Sandmeyer
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Alkylzinc reagents from unactivated alkylbromides and chlorides
I thought that some of you might find this info useful and to turn attention to zincorganics.
Highly Efficient, General Procedure for the Preparation of Alkylzinc Reagents from Unactivated Alkyl Bromides and Chlorides
ORGANIC LETTERS 2003 Vol. 5, No. 4 423-425
Abstract
The use of 1-5 mol-% I2 leads to a powerful activation of zinc metal (dust, powder, granule, shot). Alkylzinc bromides have been efficiently prepared
by the direct insertion of activated zinc metal into alkyl bromides in a polar aprotic solvent. The in situ Ni- or Pd-catalyzed Negishi crosscoupling
gave alkylarenes in excellent yields.
Fulltext: http://rapidshare.de/files/7545987/organozinc.pdf.html
edit: notice that in method posted by zealot (below thread) there is use of toluene as solvent which is in contrast to the above paper. Those of us
having trouble to get the wonderful polar aprotic solvents such as DMF need to look at NMP which is OTC. It should also be noted that keto functional
group can be present during the organozinc formation, it is tolarated, so it is worth to try using acetone in place of those more difficult-to-obtain
solvents, it might work.
http://www.sciencemadness.org/talk/viewthread.php?tid=3007&a...
edit2:
Preparation and reactions of polyfunctional organozinc reagents in organic synthesis
Paul Knochel, Robert D. Singer;
Chem. Rev.; 1993; 93(6); 2117-2188.
http://rapidshare.de/files/7547633/organozincreview.pdf.html
Organozinc compounds (R2Zn and RZnX) are one of the first classes of main-group organometallic compounds prepared. Frankland discovered, in 1849 at
Marburg, that the heating of ethyl iodide with zinc produces highly pyrophoric diethylzinc. Amazingly, hydrogen gas was used as protective atmosphere
in this preparation.  
" Conclusions and Perspectlves
Organozinc compounds have been considered for a long time as unreactive organometallics with limited applications in organic synthesis. It has become
clear within recent years that this opinion has to be revised.
In fact, the low reactivity of the carbon-zinc bond can be exploited for the preparation of a wide range of polyfunctionalized zinc reagents. The good
transmetalation ability of organozinc derivatives with soluble copper salts or palladium(II) complexes allows the in situ preparation of highly
reactive organometallic species. The reaction pathways which are now available for these transition-metal intermediates allow reactions with numerous
carbon electrophiles in excellent yields. The addition of functionalized dialkylzincs to aldehydes in the presence of chiral titanium catalysts
provides a general enantioselective preparation to polyfunctional secondary alcohols and considerably extends the synthetic utility of diorganozincs.
Their excellent functional group tolerance, their high chemoselectivity and excellent stereoselectivity in many reactions makes organozincs ideal
organometallic intermediates for the construction of complex polyfunctional molecules. "
[Edited on 13-11-2005 by Sandmeyer]
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Sergei_Eisenstein
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Zinc Alkyls, Edward Frankland, and the Beginnings of Main-Group Organometallic Chemistry
When, on July 28, 1848, Edward Frankland (Figure 1), then a 23-year-old faculty member of Queenwood College in Hampshire, England, filled a
thick-walled glass tube with finely granulated zinc and ethyl iodide and then sealed it, he did not realize that he had set up the reaction that would
produce the first main-group organometallic compounds, ethylzinc iodide and diethylzinc, our cover molecules (if you accept Zn, Cd, and Hg as honorary
main-group elements). His goal was the preparation and isolation of the ethyl “radical”. A “radical” in 1848 was the name for
a component group of a compound: i.e., a stable group of atoms that retains its integrity in its reactions and in its formation of compounds with
other atoms or groups of atoms (generally electronegative). Thus, it was regarded as an organic equivalent of a metal. Organic chemistry in the first
half of the 19th century was in a rather disorganized state, and the chemists of the day proposed various “theories” to rationalize and
systematize the known types of organic compounds. One such theory was that of “alchohol radicals”, i.e., the “R” of ROH,
proposed by Liebig and Kane. Berzelius, the great Swedish chemist, was an enthusiastic supporter of the “radical” theory. For instance, he
considered acetic acid to be a “conjugate compound” composed of the methyl “radical” (C2H3 in those days on the basis of his
atomic weight scale in which C ) 6 and O ) 8) and CO2H. The cacodyl “radical” of Bunsen, (CH3)4As2 (later (CH3)2As),1,2 was regarded by
the chemists of the day as the first “radical” that actually had been isolated, since it formed “conjugate compounds” with
electronegative groups. So the question arose: if the cacodyl “radical” can be isolated in substance, why not organic
“radicals” such as methyl, ethyl, phenyl, etc.?
A historical review of the first organozinc research by Frankland.
