napoleon9
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reduction alkyne to alkene by LiAlH4, diglyme
i want reduction alkyne to alkene by LiAlH4 in diglyme solution catalyst. If you are this experience about this event, please you will give me some
advice.
How is Mixture LiAlH4 in diglyme dropwise in mixture alkyne ? Temperature ? Do you user stirring bar ? and so on ...
How is LiAlH4 protected when we open LiAlH4 bottle ?
thanks
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DJF90
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This methodology works best with propargylic alcohols, furnishing the trans allylic alcohol. The alcohol gives the reagent a "handle" which attack on
the alkyne can be directed with. It does work with isolated alkynes as far as I remember, but I dont think the yields were comparable. I suggest you
do a literature search to find experimental details for the reaction before attempting it "blind". LAH is not a nice reagent to work with and get the
conditions wrong...
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smuv
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You know, for most practical conditions, you are dead on DJF90, and I almost just submitted a post that was essentially, yep "what he said". But now
that I think of it, I seem to remember an old paper about the reduction of styrene under prolonged reflux with LAH in a high boiling solvent (maybe it
was dioxane). I will say though, that it is not a practical method of reducing alkenes.
But then again, it was a long time ago and I don't have scifinder access atm, the reduction might have been with alane.
Also, 'the books' say (though I don't doubt some say the contrary) that alkynes are not reduced by LAH. But (as you speculated) in my hands, with the
reduction of other shit on an alpha-propynylbenzene (sidechain unsubstituted) there was definitely a good quantity of impurity which I didn't isolate,
but judging by the rough proton might have been the propenylbenzene. Then again, this was an overnight THF reflux, so on the harsh side.
[Edited on 4-1-2011 by smuv]
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napoleon9
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i want redection 9-chloro-1,1-diethoxynon-2-yne to 9-chloro-1,1-diethoxynon-2-ene by LAH in diglyme. It begins at room temperature to 130 degree C.
the resulting mixture was stirred at 130 oC for 5h. The reaction was quenched by NaOH 0.1 mol/L (2mL) at O oC followed by addition of ether (10 mL).
Is mole LAH > alkyne mole ? (right or wrong)
the end reaction, how are we washing products ?
thanks
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DJF90
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You still haven't told us which alkene geometry you're after, but assuming you're using this methodology, I assume you're aiming for the E-alkene
(i.e. trans-geometry). I'm not sure if propargylic acetals work the same as propargylic alcohols, but I somehow doubt it. Do you have any references
to say this takes place?
Is the acetal/aldehyde functionality retained in your final product (whatever you're working towards??)? It may be possible to hydrolyse the acetal,
reduce the aldehyde, reduce the resulting propargylic alcohol, and then if necessary, oxidise the alcohol back to the aldehyde, and convert back to
the required acetal. There may be reagents/methodologies for direct acetal=>alcohol and alcohol=>acetal FGI, but nothing is ringing in my head
atm. I'd also think twice about using LAH (or at least using an excess) as theres a primary (terminal) halide present that needs to be left alone -
you might consult the book on alumino- and borohydrides by Jaqueline Seyden-Penn to check chemoselectivities etc etc.
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smuv
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nah, you can't do this, that chlorine will be toast under the conditions you mention. Also, in some cases acetals are hydronolyzed by LAH (only
single reduction to an ether).
If you ONLy want reduction of the alkyne, best use Pd/c and Hydrogen @ 1atm, in THF, poisoned with either quinoline or pyridine. Read a book about
selective reduction of alkynes. This should be a sinch. Also whenever I reduce compounds with acetals, I throw a gram or so of MgSO4 into the mix;
Pd is acidic, it will deprotect acetals if water is around.
[Edited on 4-1-2011 by smuv]
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DJF90
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The issue with catalytic reduction is that by the nature of the reaction the Z-alkene is furnished - this is the opposite selectivity to that obtained
with LAH or Red-Al. If the Z-alkene will suffice, then the "standard" procedure is to use lindlar's catalyst (Pd/CaCO3/Pb(OAc)2), although I'm
confident in the conditions that Smuv suggests will work adequately. Nickel boride is selective for alkyne=>alkene and supposedly gives superior
results, so this might be one to try. The book on reductions by Hudlicky will give you details for the older procedures, whilst a literature search
should yield information on NiB reduction.
Nice trick with the MgSO4 smuv... I think I may have seen something similar before.
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smuv
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Ya, can't believe I overlooked what isomer he wanted. Lol, I just wanted to recommend a Pd based hydrogenation method . Its actually a tough compound to reduce, because of that chlorine. I can't think
of a standard reducing agent that will do it (I know nothing of NiB, so I'll take your word), except maybe 1eq of borane with a non-oxidative
workup...even then...kinda splitting hairs...
Ya, lindlars cat is usually more expensive and it works no better. Actually, if you exactly measure the amount of hydrogen that is taken up, and do
the reaction with low catalyst loading, you can get selective reduction to the alkene with plain 'ol Pd/C without any poisons added. One thing to
keep in mind is except for thiols, alkynes have the highest affinity to palladium of any functional group (and definitely the highest affinity of any
reducible functional group). In short, alkynes can always be selectively hydrogenated by Pd, no matter what else is on there (ps nitro and nitroso
follow alkynes, from there its a crapshoot).
Edit: shit, borane does cis too. So...what DJF90 said 
[Edited on 4-1-2011 by smuv]
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DJF90
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You also overlooked that an oxidative workup of an alkyl borane species gives an alcohol (using NaOH/H2O2 for example... using CrO3 gives the ketone),
whereas an organic acid (e.g. acetic acid) will give the alkyl species. Of course, the oxidation to an alcohol for a vinyl borane (which is what would
be obtained) gives the enol, which of course tautomerises to the ketone. In this case, Sia2BH or catecholborane is typically used for the
hydroboration of alkynes (at least in the examples I've just looked at). The acetic acid treatment does yield the alkene (this is one method for
mono-deuterating an alkene, using AcOD.
The attached papers detail the reaction and also other borides and aluminides that are synthetically useful reagents.
[Edited on 1-4-2011 by DJF90]
Attachment: NiB reduction of alkynes.pdf (141kB)
This file has been downloaded 2070 times
Attachment: Nickel Boride catalyst.pdf (670kB)
This file has been downloaded 685 times
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smuv
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I didn't specify an oxidative workup, I said non-oxidative  . The catecholborane
reduction you referenced is essentially the same thing as what I specified. One mistake I made though was I should have specified 0.5 eq of reducing
agent , which assumes that sterics will allow 1eq of borane to reduce 2 alkynes. I admit the good thing about using the partially oxidized reducing
agents you specified, is that there is no ambiguity about how many eq of alkyne each eq of the borane will reduce.
P.S. thanks for the refs, I'll have a look...After I stop procrastinating and do my work!
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DJF90
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Sorry smuv, the "non-" was on the line above "oxidative" and as such I appeared to miss it. I have procrastination issues also...
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napoleon9
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I want E-alkene product. I use LAH in diglyme solution. If you use Pd/C then it has Z-alkene product. I wants some this program file but I don't find.
If you have it, could you share me ?
could you tell me some advices.
thank you
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