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Unformatted text preview: 318M Krische, Lecture 30 M - 11/03/08, Alkynes Hydration of Alkynes : We recalled that alkenes undergo stereospecific anti-Markovnikov hydration upon exposure to borane (BH3) followed by aqueous alkaline peroxide. While alkynes do undergo reaction with borane, the reaction is complicated by over-hydroboration, i.e. the initial product of hydroboration can react further. To circumvent this problem, we use a bulky hydroborating agent that possesses only a single B-H bond: (sia)BH, where sia = di-sec-isoamyl. Upon exposure of a terminal alkene to (sia)BH, one obtains a terminal vinylborane, also termed an alkenylborone. Treatment of the alkenylborone with aqueous alkaline peroxide produces an enol which undergos tautomerization to afford the corresponding aldehyde. The hydroboration of terminal alkynes using (sia)BH affords the anti-Markovnikov hydration product. To obtain the Markovnikov hydration product, we perform a variation of the oxymercuration reaction. The mechanism of this reaction was discussed in class. Although it starts off oxymercuration reaction....
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- Fall '08