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Author: Subject: Phenyldichlorophosphine, Useful Reagent
Sauron
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[*] posted on 27-9-2007 at 21:56
Phenyldichlorophosphine, Useful Reagent


PhPCl2 is a trivalent phosphorus compound that is commercially available and also amenable to preparation by two main routes.

PX3 (where X = Cl, Br) is reacted with benzene over AlCl3 catalyst and the complex broken up to yield phenylphosphonous dihalide of phenyldihalophosphine. The trick is to break up the AlCl3 complex without hydrolyzing the air and moisture sensitive phosphine. The reagents of choice for this are 1) POCl3 which forms a granular complex with AlCl3 which is then filtered off, 2) pyridine, or 3) water, in amount just enough to break the complex.

The hydrolysis with water (usually as conc HCl) gives 20% yields. That with POCl3, 65-75% yields but at what cost? The pyridine method I have no details on as yet. It's a Russian reference in translation, Springer might have it.

The Org.Syn procedure for the Michaelis-FC process and using POCl3 to liberate the product is attached.

Alternatively PX3 can be reacted with phenyldiazonium fluoborate and the resulting complex reduced to PhPX2.

The diazonium complex can also be hydrolyzed to the phosphonuc acid, chlorinated and reduced, so that for example starting with PBr3 the final product can still be PhPCl2.

In general the Michaelis modified Friedel-Crafts is higher yielding than the diazo method.

Phenyldichlorophosphine is useful for the desulfurization of certain organothiophosphorus compounds. For example esters of thiophosphoryl chloride P(=S)(OR)3 are converted to P(OR)3. The byproduct is PhP(=S)Cl2 which itself interesting.

If you can't make or purchase PCl3 or PBr3, do not despair. P4O10 (phosphorus pentoxide) reacts with benzene in large excess of the latter to give PhPO(OH)2 convertible to the dichloride and reducible to PhPCl2.

Typically this reaction requires an autoclave, 275 C 24 hrs to get to the phenylphosphic acid. The reduction of the dichloride is by Mg turnings. A variety of chlorinating agents have been used, with variable results.

PhPCl2 will also reduce phosphine oxide derivatives to phosphines. Trialkyl or triaryl phosphates to the corresponding phosphites for example. Maybe just maybe POCl3 to PCl3.

While less direct and requiring special equipment, this is a better way into this system than the P-trihalides for many people.


[Edited on 29-9-2007 by Sauron]

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[*] posted on 28-9-2007 at 21:01


Maybe there is some method of recovering the POCl3 from the AlCl3-POCl3 solid complex. That would certainly be better than using up 1 mol POCl3 for every mol PhPCl2 formed.

I will look into this aspect.

Meanwhile, although I have not laid my hands on the Russian article as yet, which states that pyridine is useful for removal of AlCl3 from Michaelis-FC reactions, at least I have figured out that they are probably talking about the AlCl3-Pyr adduct. So this may be a viable way to go, pyridine is a lot more asvailable than POCl3.

There are also higher AlCl3-Pyr adducts like AlCl3-2Pyr, AlCl3-3Pyr etc.

[Edited on 29-9-2007 by Sauron]




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[*] posted on 30-9-2007 at 22:08


The reaction of alkyl halides with PCl3 and AlCl3 to form alkylphosphic acids, alkylphosphonyl dichlorides or alkylphosphonous dichlorides, depending on the workup, is obviously very closely related to the Friedel-Crafts reaction (as modified by Michaelis) in which an aromatic compound such as benzene reacts with PCl3 and AlCl3 to form the same products - again with the workup controlling which product is obtained.

Certain olefins also react the same way.

While the Friedel-Crafts reaction is more than 125 years old and the Michaelis modification still in nineteenth century, the reaction of alkyl chlorides with PCl3 and AlCl3 was discovered in 1948 by Clay and studies shortly thereafter by Kinnear and Perrin, and others.

In the simplest instance, chloromethane is reacted with PCl3 and AlCl3, no solvent, in a pressure bottle with shaking. Ethyl chloride is treated in same fashion. The complex

[MePCl3][AlCl4]

is solid, whereas [EtPCl3][AlCl4] is liquid.

These complexes are salt like (ionic) as demonstrated by the fact that a solution in MeNO2 is electrically conductive.

Hydrolysis proceeds in two stages: first to the formation of the alkylphosphonic dichloride, and then to the alkylphosphonic acid.

Alcoholysis produces alkylphosphonyl ester chloride or alkylphosphonyl diesters.

It is possible to work the complex up to the alkylphonous dichloride.

Also by treating the complex with H2S the alkylthiophosphonic dichloride can be had. The same product is available according to a US patent, by treating the alkylphosphonic dichloride with P4S10 or by reacting the alkylphosphonous dichloride with elemental sulfur.

Note that the lower alkylphosphonous dischlorides are usually highly pyrophoric so must be manipulated in an inert atmosphere.

The alkylphosphonic and alkylphosphonous dichlorides are also highly corrosive.

The esters, available from the dichlorides or from alcoholysis of the complex, often exhibit biological activity.

The identical alkylphosphonic diesters are produced by the classical Michaelis-Arbusov reaction, see Org.Syn. for the preparation of di-isopropyl methylphosphonate for example.

Tri-isopropyl phosphite is reacted with iodomethane.

P(OPri)3 + MeI -> MeP(O)(OPri)2

But I would recommend avoiding the making of any of these esters by either method because of their biological activity. While not of a military order, it is sufficient to get you in trouble, both criminally and from the standpoint of your own health.

