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11-03-08 - 318M – Krische Lecture 30 M Alkynes •...

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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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11-03-08 - 318M – Krische Lecture 30 M Alkynes •...

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