Reaction Review 14 - The Hydroboration-Oxidation of Alkynes to form Keto-Enol Tautomers PDF

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Rxn Review #14 – Hydroboration of alkynes to form Keto-Enol Tautomers This reaction is similar to the hydroboration of alkenes in that it proceeds in anti-Markovnikov fashion. It is a slower reaction since it proceeds through a vinylic carbocation (less stable). Keto-enol tautomeric products are produced, but in this case, the keto version is actually an aldehyde functional group. Here’s the basic reaction for an internal alkyne: (mechanism similar to that for alkene… syn addition)

Note: for internal alkynes (like the above), it’s OK to use BH3 (in place of R2BH) as the reagent, but not so for the more reactive terminal alkynes.

Terminal alkynes are more reactive than internal alkynes toward hydroboration. For terminal alkynes to prevent multiple additions to the antiMarkovnikov C,a boron reagent containing only one H is best used in the first step. Here are some examples:

Note: These reagents are also useable for internal alkynes.

A typical reaction and products for an internal alkyne would look like the following. Note how two constitutionally isomeric ketones are produced. There is no Markovnikov here so both are produced:

Sample reaction of terminal alkyne:...