I mention them as an example of what not to do. They are hardly a deep dark secret as they are in Org.Syn. and no one seems to be going after Wiley for aiding & abetting terrorism.

With alkyl halides of chain length C3 and up, n-alkyl halides do not give the expected products, isomerization occurs. Thus n-propyl chloride gives the same product as isopropyl chloride, namely the isopropylphosphonic acid or derivative.

Even more oddly, isoamyl chloride gives isobutylphosphonic acid, an entire methylene being lost. An extrusion rather than rearrangement.

This phenomensa has narrowed the scope of this procedure a lot more than Clay, the originator, initially expected.

The Arbusov reaction IIRC does not suffer from this nor does the Grignard, or lithiation. Nor the diazonium fluoborate route.

Somewhat analogous isomerizations do occur in Friedel-Crafts alkylations however.




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[*] posted on 1-10-2007 at 09:58


The literature has confirmed that dialkyl alkylphosphonates are cleaved to the corresponding dichlorides by PCl5, SOCl2, (COCl)2 as wel as by phosgene. Of these, oxalyl chloride is the most convenient and efficient.

Furthermore, the same reagents convert the dialkyl thiophosphonate diesters RP(S)(OR')2 into RP(O)(OR')Cl and elemental S. So oxalyl chloride may also convert triethyl dithiophosphate into POCl3, thus affording me a facile route to POCl3 from P4S10 and HC(OEt)3.

This is very good news indeed.




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[*] posted on 2-10-2007 at 22:43


This might be of some use, haven't seen but the summary

Quote:
W. Brockner, K. Grande and H.A. Øye:
Vapour Pressure of the AlCl3-POCl3 System.
Ber. Bunsenges. Phys. Chem. 91 (1987) 561-565
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[*] posted on 3-10-2007 at 00:58


AlCl3-POCl3 complex is a granular solid that is filtered off from the reaction mixture(s).

I would certainly like to have a clue as to how to recover the POCl3 from this solid, are you suggesting that it might be thermally labile?

Now how do I get my hands on this German p-chem journal?




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[*] posted on 3-10-2007 at 06:01


As I said, I've not seen the text, just the brief, so I don't know any details and if a thermal separation is possible. But it's the only thing I've seen that may have useful information.

Be nice if a decent thermal recovery was possible, certainly increase the economic viability of the process.
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[*] posted on 3-10-2007 at 06:14


Just how many times are you going to misspell "phosphorous" and its groups?

Tim




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[*] posted on 3-10-2007 at 06:16


The full title of the journal is

Berichte der Bunsengesellschaft, Physik Chemie

I have been unable to locate a DOI, or any site for this journal, which sounds like a house organ for Bunsen GmBh




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[*] posted on 8-10-2007 at 07:39


12AX7, "phosphorus" is an element. "Phosphorous" is used in some tervalent phosphorous compounds, for example phosphorous acid H3PO3 and its derivatives.

Another example hexamethylphosphorous triamide is a perfectly correct usage for the compound P(NMe2)3 not to be confused with hexamethylphosphoric triamide which is the oxide of the former and therefor, pentavalent.

There are several competing systems of phosphorus nomenclature in the literature. Some are obsolete, some are obsolescent but still commonly encountered.

Or maybe you do not care for phosphonous"

When pentavalent phosphoris has a single alkyl or aryl subsituent it is named as a derivative of phosphonic acid, and when the same is true of a tervalent phosphorus it is named as a derivative of phosphonous acid, i.e., phenylphosphonous acid PhP(OH)2 so the dichloro derivative is phenylphosphonous dichloride, Also named phenyldichlorophosphine. Or Dichlorophenylphosphine.

There are other nomenclatural possibilities that can be encountered and must be understood but those are the most current ones.

[Edited on 8-10-2007 by Sauron]




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[*] posted on 8-10-2007 at 07:51


I have to thank member Orgkemi for his assistance in obtaining a key Russian reference and translating part of it for me.

Benzene + 3 PCl3 + AlCl3 -> PhPCl2-AlCl3 + 2 PCl3

PhPCl2-AlCl3 + Pyridine -> PhPCl2 + Pyridine-AlCl3

So pyridine can be used in place of POCl3 to displace PhCl2 from the complex with aluminum chloride. The yield is 90%.

"Phenyldichlorophosphin. Benzene (36g), PCl3 (206g), and AlCl3 (70g) (molar
ratio 1:3:1) were placed in a four-necked round-bottom flask furnished with a
dropping funnel and a reflux condenser, the latter both equipped with CaCl2 pipes.
The mixture was vigorously stirred and boiled for 3 h. Abundant hydrogen chloride
was trapped with a water. The mixture was cooled, abundant PCl3 was distilled off
at 30-40 mmHg and 50-55°C bath temperature. Viscous liquid residue was treated
with stirring by 42g of dry pyridine (molar ratio AlCl3:Py=1:1) while controlling
the temperature in the region 40-50°C. The mass obtained solidifies below 60-
70°C. This was heated above melting point and stirred; volatile fraction was
distilled off under reduced pressure first. Then 83g of phenyldichlorophosphin of
technical grade was distilled off at 2-3 mmHg and bath temperature 90-155°C.
Repeated distillation gave 80g (90%) of the pure product, b.p. 58-59°C (0.8-1
mmHg) in accordance with the literature data [2]."

[Edited on 8-10-2007 by Sauron]

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