Additional Alkyne Addition Reactions

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Additional Alkyne Addition Reactions

• Bruce A. Hathaway
• CH343
• Southeast Missouri State

2
Reactions of Alkynes

• Remember the main reaction of alkenes is
  addition to the pi-bond.
• Since alkynes have two pi-bonds, they can add two
  equivalents of a reagent.
• The wiggly lines in the middle structure means a
  mixture of E and Z isomers can be formed,
  depending on the exact reaction conditions.

3
Review Addition of H-X

• Just like with an alkene, H-X adds to the
  pi-bonds of an alkyne. The first equivalent adds
  to make a haloalkene, and the second equivalent
  adds to make a dihaloalkane.
• Markovnikovs rule is followed as well, so the
  major product of the reaction of a 1-alkyne is
  easy to predict.

4
Review Addition of Water

• Water adds to an alkyne in a similar way it adds
  to an alkene. An acid catalyst is required.
  Usually, a mercury (II) salt is also added as a
  catalyst to speed things up, but is not always
  required.
• Addition of water to a 1-alkyne follows
  Markovnikovs rule to make the ketone, rather
  than the aldehyde.

5
Review Addition of Water

• The addition of one equivalent of water results
  in the formation of an enol.
• Enols are less thermodynamically stable than the
  isomeric ketones, so they tautomerize to the
  ketones.

6
Review Addition of X2

• Addition of one equivalent of halogen often forms
  a mixture of cis and trans isomers.
• Addition of the second equivalent forms the
  tetrahaloalkane.

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Review Reduction of Alkynes

• Complete reduction of an alkyne to an alkane
  requires 2 equivalents of H2.
• The usual catalysts are Pd, Pt, or activated Ni.

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Partial Reduction of Alkynes

• The partial reduction of an alkyne to a
  cis-alkene and be accomplished by using hydrogen
  and a special catalyst.
• Two common catalysts are the Lindlar catalyst
  (Pd/BaSO4 with added quinoline) and P-2 nickel
  boride (which is easily prepared from sodium
  borohydride and nickel acetate).
• These catalysts react much faster with CCs than
  with CCs, although they will reduce CCs
  slowly.
• Usually, the amount of hydrogen reacted is
  monitored, and stopped when one equivalent of
  hydrogen has been used.

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Parr Hydrogenator (Wade p. 348)
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Partial Reduction of Alkynes

• Reduction of alkynes to trans-alkenes is usually
  accomplished by treating the alkyne with sodium
  in liquid ammonia.
• See page 399 in Wade for the mechanism.

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Hydroboration-Oxidation of Alkynes

• Alkynes can be hydroborated in a similar manner
  to alkenes, and the resulting organoborane
  intermediates can be oxidized. In this case, CO
  compounds are the isolated products.
• In the case of 1-alkynes, aldehydes are the major
  products.
• Usually, large and bulky dialkylboranes are used
  as the hydroborating agents, to maximize the
  selectivity of the addition reaction.

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Hydroboration-Oxidation of Alkynes
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Oxidation of Alkynes with KMnO4 and Ozone

• Internal alkynes can be oxidized at the CC by
  KMnO4 under mild conditions to form a diketone.
  Some examples are shown on the next slide.
• PTA stands for phase-transfer agent, which is a
  compound that allows the permanganate ion to
  travel into the organic layer of the reaction to
  react with the alkyne.
• Ozone (O3) can do a similar oxidation, as long as
  a reductive work-up procedure is used (Zn in
  acetic acid, or dimethyl sulfide)

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Oxidation of Alkynes with KMnO4 and Ozone
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Oxidation of Terminal Alkynes

• Oxidation of 1-alkynes usually forms the
  carboxylic acid with one carbon fewer, due to
  loss of the terminal carbon as CO2.

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Cleavage of Alkynes to Carboxylic Acids

• Ozone cleaves internal alkynes to carboxylic
  acids, when an oxidative workup is used (H2O2),
  as does vigorous oxidation with KMnO4.

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A Synthesis Problem
Answer
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Another Synthesis Problem
You (should) now know at least three ways to do
the following reaction.
Answer