Organometallics 20 (2001) 2940-2955
Full article: http://rapidshare.de/files/9992856/organofrank.pdf.html (390 KB)
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garage chemist
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I found an article by Edward Frankland on the internet, in which he describes the preparation of tin and zinc alkyls:
http://web.lemoyne.edu/~giunta/frankland.html
I have recently come across zinc alkyls as a possible precursor to other organometals (trialkylboranes). Trialkylboranes can be prepared from a
grignard and BF3, but BF3 has to be used as the gas (or dibutyl etherate), the diethyl etherate is unsuitable because diethyl ether boils too close to
the product.
Zinc alkyls, on the other hand, can not only be obtained free from solvent, but they also react with plain borate esters to the desired
trialkylboranes.
The reactions of zinc alkyls are similar to those of Grignards- they react with water to alkane and zinc hydroxide, and add to carbonyl compounds.
Their lower reactivity compared to Grignards could offer the possibility of selective reactions that are impossible with Grignards- like coupling with
acyl chlorides or anhydrides to directly form ketones. Look at Rhodiums method for P2P using dibenzylcadmium- I think this method of ketone synthesis
can be done with zinc alkyls as well.
And the best thing about zinc alkyls is how easy they are to make!
Look at Brauers preparation of Diethylzinc. Ethyl iodide is refluxed with excess zinc powder and the formed intermediate ethylzinc iodide is subjected
to distillation, causing it to decompose into diethylzinc and zinc iodide.
Diethylzinc is obtained as the distillate, a colorless liquid boling at 117°C.
The preparation is done under protective gas since diethylzinc is pyrophoric, but dry CO2 does the job according to Brauer.
No solvent is needed.
Ethyl iodide is reasonably easy to prepare, look at the methyl iodide thread- one possibility would be ethanol, KI and H3PO4.
If red Phosphorus is available, one would choose the method with ethanol, iodine and red P since it gives superior yields.
Brauer says that EtI can be partially replaced by the cheaper EtBr if a Zn-Cu-alloy instead of pure zinc is used.
Sandmeyer also mentioned the catalytic effect of iodine upon the reaction of alkyl bromides with zinc.
Diethylzinc is something I want to prepare at some time in the future- I could imagine doing impressive demonstrations of its pyrophoric nature.
A quote from the site I posted:
"On cutting off the upper portion of the decomposition tube and pouring cold distilled water upon the mobile liquid and white mass of crystals just
mentioned, a very violent action ensued, and a column of flame several feet high shot up momentarily from the mouth of the tube (...)"
This was with dimethylzinc, which boils at 46°C and whose preparation is not included in Brauer. It may require a pressure tube for its preparation,
it would also be very hard to separate it from any unreacted methyl iodide.
About diethylzinc:
"(...) its affinities are also somewhat weaker than zincmethylium, and it only takes fire in air spontaneously when exposed in considerable quantity."
So it is not as violently pyrophoric as dimethylzinc, but still spontaneously flammable.
What I dont like about the otherwise interesting metal alkyls is their sometimes dreadful toxicity.
Since zinc alkyls hydrolyse rapidly with water I would expect them to be not very toxic, but they might have some action upon the lungs due to the
zinc hydroxide released upon hydrolysis.
Does someone have reliable toxicity data for diethylzinc?
It has the hazard symbols corrosive, flammable and ecotoxic on the Aldrich website, so no specific symbol for "toxic". But it would be better to have
reliable data.
Frankland mentions dimethylzinc having a "peculiar penetrating and insupportable odor".
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Sandmeyer
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IIRC, long ago, I have stated elswhere (another thread) that arylzinc reagents can be prepared by direct insertion of zinc, in DMF, with catalytic
amount of iodine -- sort of like the usual Grigards. Well, I was wrong since I mixed up the concepts of alkyl and aryl (ups!). But, since I read about
Zealots sucsess I gave it a try, following the protocol presented in ORGANIC LETTERS 2003 Vol. 5, No. 4 423-425 (referenced above) only using
aryliodides instead of alkylhalides like the original authors did, but I never observed zinc insertion, iodine colour did dissapear, just like in
Grignards, but nothing else happened. [It remains a mystery to me how Zealot got it to work (there is a link to his procedure in the first post of
this thread)]. So, as the paper implies, it seems to only work on alkyl halides. Zinc metal can be inserted directly, but thus far only by the use of
Rieke Zinc, by electrochemical methods, or using sodium naphtalenide, to my knowledge, otherwise one has to transmetallate an Ar-MgX or Ar-Li with
zinc salt. It would be cool to make the direct insertion of zinc on aromatic systems more practical (and OTC), how about ultrasonic bath  ? I'm away from the litterature and litterature searching possibilities over weekend,
so this is written from poor memory, lack of sleep. and general dislike of organometallic chemistry. Anyways, I do have some alkyl/aryl zinc related
papers which I once found interesting, I can regather and post them if there is interest...
[Edited on 2-12-2007 by Sandmeyer]
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