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Topic: borane reduction & imine-enamine equilibrium  (Read 10191 times)

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Offline g_orbital

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borane reduction & imine-enamine equilibrium
« on: April 18, 2006, 04:19:49 AM »
I have two questions:

1) I've encountered a reaction (attached herewith as image file) in which one of the double bonds of the pyrrole subunit in indole (actuallly, in tetrahydrocarbazole) is reduced by BH3-THF + TFA. My question is: what is the mechanism of this borane reduction? I assume that it starts like a normal hydroboration (a syn addition of BH2 and H etc.), but instead of oxidative workup (H2O2 + OH-) it goes through a different pathway.

2) As known, the famous Fischer indole synthesis contains an essential step of imine-enamine tautomerization (after imine formation by aryl-hydrazone and ketone). However, If I start with an asymmetric ketone (the example is attached as image file), the double bond of the enamine could be placed apparently in two positions. Of course, the position of the double bond has crucial implications on the synthesis of the indole product, since the [3,3] sigmatropic rearrangement occurs in a different manner. My Question is: how can one control the tautomerization to the preferred enamine?

Many thanks!

Offline movies

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Re: borane reduction & imine-enamine equilibrium
« Reply #1 on: April 19, 2006, 12:38:38 AM »
Those kind of indole reductions go through an iminium ion when you have TFA in the pot.

Offline Mitch

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Re: borane reduction & imine-enamine equilibrium
« Reply #2 on: April 19, 2006, 12:53:26 AM »
I didn't know what TFA was, but then I was like, "Chemicalforums must have the answer". http://www.chemicalforums.com/index.php?page=organicabbreviations ;)
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Re: borane reduction & imine-enamine equilibrium
« Reply #3 on: April 19, 2006, 03:38:34 PM »
I didn't know what TFA was, but then I was like, "Chemicalforums must have the answer". http://www.chemicalforums.com/index.php?page=organicabbreviations ;)


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