10-31-08 - dibromide with 2 equivalents of NaNH 2 •...

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318M – Krische, Lecture 29: F - 10/31/08, Alkynes Terminal alkynes are weakly acidic (pK a 25-26). Upon deprotonation, the anionic lone pair will reside in an sp hybrid orbital. Greater S-character helps to stabilize the negative charge, because the electrons reside in an orbital that is closer to the positive charge of the nucleus. A strong base, such as sodium hydride (NaH) or lithium diisopropylamide (LDA), must be used to form acetylide anions. Acetylide anions are good nucleophiles. Internal alkynes can be prepared from terminal alkynes by (1) deprotonation with strong base to form an acetylide anion, then (2) S N 2 reaction with an appropriate electrophile. Alkynes can also be prepared upon treatment of a vicinal dihalide with a strong base (typically sodium amide, NaNH 2 ). The reaction mechanism involves two successive β -elimination (E2) reactions. Recall that a vicinal dihalide can be prepared by halogenation of an alkene. Thus, an alkene can be converted to an alkyne by (1) bromination using Br2 followed by (2) treatment of the resulting vicinal
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Unformatted text preview: dibromide with 2 equivalents of NaNH 2 . • Alkynes can be reduced to alkanes by treatment with H 2 in the presence of a Pd or Pt catalyst. The alkene that is formed after addition of 1 equivalent of H 2 cannot be isolated because it is still reactive under these conditions. It is possible to stop the reduction at the stage of the alkene if a special catalyst called the Lindlar catalyst is used. Use of the Lindlar catalyst leads to the stereospecific formation of a cis-alkene. • In the dissolving metal reduction of an alkyne, a trans-alkene is isolated as the product. In this reaction, the alkyne is treated with sodium metal in liquid ammonia (NH 3 ). You should understand the mechanism of this reaction, and the basis for the stereoselectivity. • Utilizing these methods we were able to convert cis-2-butene to trans-2-butene. Steps: (1) Br2; (2) NaNH2; (3) Na/NH3. Also, we were able to convert trans-2-butene to cis-2-butene. Steps: (1) Br2; (2) NaNH2; (3) H2/Lindlar’s Catalyst....
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