Alkynes: An Introduction to Organic Synthesis

1
Alkynes An Introduction to Organic Synthesis
2
Topics to discuss

• Electronic structure
• Naming
• Preparation
• Addition of HX and X2
• Hydration
• Reduction
• Oxidative Cleavage
• Acidity and Acetylide Anions
• Alkylation

3
Alkynes

• Q What is an alkyne?
• Q Where do we find them?
• Q Why do we care?

4
Electronic Structure of Alkynes

• Q What is the C?C structure?
• Q How do the C?C form?

5
Naming Alkynes

• General hydrocarbon rules apply with
• Numbering of chain with triple bond is set so
  that

6
Dyines, Enynes, and Triynes

• A compound with two triple bonds is a
• Number from chain that ends nearest a double or
  triple bond

Alkynes as substituents are called alkynyl
7
Preparation of Alkynes Elimination Reactions of
Dihalides

• Alkynes from vinylic halides
• Alkynes from alkenes

8
Reactions of Alkynes Addition of HX

• Since alkynes contain 2 p bonds, they should
• REGIOCHEMISTRY
• STEREOCHEMISTRY

9
Reactions of Alkynes Addition of X2

• REGIOCHEMISTRY none
• STEREOCHEMISTRY anti (trans-)

10
Hydration of Alkynes

• Q What is hydration?
• Q How does it occur?

11
Mercury(II)-Catalyzed Hydration

• Alkynes do not typically react with
• Initially forms an
• Mechanism is similar to that of

12
Mechanism of Mercury(II) Hydration

• 1. p e- attack Hg2, to form a CC-Hg
  intermediate.
• 2. Nucleophilic attack of H2O on C yields a
  protonated Hg2 enol.
• 3. Removal of H from protonated enol gives the
  organomercury compound.
• 4. Replacement of Hg2 by H gives a neutral
  enol.
• 5. Enol tautomerizes to ketone.

13
Keto-enol Tautomerism

• Q What is Keto-enol tautomerism?

14
Hydration of Unsymmetrical Alkynes

• This reaction works best when a terminal alkyne
  is used, so that
• When an internal alkyne is used

15
Hydroboration/Oxidation of Alkynes

• BH3 (borane) adds to alkynes to give a
• Oxidation with H2O2 produces an
• Process converts alkyne to a

16
Comparison of Hydration of Terminal Alkynes

• Hydroboration/oxidation
• Mercury (II) catalyzed hydration

17
Reduction of Alkynes

• Q Can we convert alkynes to alkanes?
• NOTE
• STEREOCHEMISTRY

18
Conversion of Alkynes to cis-Alkenes

• Q Can we stop the reaction to give only the
  alkene?
• NOTE
• STEREOCHEMISTRY

19
Conversion of Alkynes to trans-Alkenes

• Q Can we control the stereochemistry to produce
  the trans-alkene?
• NOTE
• STEREOCHEMISTRY

20
Oxidative Cleavage of Alkynes

• Q Can we cleave C?C like we cleave CC?

21
Alkyne Acidity Formation of Acetylide Anions

• Terminal Alkynes are
• Reaction of strong anhydrous bases with a
  terminal acetylene produces an
• Acetylide anion has more s character than
• More s character

22
Alkylation of Acetylide Anions

• Q Why do we care about acetylide anions?

23
Limitations of Alkylation of Acetylide Ions

• Q What is the limitation?
• Q Why is this?

24
Intro to Organic Synthesis

• Q What is organic synthesis?
• Q Why do we care?
• Pharmaceutical Industry
• Chemical Industry
• Academic Industry

25
Synthesis as a Tool for Learning Organic Chemistry

• In order to propose a synthesis you must be
  familiar with reactions
• A synthesis combines a series of proposed steps
  to go from

26
Strategies for Synthesis

• Compare the
• Consider reactions that efficiently produce the
  outcome. Look at the

27
Summary

• Electronic structure
• Naming
• Preparation
• Addition of HX and X2
• Hydration
• Reduction
• Oxidative Cleavage
• Acidity and Acetylide Anions
• Alkylation