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Topic: TLC and BOC migration  (Read 6749 times)

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Offline JeffG

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TLC and BOC migration
« on: March 04, 2010, 05:13:12 PM »
Hi everybody.

I dont know if this question should be posted in analytical chemistry or here. Sorry.

I have tried to protect an amine, by di boc´ing it. But when I follow it on TLC I see two spots lying allmost onto each other, I guess it is the mono boc´ed and di boc´ed. But I dont know which is what? I am thinking that it maybe is the di boc´ed, which is placed at the bottom on a normal TLC, since it maybe will have greater affinity for the plate and will migrate slower because of the bigger size? But dont know?

Hope somebody can help.

Thanks in advance.

Offline johnmalkinson

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Re: TLC and BOC migration
« Reply #1 on: March 05, 2010, 02:02:54 PM »
Hi JeffG

I assume you are trying to introduce two Boc groups onto the same primary amino group. As a general rule, you would expect the di-Boc protected amine to have a higher Rf value than the mono-Boc protected amine, for a normal (straight) phase TLC plate. What is the stationary phase? A normal phase plate has silica as the stationary phase. Lipophilic compounds will have higher Rf values (and migrate further up the plate). The mono-Boc compound is very likely to have better H-bonding potential and so should partition very slightly better into the stationary phase (have higher affinity for it) so will migrate less. [If you are using reversed phase TLC, the order will be opposite, with the di-Boc migrating less.]

John

Offline JeffG

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Re: TLC and BOC migration
« Reply #2 on: March 05, 2010, 03:21:06 PM »
Thank you very much John.

Yes I am trying to di protect a primary amine.

I thought that the extra boc group would make it a little more polar, because of the ester and then have better affinity for the silica making it migrate less, also because of the bigger size if that contributes.

Thanks for the answer.




Offline johnmalkinson

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Re: TLC and BOC migration
« Reply #3 on: March 05, 2010, 06:21:38 PM »
No problem.
The H-bonding potential of the NH should out-weigh the presence of the additional electronegative oxygen atoms of the carbamate (urethane). H-bonds with silanol groups on silica is usually dominant effect (in absence of charges). Size unlikely to be an issue (two molecules are comparable). Bulky tBu of Boc also inhibits availability of H-bonding sites. Also need to consider (likely improved) solubility of your products in the mobile phase.
John

Offline cpncoop

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Re: TLC and BOC migration
« Reply #4 on: March 05, 2010, 07:04:52 PM »
I think John is right with the TLC analysis.  I 'm curious if you could convert everything to the di-boc product by adding additional boc2O, and therefore not needing to worry much about purification outside of an extractive work-up.

Offline JeffG

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Re: TLC and BOC migration
« Reply #5 on: March 07, 2010, 01:44:48 PM »
I used about 3-4 eq. boc which worked fine with my other amine. But with this one there was a problem. There shouldnt be any steric problems, so I am a bit confused.

Offline johnmalkinson

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Re: TLC and BOC migration
« Reply #6 on: March 09, 2010, 09:46:37 AM »
I would add catalytic DMAP to the reaction. Should help the second Boc group to go on. Sometimes Boc2O is not reactive enought for di-Boc protection.
John

Offline JeffG

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Re: TLC and BOC migration
« Reply #7 on: March 09, 2010, 01:01:16 PM »
Thanks for reply.

I have used a catalytic amount of DMAP, but it does not seems to work. Maybe I will try with a stronger base soon.

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