panziandi
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Preparation of Alkyltriphenylphosphonium Bromides
Today I thought I would attempt to prepare butyltriphenylphosphonium bromide.
Method (I came up with this myself)
Triphenylphosphine (6.56g, 0.025mol) was added to a 100mL rbf. Acetonitrile (25mL) and 1-bromobutane (4.57g, 0.033mol) were added to the flask and the
flask set up for reflux. The mixture was refluxed gently for 30minutes. The reaction mixture was transferred to a distilling flask and the majority of
the acetonitrile was distilled off at atmospheric pressure. The reaction mixture was allowed to cool slowly and a compound crystallized out as fine
white shiny crystals.
Paper Chromatography
I took a small quantity of the supernatent from the crystals and spotted onto a clean strip of filter paper. I dissolved a small quantity of
triphenylphosphine in acetonitrile and spotted this onto the same strip of paper as a reference. Eluted with i-PrOH and developed with iodine vapour.
The result was a poor messy chromatogram! Revealed one long splodge in the Product lane (obviously too strong a sample) and a similar although small
splodge in the Reference lane. In conclusion the reaction did not occur.
Acetonitrile was chosen as it is what I use for making ionic liquids in (and it works great). I am going to try this again tomorrow by adding a
greater excess of 1-bromobutane to todays resulting crystals and refluxing for longer.
Does anybody have any experience in preparing phosphoniums? Orgsyn used molten phenol and said acetic acid can work for making a
phenoxyethyltriphenylphosphonium bromide which is something I'm not too keen on using! I'm searching J. Am. Chem. Soc. currently but would love a bit
of help.
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Klute
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I would follow any common procedure for a wittig reaction, I am sure ther eis plenty in the litt, generally the PPh3 is reacted with RX in THF or
simialr, and the strong base added. The carbonyl is added to the formed ylide. I guess you are interested only in the first step, or do you want to
isolate the alkylated phosphine before a wittig or simialr?
EDIT: paper chromatography sucks  Use proper TLC plates. Maybe IPA is too poalr
for such a compound, better start from something much less poalr and increase polarity in gradient: you can use the same plate as long as the product
hasn't eluted (much or at all): this way you can try seevral solvents systems without changing plates each time..
you can find several refs in the Wiki article, most of which will surely prepare the alkylated phosphine before hand, and give you a pannel of
conditions:
Wittig Reaction
Even better, from OrgSyn:
| Quote: | 3-Phenylpropyltriphenylphosphonium bromide (1) was prepared using the following procedure: A
solution of 1 equiv of 1-bromo-3-phenylpropane and 1.05 equiv of triphenylphosphine (both obtained
from Aldrich Chemical Company, Inc.) in dry toluene is heated at reflux for 50 hr. The resulting solids
are collected by vacuum filtration, washed on the filter three times with dry pentane, and dried at 100°
C/1 mm for 6 hr affording 1 in 87-92%yield. |
Wittig olefination
| Quote: |
2. Ethyl (triphenylphosphoranylidene)acetate is available from Fluka AG and Tridom Chemical Inc. under the name
(ethoxycarbonylmethylene)triphenylphosphorane and from Aldrich Chemical Company, Inc. under the name (carbethoxymethylene)triphenylphosphorane. The
reagent may be prepared from triphenylphosphine and ethyl bromoacetate by the following procedure.3
A 1-L, two-necked, round-bottomed flask fitted with a dropping funnel and a mechanical stirrer is charged with 131.0 g (0.5 mol) of triphenylphosphine
(Fluka AG, purum) and 250 mL of benzene (Merck, pro analysi). The solution is stirred vigorously while 83.5 g (0.5 mol) of ethyl bromoacetate (Fluka
AG, practical-grade) is added dropwise at a rate that maintains the reaction mixture at, or slightly above, room temperature. After a total of 2 hr
the reaction is complete and the colorless phosphonium salt is filtered. The salt is washed with 300 mL of cold benzene and 200 mL of pentane and then
dissolved in 3 L of water at room temperature. Some further organic impurities are removed by extraction with ether, after which 2 drops of 2%
alcoholic phenolphthalein are added. The aqueous solution is stirred vigorously and cooled in an ice bath as 2 M aqueous sodium hydroxide is added
slowly until the pink endpoint is reached (pH 8–10). The crystalline phosphorane is collected by filtration, washed thoroughly with cold water, and
dried, first with a rotary evaporator under reduced pressure at 60°C and then overnight in a drying oven at 180 mm and 70°C. The white to
cream-colored crop of ethyl (triphenylphosphoranylidene)acetate, mp 124–126°C, weighs 150–156 g (86–90%) and may be used for the preparation of
α-allenic esters without further purification. |
OrgSyn VII, 232
So it looks like toluene would be the perfect solvent, the phosphonium directly precipitating out of solution.
