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Author: Subject: the right choice of PTC catalyst
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[*] posted on 2-1-2018 at 20:38
the right choice of PTC catalyst

Hey, there is something that is very unclear to me.
With PTC, in this case, only quarternary ammonium salts are meant, to make that clear first.

I am a little confused as to which substituents/substituent combination has influence on the outcome of the reaction they´re used in?
I want to attempt a reaction where especially tetrabutyl is used in the only example found, and I wonder, if this can be substituted under certain circumstances?
Somewhere I´ve read that the butyl groups have the ideal length, but said source sounded like they just wanted to claim how superior another "aliquot #465666" is, indeed every source who claims superior PTC-qualities with a "new" quat. sounds just like this.

To me, it seems that their properties aren´t that different and they are mostly exchangeable without much difference in the outcome, or is this a wrong impression I have?

For example, for my intended reaction, tetrabutyl-groups work better because of their length than tetraethyl(toxic:(), but how about a triethylpentyl ammonium salt? Would the long alkyl chain suffice to make it a better PTC for my reaction, like the trimethyloctylammonium salt?
I just guess the octyl chain makes it more efficient to work with a wide range of substrates, while the methyl groups are responsible for the hydrophilic effects, thus being very effective to have polar and nonpolar functional groups?

Specifically, I want to N-alkylate indole in aqueous alkaline media with a PTC, and lacking knowledge about them, nor want to buy this special one called for, just want to substitute it with another one if possible.
For example, I already have TEBAC, but would it even work at all?
I could easily make TEPeBr(triethylpentyl-NH4Br), or Tetraethyliodide, and the Tetrabutyl substituted salt could be made too.
But I don´t want to buy anything new for only this single reaction.
In fact I would rather like to understand those quarternary ammonium salts and how their substituents affect a reaction, like, are they all useful, is it only in reaction time/efficiency, or are there really special better substituents?

If there´s something that has a brand name and is patented under it(the Aliquot 0xx´s), I tend to become a little mistrustful.
They´re seemingly only deserving name and paper because the inventors want to make some advice for their PTC, while preventing others from making profit with it...

Or am I totally wrong and the choice of PTC´s and their substituents is more important than it seems?
I think, the choice of PTC matters much less as the choice between random PTC´s and no PTC at all, right?

Can somebody shed some light on these very useful compounds, especially on how to choose the right PTC reagent? I guess it depends on the polarity of the substrates which should react with each other, but is there maybe a general rule after which to decide?

My hope is, that not-so ideal catalysts still work, and only prolong the reaction time, not more? That would pose no problem, TLC would help to determine the progress of reaction, as long as it happens anyway.

Obviously I´m completely clueless, I´ve used PTC´s before, but only exactly like specified in my synthesis, now I´m trying to replace one, realising painfully how little I understand these reagents... Always assumed they´re all replaceable more or less, doubt it now... :)

And how important is the anion at all? The PTC used in said application isn´t even an halide salt, but a sulfonate(I guess). Edit: No, an ammonium hydrogen sulfate salt, no sulfonate.

Thank you all!
If this fits better in the S-Q thread, please put it there, no worries :)

PS:(second attachment on page 13)

[Edited on 3-1-2018 by karlos³]

Attachment: n-pentylindol PTC.pdf (68kB)
This file has been downloaded 26 times

Attachment: ptc alkylation indol.pdf (324kB)
This file has been downloaded 40 times

[Edited on 3-1-2018 by karlos³]
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[*] posted on 3-1-2018 at 10:39

A company years ago made thousands of combinations of the alkyl groups to find disinfectants, from the little data that I could see, similar ones behave similarly, thus tributylpentyl would work as well as tetrabutyl or tributylpropyl... But benzyltrimethyl would be quite different than benzyltripalmoyl... As long as you have enough carbons on the nitrogen, it will be lipopohilic...
Most are made in order to be cheap to make and easy to handle, thus the simples solution is to use cheap SM like n-butyl-chloride, fatty acid derived C12-C16 chains, and simple methyl and ethyls. If all of the chains are the same, the analytical work will be simpler, thus tetrabutyl being popular and tributylpentyl not being so...
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[*] posted on 3-1-2018 at 17:29

I can't claim to know much about it, but since this is the internet, allow an ignorant hack to wildly speculate. :-) My understanding is that the PTC is acting as a ferry, picking up the reagent from the aqueous layer and 'dropping' it in the organic layer when it dissociates by chance. Once dissociated from the PTC, the reagent is free to react with whatever you have in the organic layer.

Looking at it this way, my assumption would be that the system is reasonably insensitive to the particular PTC used, with the primary factor being the relative solubility of the PTC (and the PTC-reagent complex) in each phase. For instance, a PTC with large, non-polar alkyl groups might be particularly effective at transporting the reagent into the organic layer, but at the same time might be less able to migrate into the aqueous layer to pick up that reagent in the first place. As a result, there might be a sort of zero-sum game at work, limiting how much variation in effectiveness you get.

Assuming the PTC fairly easily dissociates from its anion, it should simply be a spectator ion in the reaction.

Which is all to say I would be quite surprised if you couldn't use virtually any of the common commercial products with, as you say, some variation in reaction speeds.
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[*] posted on 8-1-2018 at 22:37

Any study on PTC should begin here........

'Phase Transfer Catalysis' by Starks, Liotta and Halpern


MSDS Addict and OTC Slut
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[*] posted on 9-1-2018 at 04:13

Thank you all!

I´ve decided to alkylate triethylamin with both bromopentane as well as ethyliodide, and try them both.
Since I am going to modify the planned reaction according to a friend, the organic phase will be only the melted substrate and no solvent besides this will be used.

But the abundance of possible applications for PTC´s is very encouraging, I am already planning other uses, phenolic ether alkylation for example.
It seems to me, that especially reactions which are under normal circumstances difficult to carry out, suddenly become easy, fast, and high yielding if a PTC is used. Or even become possible at all!
I wonder why these reagents aren´t more popular amongst hobby chemists?
I think they are, if not readily purchaseable, at least easily prepared OTC, no?

CJ´s book is sadly not downloadable for me.
But I´ll look in other places, maybe I´ll find it anyway.
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