[rita87]

Hi,

a)Are any of the steps in the following mechanism chemoselective or regioselective?
b)Is there any stereoselectivity in formation of compounds B and D?

Thank you

[macman104]

You need to give us the picture that you are working from.  Right now, we don't know what the reaction is, or what  compounds B and D are.

[rita87]

Sorry, I attached the picture but i don't know what happend to it,anyway This is the reaction.
cheers

[macman104]

Ok, good.

Now, we don't just hand out answers, we'll help you through it.  You also need to try and show some work on the problem, or at least explain what you don't understand.

Can you tell me what it means if a mechanism is chemoselective?
What does it mean if a mechanism is regioselective?

[rita87]

well, as far as i know in a regioselective reaction,bond formation occurs in one direction, and chemoselective is when a reagent prefers to react with a particular functional group.but what i know is just the theory and i am not sure how it works in a reaction like this.

[macman104]

well, as far as i know in a regioselective reaction,bond formation occurs in one direction, and chemoselective is when a reagent prefers to react with a particular functional group.but what i know is just the theory and i am not sure how it works in a reaction like this.

Ok good!  Now, in order to answer this question, you need to be able to draw the mechanisms for these reactions.  For each reaction, you need to consider two things:

To consider regioselectivity, you need to look at where bond formations occur, and see if it could occur in a different direction.  This is typically things like markovnikov addition, and such.  However, wikipedia uses the example of a strong base and the choice between two different protons to abstract (however, I've always heard this referred to as chemoselectivity).

To consider chemoselectivity, you need to look at what the reagent is reacting with.  See if there are any other locations or groups that the reagent could react with.

Don't worry about the whole reaction, just take each reaction step-by-step.  Provide your guesses and thoughts and we can go from there.

EDIT:  Also, how does the transformation from A to B happen?  If this is how the information was handed out from your teacher, I would instead suggest you speak to them, as there is information missing.

[rita87]

B is produced by repeated methylation of A.
By looking at the mechanism, i think formation of B,D and E is regioselective as in A methyl and dimethyl could be in a different direction,same for other D aned E.
and i guess formation of D is chemoselective as it could have been ortho.
Thats what i think, but i am not sure at all.
Thank you so much for helping me out.

[macman104]

Regioselective:

B - The second methylation can occur at either the methylated site, or the non-methylated site.
D - Ortho or Para reaction, I would consider that regioselective, because there really isn't a chemoselective difference between.
E - I don't see as regioselective.  There is really only one spot that alkyne anion would attack (but see the note below).

There is one problem, that I believe to be a synthetic issue, or for your course it may be a non-issue.  I think the alkyne proton has a rough pKa of 25 (actually, probably higher due to the electron push of the o-methyl group).  The alpha-proton on the monomethylated site is probably going to be near 21-22 or so.  That is at least 1000 fold reactivity difference.  So, I imagine the NaNH₂ would abstract the alpha-proton, or at least the alkyne anion would abstract it.  I don't see how that reaction would happen...

Or maybe that is supposed to be your chemoselective issue.

[rita87]

I think Na attacks the alkyl, but don't know what happens next.
Do you have any idea about the stereoselectivity in formation of E from B and D? I think there is a stereoselectivity in this reaction but don't know the reason.

Thanks,

[macman104]

Ah, sorry I missed that part.  Yes there might be some slight stereoselectivity in the formation of E.  For the NaNH₂ the NH₂ is quite a strong base and will remove a proton.  I was just debating where that removal would happen.  But it's not that important.

[rita87]

yes, Thank you so much for your help,macman104 .

[rita87]

Hi,
I still dont know why there is some stereoselectivity in the formation of E from B and D.

[sjb]

Well, you haven't drawn any, so I (for one) don't know what answer you're looking for, but consider the attack of the acetylide anion on the carbonyl. What factors could affect which face of the carbonyl is attacked?

[rita87]

what should i draw?the picture is given.but to be honest i don't know what factor affects which face is attacked. the only thing i think is that reaction of B and D can form 2 stereoisomers.but why that happens, i don't know. have gone through all my lecture notes, but still don't get it:-[

[sjb]

You're essentially right, as far as I'm concerned. I'd suggest that there are two pairs of diastereomers formed, rather than just two stereoisomers. Here's a pared down sketchet of what's happening. Although I have drawn the mono-methyl side of the carbonyl as (R) there is nothing I can see in your conditions to say that it could not be (S) instead, so you have a racemic mixture there. Now, if you consider things like Burgi-Dunitz (http://en.wikipedia.org/w/index.php?title=B%C3%BCrgi-Dunitz_angle&oldid=256841485 ) there are two possible angles of attack of the acetylide on the carbonyl - perhaps one is favoured? Can you suggest which one, and why?