Zealot: a new zincorganic synthesis of P2P's

[Antoncho]

The following is a verbatim translation of

Post 345309 (missing)

(zealot: "Ñèíòåç Ð2Ð ÷åðåç öèíêîðãàíèêó.", Russian HyperLab):





Zincorganic synthesis of P2P's.



2C6H5I                  +                2Zn                  =              (C6H5)2Zn              + ZnI2

M=204                                     M=65                                M=219
m=408 g                                  m=130 g                            m=219 g
m=63 g                                    m=22  g                             m=34 g
D=1,83 g/ml
V=34,5 ml

(Ñ6H5)2Zn         +           2BrCH2C(O)CH3      =       2C6H5CH2C(O)CH3    +    ZnBr2

M=219                             M=137                                 M=134
m=219 g                           m=274 g                              m=268 g
m=30 g                             m=37,5 g                             m=37 g
                                         D=1,6 g/ml
                                         V=24 ml





Almost forgotten until recently, zincorganic comp'ds beecome now increasingly often used in synthesis once again. They are less reactive than Grignars, but in many cases - e.g., when it is necesary to preserve a functional group, this is an advantage rather than a drawback. For magnesiumorganics the more stable form is the asymmetric one, which exists in form of associate comp'ds (etherates); between the two  forms, RMgX and R₂Mg, there exists a Schlenk (sp?) equilibrium. OTOH, for zincorganics the more stable is the symmetric form, which is conveniently obtained from the asymmetric one by heating. In addition to that, R₂Zn species aren't prone to association and have a linear structure.

It remains a mystery for me as to why the well-known sources are full of methods making use of
cadmium-organics, while everyone knows of their extreme toxicity; beesides, the only way of obtaining those is thru exchange between Cd salts and Grignars, whilst Zn-organic compds can bee obtained directly from alkyl or aryl halides and are non-toxic. The traditional method of making them is from alkyl halides and Zn dust or foil. Later it was shown that alkyl bromides may bee used in this rxn as well as iodides, and only for the less active phenyl halides, iododerivatives must bee used.

The solvts employed in this rxn may bee ethers (diisopropyl, dibutyl) , toluene, benzene, xylols.


Synthesis:

20g Zn dust is washed w/5% HCl, then 2x100mls water, 2x50mls acetone and 2x50mls ether (Zealot keeps his ethers over KOH, which is reported to give even better results that traditional Na drying - proc. has been posted here). All is placed in a 500ml RBF w/a reflux condenser and 75 mls toluene is added. A mag stirrer is turned on, there's added several iodine crystals, and the system is flushed with Ar. Thru a dropping funnel there's added a little of the mixture of 75mls toluene and 34,5mls iodobenzene and the flask's bottom is heated to 80-90 C on oilbath. A vigorous reaction takes place, and if the condenser begins to choke up, heating and addition is disontinued for a while. A following nuance should bee noted: if in preparations of Grignars the faster the reagent is prepared, the better it is; with Zn comp'ds, after all the PhI has been added, the mixtr needs to bee boiled for additional 3-4 hrs to allow full disproportionation to ZnPh₂. This is accompanied by heavy precipitation of ZnI. Thus obtained solution of diphenylzinc is as quickly as possible decanted from the precipitate, the latter is washed w/2x50mls toluene and all is placed into a dripping funnel.

The mixtr is gradually added to 24mls bromoacetone (see

Post 285923

(Antoncho: "Bromoacetone Synthesis", Novel Discourse) for the best synthesis of bromoacetone, also by Zealot) in 50mls toluene, ZnBr2 precipitation occurs. The liquid phase is decanted and the solvent is evap'd , the residual oil is dissolved in 80mls MeOH and 20mls 40% aq. alkali and refluxed ~15 mins until the complete disappearance of lacrymatory bromoacetone smell. MeOH is removed and P2P is xtracted with ether.

Yields are usually around 70-80%.

The reaction also works well for preparing substituted phenylacetones.

A necessary note on the so-much-dreaded lacrymogenicity of bromoacetone: there were times when i successfully worked w/it at home (wearing an ordinary gas mask ).






Antoncho

[Antoncho]

The reaction is moisture-sensitive, but there's a pleasant advantage over Grignars: it is carried in non-etheric solvts which can bee dried by simple distillation. Personally i use chromatographic grade solvts - they don't need to bee dried at all.
The principal concern is to wash and dry the Zn dust very thoroughly; if toluene gets turbid after its addition, that's a sure sign that there'll bee problems with starting the rxn and/or the yield.

This has been successfully performed on: p-iodoanisole, p-iodotoluene, p-iodobenzodioxole, iodomesitylene - the last one gave the lowest yield and the resulting amine was completely inactive.




Translated by
Antoncho

[Mountain_Girl]

Just wanna point out for those who don't know:

Iodobenzene can be prepared directly from C₆H₆ + I₂, or from aniline + KI + NaNO₂.
Both methods are described in Vogel and TFSE.

[PrimoPyro]

Silver benzoate + halide --> halobenzene. It's called the Hunsdieker Reaction and can be referenced at

http://themerckindex.chemfinder.com/TheMerckIndex/NameReactions/ONR200.htm

PrimoPyro

[Nemo_Tenetur]

A few weeks ago I've looked for zinc diethyl preparation. Org. Syntheses told that a zinc/copper couple (best freshly prepared) is necessary and at least a bromoethane/jodoethane mixture for good results. In your writeup they use only washed zinc dust and bromo compounds. Does this really works well?

