Author Topic: Mechanism? P2P from PAA & AA  (Read 2647 times)

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arrowsmith

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Mechanism? P2P from PAA & AA
« on: August 17, 2000, 03:56:00 AM »
I've dreamed this one twice...it's a decarboxylation of a rearranged version of the mixed anhydride. But, what shifts?
Anyone feel like enlightening me?

BTW, I've dreamed two other routes. Both, IMHO, suck. I tried via benzyl Grignard and got pathetic yield after one horrid "volcano reaction" failure.. It is very very hard to make ZnCl2 anhydrous. Also, when you add it, it results in a gooky Mg compound forming in the rxn vessel that makes stirring very difficult.I also tried the destructive distillation of PAA and BaOAc; again, pathetic yield, and a ruined flask to boot.

BTW, a million, million thanks to Rhodium for the great info, without which these dreams would not have been possible.


"On that Tower of Babel they knew what they were after."
P.Smith

Beaker

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Re: Mechanism? P2P from PAA & AA
« Reply #1 on: August 18, 2000, 06:27:00 AM »
1. Equilibrium formation of mixed anhydride

C6H5-CH2-CO-OH + CH3-CO-O-CO-CH3 <-> C6H5-CH2-CO-O-CO-CH3 + CH3-CO-OH

2. Attack of enolized form of acetic acid on the mixed anhydride to give the beta-keto acid

C6H5-CH2-CO-O-CO-CH3 + CH3-CO-OH -> C6H5-CH2-CO-CH2-CO2H

3. Decarboxylation

C6H5-CH2-CO-CH2-CO2H -> C6H5-CH2-CO-CH3

Lots of alternate pathways to give the same thing and a few giving byproducts, but that's the general mechanism.

Xaja

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Re: Mechanism? P2P from PAA & AA
« Reply #2 on: August 30, 2000, 08:13:00 PM »
NaOAc is also required in this reaction (or did you get product without it?), I guess to split the anhydride or remove the hydroxyl leaving group from the PAA (or both) because the mixed anhydride would otherwise be very slow to reach equilibrium (for steric reasons). Probably more to remove that OH- because the other way to do it is to use pyridine instead of NaOAc, so it must need the base to (at least) initiate the rxn.                                     The Grignards: Use bromobenzene, absolute ether, oven-dried glassware, oven-dried Mg filings and a propyl ester. No stirring needed, just an ice bath to keep it under control. Scratch the Mg filings against flask bottom and heat (gently) if the rxn fails to initiate. Plug condenser with drying salts too, don't forget.

terbium

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Re: Mechanism? P2P from PAA & AA
« Reply #3 on: September 09, 2000, 06:38:00 AM »
Don't know the mechanism but it works with no special skills needed. You just need to be able to obtain the PAA and relatively large quantities of AA. I used to use (anhydrous) potassium acetate, it has a much better solubility in AA than the sodium salt. Running the reaction in a 1L boiling flask you can isolate 50+ grams of P2P. The hardest part is getting the P2P out of the AA/KOAc mix.

CHEMMAN

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Re: Mechanism? P2P from PAA & AA
« Reply #4 on: September 12, 2000, 12:11:00 PM »
Ill have to agree on that one .The reaction goes well.Then its a case of simple distillation
to remove the lower bp stuff.The next bit is where you succeed or stuff it.
Options are .1.Vacuum distill twice.2 Go to a proper frac distillation3 .Steam  dist it out.
# 3 didnt work so well in this case, so its one or two.I didnt get a second chance, so im all ears on this one.

arrowsmith

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Re: Mechanism? P2P from PAA & AA
« Reply #5 on: September 12, 2000, 11:32:00 PM »
Sorry for having taken so long to reply - I only post from terminals that are completely anonymous.

Yes, I used sodium acetate. Unlike ZnCl2, this is very easy to make anhydrous. And I agree that the workup is the most challenging part. I did not have a very good heating setup the last time I did this - the acetic anhydride was very slow to distill off. And at the end I got a lot of p2p coming over with it. It would have been helpful to use a fractionating column.

I dreamed yet another Grignard, a la the procedure on Rhodium's page. BTW, thanks for the helpful advice, but my trouble was NEVER with getting the Grignard to form. In fact, the first time I dreamed this, it formed so merrily that benzyl chloride and THF squirted out of the condensor and all over the place. (using "3x the molar quantity of ether or THF" is *terrible* advice; this is far too little solvent.) My trouble has always been with the ZnCl2. After this last experience (I heated the stuff to like 200 C under vacuum for a good 20 minutes and it *still* had fucking water in it. I think perhaps you have to buy anhydrous ZnCl2, or make it by gassing a suspension of granulated zinc metal in THF with anhydrous HCl.

I dimly recall some procedure where you just use the Grignard, without converting to the organo Zn or Cd compound, and react it with ethyl acetate or acetonitrile. Anybody hear of such a procedure? Even if I gotta do a Collins oxidation on p2p ol, it might be worth it - I love the alchemical magic of the Grignard!

I think my next dream is going to be via benzaldehyde & nitroethane. There's a one pot procedure in "Organic Syntheses" for o-methoxy p2p from the benzaldehyde where the aldehyde, the nitrocompound, and n-butyl amine are reacted together in toluene, then the nitropropene reduced in toluene without isolation with Fe/Hcl. If you folks would like me to post this, say so and I will.


"On that Tower of Babel they knew what they were after."
P.Smith

arrowsmith

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Re: Mechanism? P2P from PAA & AA
« Reply #6 on: September 12, 2000, 11:33:00 PM »
Thanks for your help, my friend.


"On that Tower of Babel they knew what they were after."
P.Smith

Rhodium

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Re: Mechanism? P2P from PAA & AA
« Reply #7 on: September 13, 2000, 12:45:00 AM »
Yes, arrowsmith, please post thay o-methoxy P2P synthesis...


http://rhodium.lycaeum.org


terbium

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Re: Mechanism? P2P from PAA & AA
« Reply #8 on: September 13, 2000, 09:27:00 AM »

Ill have to agree on that one .The reaction goes well.Then its a case of simple distillation
to remove the lower bp stuff.The next bit is where you succeed or stuff it.
Options are .1.Vacuum distill twice.2 Go to a proper frac distillation3 .Steam  dist it out.
# 3 didnt work so well in this case, so its one or two.I didnt get a second chance, so im all ears on this one.


I used to steam distill. I would add several volumes of water (destroying all the acetic anhydride, but it was available in large quantities at the time) and then steam distill. I wouldn't acutally add any extra steam just raise the whole flask to boiling with a modified Dean-Stark trap and Friedrichs condenser fitted to the flask. The modified Dean-Stark trap I had added a glass tube from near the bottom of the collection tube to near the top of the reflow tube so that the water (which is heavier than the P2P) would flow back into the boiling flask and the P2P would collect in the trap. As the trap would fill with P2P I would open the stopcock to transfer it (and a small amount of water) to a sep. funnel.