[Edited on 29-9-2008 by Klute]
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panziandi
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Thank you very much Klute! Indeed I am only interested in the first step of preparing the alkylated phosphine atm. I will be doing the Wittig later
(when I decide what to make).
I thought acetonitrile would be fine and thought I'd experiment rather than just following instructions, I don't see why it wouldn't be, I had
anticipated the product would be soluble in it. Reaction in toluene may be better though because a visible change would occur.
I now have 1L of toluene (arrived other week finally) so I may try that save on work up if the phosphoniums are insoluble!
I know paper chromatography sucks but... I couldn't be bothered to make a TLC plate I wanted a clue to whether the reaction had worked or not. I chose
the IPA because it said that Ph2P can be purified from Ph3P=O impurities by recrystallizing from hot ethanol (so figured IPA would dissolve Ph3P more
but not so much the phosphonium, perhaps a mixed solvent of hexane/IPA or similar would work.
I'll try later and post updates. Thanks again Klute
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Nicodem
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Isopropanol is way to polar to use it as the mobile phase for TLC for most imaginable compounds, at least on silica based TLC. Ph3P is so nonpolar
that it is actually soluble in petroleum ether and this property you can use to remove the unreacted Ph3P and BuBr from the phosphonium salt. For the
mobile phase you should use a mixture of ethyl acetate and petroleum ether - try mixtures of different proportions.
Acetonitrile has a too low bp for this reaction. I guess it would work with the more electrophilic alkyl bromides (like BnBr, allyl bromide etc.), but
for BuBr I would stick to toluene.
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scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
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Klute
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Especially with paper chromatography, which doesn't have the same adsorption properties as silica... AcOEt/Pet ether is surely on the most universal
eluant systems used, I use it in variosu proportions 80% of the time, and usually it works great for the type of chemistry I do (mostly carbonyl,
esters, ethers).
What carbonyl to you plan on using for the Wittig? And what base? That's one reaction I've always wanted to do, but never found a product I wanted to
make requiring it (although I haven't looked specificly for a wittig reaction).
I'd love to hear how the reaction goes
Do you have problems to obtain toluene? It's usually dead cheap, and very usefull... I guess xylenes could be used if pur enough.
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
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Nicodem
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The Journal of the Brazilian Chemical Society, 18 (2007) 284-290 describes the preparation of butyltriphenylphosphonium bromide in 70-80% by
a 10h reflux in toluene (the paper is freely available; and supplementary material). The general procedure:
| Quote: | | The alkyltriphenylphosphonium bromides were prepared by refluxing a solution of triphenylphosphine and the alkyl bromides (1:1) in toluene, under
nitrogen atmosphere for 10 hours. The solids were separated by filtration, washed with dry diethyl ether and dried under reduced pressure (70 to 80%
yields). |
Check also Wittig's entry in Org. Synth.: http://www.orgsynth.com/orgsyn/pdfs/CV5P0751.pdf
…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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panziandi
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The base will likely be something like butyllithium or sodium hexamethyldisilazide.
I have a few carbonyls so I don't know what to use. Perhaps cyclohexanone. I have no planned product I'm just doing this for the hell of it. I decided
to make butyltriphenylphosphonium because the alkyl halide is cheap and using it as a practise is handy.
Cheers for the links will look into using toluene now. I had no problem getting toluene only I didn't need it for ages and only just purchased some
(after a few years of not having any)
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Klute
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I am correct in assuming NaH would work also?
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panziandi
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Sounds probable since hydride is a strong base.
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