[zealot]

Use of zinc - copper pair is connected with  unpredictability of reaction. You can heat up reaction to rather high temperature and it all the same will not begin, and then suddenly Rm sharply boils also contents of a flask is thrown out outside. The zinc - copper pair simultaneously serves as the catalyst and for soft current of reaction. If will make zinc diethyl  from jodoethane do not use a zinc dust but only a foil differently reaction can be finished by explosion. And it is even better bromoethane and a zinc foil. And still phenylbromide do not react with zinc, only phenyliodide.

[Antoncho]

First of all - thank you all for the input!

Actually, there is a document on Rh's that beats all of the iodination methods i've seen thus far with its simplicity, yields and OTCness -

https://www.thevespiary.org/rhodium/Rhodium/chemistry/aromatic.iodination.i2-nitrate.html

- all it takes is I2 and KNO3; and it works on all sorts of cpds, save unsubst'd and halogen-subst'd benzene.

A HUGE scope of possible comp'ds to research - yet another good news is that iodination is very para-selective (xcept w/toluene, where it gives 61% of p-isomer of overall 61% yield).

Primo: actually i' afraid that Hunsdiecker (also Borodin ) rxn works only w/Cl and Br. With iodine it proceeds completely differently - see linked rxns from that Merck entries.

Borodin who co-authored this rxn was also a famous composer. He, among other things,  wrote 'Prince Igor' opera - if you're familiar w/classical music



Antoncho

[josebob69]

[ClearLight]

I made it when I was 15,  by putting br in acetone in sunlight... it is toxic but at such high levels, you'll disappear before you get that much.. a simple surplus military gas mask will work... it was one of the best lachrymators  until brombenzyl cyanide in wwI..
 does polymerize though...


Infinite Radiant Light - THKRA

[raffike]

Halobenzyl cyanides are heavily watched as they use em in chemical weapons.
And you ,who said he had a gallon of P2P and now wants to know what to do with it,i really doubt you have P2P.
A friend with speed is a friend indeed

[El_Zorro]

So would this work on a benzyl chloride?  And if so, would the resulting compound be reactive enough to give decent yeilds of the corresponding P2P when reacted with acetonitrile and hydrolized? 

https://www.thevespiary.org/rhodium/Rhodium/chemistry/grignardnitrile.html

  The link says that the nitrile is already a poor reacting reagent for the reaction, will the reduced reactivity of the Zn compounds hurt this reaction any more?
Who is that masked man?

[hermanroempp]

Should work. The problem is not only the poor reactivity of the nitrile, the problem is also the unwanted side reaction of the ketimine magnesium halide (intermediate) with the Grignard reagent (benzylmagnesium chloride) where the imine can be converted into an amine, resulting in poor yield of P2P. This can be overcome by using a great dilution of the reagents and by inverse working => adding the solution of the Grignard reagent slowly to the ethereal solution of the nitrile so that the nitrile is always in excess over the organomagnesium compound. Yield of the desired product P2P is also increased by using a benzene/ether mixture as solvent, like stated on Rhodium's page.
The use of the (less reactive) zincorganic compound in this reaction should keep side reactions at a minimum, should allow the use of normal working technique (no inverse working), but will need longer reaction time and/or higher temperature.
Check out

https://www.thevespiary.org/rhodium/Rhodium/chemistry/phenylacetone.html

Quidquid agis, prudenter agas et respice finem!

[pHarmacist]

Procedure:

In a 1-l. three-necked flask fitted with mechanical stirrer, reflux condenser, and separatory funnel, are placed 381 g. (1.5 moles) of iodine and 400 g. (455 cc., 5.1 moles) of benzene. The mixture is heated to about 50° on a water bath, and 275 cc. (6.15 moles) of nitric acid (sp. gr. 1.50) is added slowly from the separatory funnel; the time required for the addition should be about one and one-quarter hours. A copious evolution of oxides of nitrogen takes place, and the gases are carried off from the upper end of the condenser to an open window or hood, or absorbed by means of a gas trap (Fig. 7 on p. 97). The reaction proceeds smoothly (Note 1), and the temperature rises slowly without the application of heat until the mixture boils gently. When all the nitric acid has been added, the solution is refluxed for about fifteen minutes. If iodine still remains, more nitric acid should be added slowly to the warm solution until the purple color of the iodine has been discharged and the solution becomes brownish red.
The lower reddish oily layer is separated, mixed with an equal volume of 10 per cent sodium hydroxide solution, and steam-distilled from a 2-l. flask until no more oil passes over. Towards the end of the distillation a yellow solid begins to collect in the receiver; this consists of nitro compounds, which are removed by vigorously stirring the oil for about three hours with 20 cc. of concentrated hydrochloric acid, 300 cc. of water, and 200 g. of iron filings in a 2-l. flask connected with a reflux condenser.
The mixture is allowed to cool and is then filtered. The filtrate is rendered distinctly acid to Congo red with hydrochloric acid and again distilled with steam. The oil so obtained is separated and distilled under normal pressure with the use of a fractionating column (Note 2). The fraction boiling at 180–190° is redistilled, and the pure compound is collected at 184–186°. The yield is 523–531 g. (86–87 per cent of the theoretical amount) (Note 3).

http://www.orgsyn.org/orgsyn/prep.asp?prep=cv1p0323

"Turn on, Tune in and Drop Out"

[lugh]

Early workers with organozinc compounds used a carbon dioxide atmosphere  

Perkin's description of the preparation of the zinc-copper couple:

Granulated zinc coated by a thin layer of copper by immersion in a dilute solution of copper sulphate and subsequent drying