sections 9.5-9.8
Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period
Klein, Organic Chemistry 2e

41. Prep for Day 26 Must Watch videos: Other helpful videos:
(anti dihydroxylation)
(syn dihydroxylation)
(ozonolysis)
(naming alkynes)
(reactions of alkynes)
Other helpful videos:
(lecture 9)
Read sections ,
Klein, Organic Chemistry 2e