a' Alkoxymercuration

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Ether Synthesis
1. Alkoxymercuration/demercuration
a. Alkoxymercuration
Cyclic Intermediate
1. 2.
b. Demercuration
Hg0 -OAc
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2. Industrial Source of Ethers Dehydration of
symmetrical ethers
Mechanism Bimolecular (SN2)
Millions of lbs of ether produced annually by
this method.
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3. Williamson Ether Synthesis
Recall this reaction from Chapter 17.
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The alkoxide is a good nucleophile and will react
with methyl or primary alkyl halides in an SN2
reaction.
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How would you synthesize the following ether?
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This reaction works will phenols as well.
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Na
CH3I
Cleavage of Ethers
Ethers can be cleaved but strong acid and heat
are required.
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Cyclic Ethers
THF
These cyclic ethers are used as solvents.
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Synthesis of Epoxides
1. Epoxides from Halohydrins
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2. Epoxides from peroxyacids and alkenes
Reagent m-chloroperbenzoic acid (MCPBA)
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Ring-opening of Epoxides a. Acid-catalyzed
process
H, H2O
ethylene oxide
Mechanism
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H2O
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SN2
H2O
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H3O
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Ring-opening results in trans stereochemistry
with cyclic epoxides, i.e. this reaction is
occurs with anti-addition.
H, H2O
trans product ( plus enantiomer)
Write a mechanism for this reaction. Why do the
OH groups end up trans to each other and not cis?
What about asymmetic epoxides?
H, H2O
Water attacks the more substituted carbon. Why?
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H2O
Protonated Intermediate
Water attacks the carbon with the greater
positive charge, which is the more substituted
carbon.
Variation with methanol as the nucleophile.
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b. Base-catalyzed process
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OH , H2O
Mechanism
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OH
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OH
What about asymmetic epoxides?
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H
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OH
Hydroxide attacks the least hindered carbon, in
this case the secondary carbon.
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Variation with methoxide as the nucleophile.
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H
Different?
Compare product with that obtained under acidic
conditions
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c. Addition of Grignard Reagent to Epoxides
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R-Mg-X
HCl
Grignard reacts much the same way as hydroxide in
this reaction, i.e. the least hindered carbon is
attack by the Grignard reagent.
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There are other synthetic approaches to alcohols.
They will be covered in later chapters.
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Crown Ethers

• Large rings consisting repeating (-OCH2CH2-) or
  similar units
• Named as x-crown-yx is the total number of atoms
  in the ring
• y is the number of oxygen atoms
• 18-crown-6 ether 18-membered ring containing 6
  oxygens atoms
• Central cavity is electronegative and attracts
  cations

K