contrived
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organolithium reagents
I need to use a strong nucleophile and am considering an organolithium compound. I've been reading some articles in which BuLi and Ph-Li are used
with no special precautions mentioned. If you've worked with these could you tell me what I need to do to store the two I mentioned after opening,
like should I blow the bottles out with N2? Is that enough? Do I need to flame all the glass before using the compounds? How heroic do I have to be
in drying solvents?
Thanks!
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Nicodem
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Working with BuLi requires knowledge and skills which are assumed in advance, so that no paper goes into the details on how to work with
organolithiums. Find someone with the know-how to show you or at least be present during your first attempt.
But if you think you know how to deal with it:
- all glassware must be flame dried and allowed to cool over argon flow;
- THF, diethyl ether or whatever solvent you will use needs to be the most dry possible (meaning: distillation from sodium benzophenone ketyl!) unless
your reaction allows the use of excess BuLi (in which case THF from over molecular sieves or Et2O over sodium wire might do);
- use argon for inert atmosphere (N2 is not heavy enough and might not flush all the oxygen out, not to even mention that N2 is not even that inert
toward organolithiums!);
- when pulling out BuLi through the septum of the reagent bottle, you need to have an overpressure of argon in the bottle (this is done by either
attaching a argon baloon with another needle or filling the syringe first with argon, injecting it and retrieving BuLi); the overpressure in the
bottle allows for good storage since otherwise the subpresure slowly sucks air trough the hole in the septum, thus fucking up your BuLi;
- organolithium reagents slowly disproportionate so be sure your reagent is still what you think it is and of the concentration you expect it (BuLi
that has been in the freezer for any longer than half year can be suspected to be of poorer quality);
- BuLi is pyrophoric, so be careful when transferring the filled syringe from the reagent bottle onto the flask septum (a drop or two of spilled BuLi
will generally not catch fire, but larger amount actually can do so!); there are simple techniques by using a needle through a rubber cork to close
the needle tip during the transfer (see in books about laboratory techniques);
- lithiations of non-acidic C-H bonds generally require more or less time (depending on their pKa and, of course, the neighboring chelating group); at
-78°C this generally means several hours, but often it can be speeded up by raising the temperature to, for example, -20°C or even 0°C (simply by
raising the flask out of the acetone/CO2 bath); methatetical lithiation of C-halogen bonds is much more rapid.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
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syntelman
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My experience with commercially available organolithiums is that they rapidly decline when stored in their original flasks due to that the septas in
reality won't stand for several needle punctures. Hence I always transfer the reagent to a round bottom Schlenk flask and store over nitrogen which
makes it last much longer and also allows easier retrieval. The molarity of BuLi also varies between different brands and batches, so if you're
reaction is very sensitive it is recommended that you titrate before using.
The website "Not Voodoo" has a lot of good organic chemistry tips, and tutorials both on how to titrate alkyl lithiums and how to use syringes. The
URL is http://chem.chem.rochester.edu/~nvd/index.html or more specifically http://chem.chem.rochester.edu/~nvd/howtotitrate.html and http://chem.chem.rochester.edu/~nvd/howtosyringe.html.
Finally, be very careful. Butyl lithium can be used safely under the proper conditions (read: laboratory fume hood) but one must take all the
necessary precautions and know what one is doing. Good luck!
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Sergei_Eisenstein
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| Quote: | | Hence I always transfer the reagent to a round bottom Schlenk flask and store over nitrogen which makes it last much longer and also allows easier
retrieval. |
I have no practical experience with organolithium compounds, but I read that nitrogen is an inapropriate choice as inert gas. Organolithium compounds
such as BuLi can actually react with nitrogen to form lithium nitride.
damnant quod non intelligunt
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syntelman
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Although LN3 can form in the reaction between metallic lithium and the nitrogen content of air, it is preferrably synthesized at elevated temperature
and or pressure. I have never witnessed the formation of lithium nitride in BuLi solutions stored over nitrogen neither by visual indication or
titration. However in our laboratory a bottle of BuLi usually lasts up to 1 or 2 months so it may become a problem first at long term storage.
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chemrox
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CTR and I really appreciate the comments, especially Nicodem and Syntelman.
@syntelman- that was great, thanks! @Nicodem, thanks for all the very specific information. Can you recommend a book or lab manual (in addition to
the urls syntelman offered?). We suspected there were assumed experience and protocol involved. I think a set of septa , appropriate syringes and
Shlenk flask are necessary aquisitions for us. Also aren't there storage bottles made for these that allow repeated Argon recharge? If the stuff is
new it should be dated. Is ph-Li any easier to deal with bu-Li? I wonder if there are ocaisions when Grignard can be subbed in at a sacrifice in
yield. Anyway, relative reactivity of bu v. ph Li?
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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contrived
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@Nicodem & Syntel: Thank you very much!
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chemrox
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confusion about handling Li from paper
Please read the follwing and comment on the process. It doesn't look like anything is missing or assumed here and its not very heroic in the handling
of Li wire. Where it gets specific with what the Li reagent is combined with I stopped to avoid side discussions. It's an interesting syn on its
own. I will say that the fask were not flushed with gas before the next reaction.
One liter of dry ether and 15 g. of lithium wire cut into small pieces were placed in a 3-liter round-bottom flask provided with a condenser, stirrer,
and dropping-funnel. Bromobenzene (172 g.) was placed in the dropping funnel and 10 cc. added at one time to the flask. Gentle warming was employed
until the reaction started and then the bromobenzene was added from the dropping-funnel at such a rate that the ether refluxed vigorously. At the end
of the reaction the dropping-funnel was rinsed with 100 cc. of ether and the flask cooled to -5 deg C.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Sandmeyer
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What do you meen by BuLi? t-BuLi is very reactive stuff, be very careful when taking it up from the ampoule over to the rxn flask (it almost always
ignite for me), it's not like n-BuLi which is relatively safe, don't remember ever using sec-BuLi.
[Edited on 24-12-2007 by Sandmeyer]
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Swany
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Sandmeyer is correct about the differences in BuLi reactivity, t-butyl is comparitively scary and tends to ignite if you squirt some at the back of
the hood, n-butyl is more forgiving in that department. I have had successive reactions pwn'd by n-BuLi that was old and wasn't titrated, and n-BuLi
that wasn't titrated but was used straight from the bottle assumed to be what it said. There may have been other factors, but we believe the BuLi was
to blame, its usually a good excuse.
We always used solvents distilled from sodium benzophenone ketyl, and we usually used nitrogen. I would recommend argon, in hindsight.
When handling; septa, syringes, Schlenk line, and a bit of extra respect.
It's neat stuff. 
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