Jor
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rxn alcohols with phosphorus halides
I have recently been doing some more experimenting on synthesis of metal complexes at home (yesterday I prepared both the complexes Cr(acac)3 and
VO(acac)2).
I was reading on trimethyl phosphite as a ligand. A quick read on wiki yielded the fact that when PCl3 is reacted with MeOH, trimethyl phosphite is
formed. However when P+I2 (PI3 in-situ) is reacted with MeOH, MeI is formed. So what determines if the ester is formed, or the alkyl halides? I assume
PCl3 and PI3 react in the same manner. Is it the fact that HI can react with MeOH, but HCl cannot, because it is not acidic enough to protonate the
OH-group?
I was thinking of easily preparing some trimethyl phosphite by adding bromine slowly to a suspension of red P (slight excess) in MeOH, and afterwards
distilling off the MeOH, followed by the trimethyl phosphite. Or does this form MeBr? If so, can a base be added to neutralise the HBr formed to
favour the formation of the ester?
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Picric-A
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I believed methanol must be reacted with POCl3 to form trimethyl phophite.... are you sure it quotes PCl3?
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DJF90
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Of course it quotes PCl3, because he's making the phosph*I*te.
The phosphite esters are formed from PX3, the alcohol, and a base. In the absence of base, the HX formed as a byproduct goes on to form RX + H2O,
which may then react with more PX3 to give the hydrolysis products H3PO3 and HX, which as we saw just a second before will react with your alcohol to
give RX and more H2O.
Personally I'd form the phosphorus halide first (in situ), add an (excess of) appropriate base (Et3N perhaps?) and then the 3eq. of alcohol.
[Edited on 23-11-2010 by DJF90]
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garage chemist
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PCl3 with alcohol in presence of 3 equivalents of base (pyridine) forms your desired trimethyl phosphite.
Without base, it forms dimethyl phosphonate O=PH(OCH3)2 ("dimethyl hydrogen phosphite", as it is wrongly called).
Read about the preparation of the isopropyl analogs in the book "Toxic fluorine and phosphorus compounds" by Saunders, I think it's available for
download somewhere here.
PBr3 reacts differently- with alcohols, it mostly gives phosphonic acid O=PH(OH)2 and three molecules of the alkyl bromide. I have done this reaction
myself to prepare an alkyl bromide from an alcohol that is unstable towards elimination of H2O and mostly gives the alkene upon treatment with HBr.
With PBr3 (premade, not in situ!), the alkyl bromide can be obtained in high yield.
PI3 exclusively gives the acid and alkyl iodide, as seen in the preparation of methyl iodide.
So it is important which one of the phosphorus trihalides is used, and you have to use PCl3 and excess pyridine if you want to make trimethyl
phosphite.
EDIT: Here's the prep of triethyl phosphite, the methyl version ought to work analogously:
http://www.orgsyn.org/orgsyn/pdfs/CV4P0955.pdf
Diethylaniline is used instead of pyridine.
[Edited on 23-11-2010 by garage chemist]
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DJF90
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Looks like I was stupid enough to generalise the phosphorus halides. garage chemist is correct when he mentions that the phosphite can only be made
from the chlroride. Diethylaniline is used as the base because it forms a non-hygroscopic salt. unlike pyridine or dimethylaniline.
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