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Offline moov

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Azidation reaction
« on: June 05, 2010, 06:55:45 AM »
Any one can give me a suggestion  ...On this Regards
"During the azidation reaction (NaN3/DMF)controlling of alkene impurity for the conversion of Secondary alkyl halide to alkyl azide"
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Offline bromidewind

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Re: Azidation reaction
« Reply #1 on: June 09, 2010, 03:17:21 AM »
That sentence is confusing at best. Is there any context to it? Regardless, I'll give it my best shot.

Let's say you have an alkyl halide compound that you believe to be contaminated with some alkenes. To selectively convert the alkyl halides to alkyl azides, you would just perform an azidation (is that the right word?) reaction, which replaces the halide group with an azide group. You can then use a solvent like DMF, as suggested here, or perhaps ethyl acetate, to extract the alkyl azides.

Offline moov

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Re: Azidation reaction
« Reply #2 on: June 09, 2010, 03:52:48 AM »
 Thanks a lot for your response...,
Actually what is the exact condition is charging of our halolactonized compound(1eq),water(2.5 v),sodium azide(5 eq),DMF
(7vol)into flask at RT for 24 Hrs.From that conclusion is 50% azide(product) 30% dehalide product(alkene-confirmed by LC-MS) compound,12% sm(starting material) remining small impurities.
 I tried same as such above condition with TPG(tri propylene glycol) except DMF at 80c also but this condition some what better than the previous exp..here i am getting 13% of dehalide (alkene).
If any other option is there ? that will helpful to me
 
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Offline bromidewind

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Re: Azidation reaction
« Reply #3 on: June 09, 2010, 04:25:34 AM »
What are you using as your haloactonizer? I would recommend using 1-methyl imidazolium tetrafluoroborate if you can get your hands on some. It's a green (the eco-friendly kind of green) compound, so you should be able to reuse it after the reaction is finished. I'd also recommend using ethyl acetate to purify it out. You might get slightly better yields.

Some questions though... what are your starting products? are there alkenes present in the compound before you run the reaction?


Offline moov

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Re: Azidation reaction
« Reply #4 on: June 09, 2010, 09:55:05 AM »
My starting compound is Aromatic bromo lactone,<  5-(1-Bromo-3-(4-methoxy-3-(3-methoxypropoxy)benzyl)-4-methyl pentyl)-3-isopropyldihydrofuran-2(3H)-one.   >.there is no alkene itself before going to run.
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Offline Doc Oc

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Re: Azidation reaction
« Reply #5 on: June 09, 2010, 10:11:50 AM »
Bromidewind, I think you're misunderstanding what he's asking.  He's trying to do an SN2 on an alkyl halide, but he's also getting competition from the E2 pathway.

Moov as far as your reaction, maybe a new solvent will help?  It sounds like your bromide is in a position that will lead to an a,b-unsaturated lactone after elimination, which would make things tricky since that can be quite favorable.

Offline moov

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Re: Azidation reaction
« Reply #6 on: June 09, 2010, 11:03:49 AM »
Thanks J-bone...i tried DMSO,Toluene,Acetonitrile.. also..eventhough i used catalytic amount of Tricaprylmethyl ammonium chloride also but that is not favorable for me...if any other possibilities/options for avoiding E2 pathway ? 
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Offline OC pro

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Re: Azidation reaction
« Reply #7 on: June 09, 2010, 02:00:59 PM »
50% yield of product is acceptable for this reaction. If purification is easy I would go further in the synthesis and not loose time with optimization although I know that many supervisors want yields >90%   :-\.

Offline orgopete

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Re: Azidation reaction
« Reply #8 on: June 09, 2010, 02:18:45 PM »
Since the starting material is a secondary bromide, competition between substitution and elimination should be expected. If you were to allow that E2 elimination reactions are not as synchronous as implied by textbooks, then Zaitsev product can be the result of an E1-like elimination and Hoffmann to the result of an E2-like elimination. Polar protic solvents like ethanol lead to Zaitsev products and extraction of the most acidic hydrogen result in Hoffmann products.

Although the description of the reaction do not favor elimination or are overtly E1-like, I would avoid any presence of water. I think water would help to polarize the C-Br bond and increase participation of neighboring C-H bond electrons. Once that can occur, it will interfere with nucleophilic attack by the azide. The large excess of nucleophile and room temperature are consistent with obtaining the SN2 product.
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Offline moov

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Re: Azidation reaction
« Reply #9 on: June 11, 2010, 04:14:02 AM »
Thanks a lot for all for given such an idea....eventhough i further plan to isolation with 60% purity....
Moovendan

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