Question

[blazinfury]

Sorry to bother you guys, but I have a multiple choice organic question that I was hoping you guys could help me with this attached problem. I understand that the first step will turn the OTMS into an OH, but I am not sure what role the H+ will play in this rxn mechanism. Thanks.

[ecoli]

I believe that the flouride ion 'displaces' the TMS group, forming (+)O-F. The hydrogen ion attacks the double bond, to form a ketone.

 

don't quote me on that though. It's been a while.

 

PS - is this the same blazinfury from the student doctor forums?

[blazinfury]

Yea. Thanks ecoli.

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This makes so much sense now. You are freakin awesome. I would have never thought of that.

[UC]

After the TMS group is clipped off, you are left with a non-trapped enolate, which tautomerizes and snatches up a H+ to make the more stable keto form.

 

It might happen how I drew it, or the enolate may first convert to an enol, then tautomerize. (This way was less drawing )

[blazinfury]

Thank you so much UC for providing further insight. I understood what ecoli was saying and had your drawing in my head after I read it, but thanks for further explaining it and reinforcing this topic. I greatly appreciate it.

 

Also sorry for not giving this topic a more specific name but I just was unsure of what this type of reaction is. Would you have any good links to understanding tautomerization reactions and their uses?

[UC]

No problem. Things click for me when I draw them, and I rather like playing around with the drawings. Even wikipedia gives a reasonable explanation of tautomerism. To make a TMS ether, you need a powerful base to drive the formation of an enolate. Say, an organolithium, which is effectively a carbanion. The conjugate acid of the carbanion (alkane) is formed and the enol is deprotonated to a lithium enolate. Then TMS chloride is added, and the reaction proceeds like any other example of the williamson ether synthesis. LiCl is the side product.

 

Somethign like "TMS ether removal mechanism" would have been fine. Chances are people will see it anyway and if they can help, will post.

 

Edited January 21, 2009 by UC

[blazinfury]

What do you use to draw them-- the ACDC Labs orgo drawing program? At my school, we use a similar thing except its an online java enabled one: http://www.ic.sunysb.edu/Class/orgchem/marvin/examples/applets/sketch.html

 

Thanks for the help. I greatly appreciate it.

[UC]

Yes, I use ACD Labs chemsketch, which I'm rather fond of at this point. I just need to convert files I save as .bmp into .jpg with another program, lest they eat up all my attachment storage space on this forum.

 

Again, no problem. Did you get the one on cyclic esters? (called lactones).

[ecoli]

Thank you so much UC for providing further insight. I understood what ecoli was saying and had your drawing in my head after I read it, but thanks for further explaining it and reinforcing this topic. I greatly appreciate it.

 

I was wrong on the first step anyway... I had the fluoride replacing TMS, which didn't make much sense upon reflection because it leaves the oxygen with a + charge when there should be extra electron density around it.