[meshohel]

Reductive amination of 3-aminophenol with D-ribose in presence of NaCNBH3 yeilded a complex mixture. The reference used AG50W cation exchange resin to separate the compounds and elution with 1% NH4OH.

But the problem is how to quench excess NaCNBH3 and what is the basic way to pack a column with AG50W?

[Dan]

1. Can you provide a citation for the reference?

2. I think cyanoborohydride is typically quenched with dilute NaOH. However, you should not put NaOH on an acidic cationic resin because it will neutralise it. I doubt you could add NaOH(aq) and then extract out your product from your reaction mixture prior to the resin column because it will be very polar. I would need to see the reference, but I suspect they are just loading the crude reaction mixture directly onto the resin, which will generate HCN so be careful and try to find an alternative reducing agent if possible (like triacetoxyborohydride or H₂, Pd/C).

3. I've never worked with this specific resin, but I have used Dowex 50W resins for amine purification, which are similar (solid supported sulfonic acid).

i) First suspend the resin in water and decant off any froth or small resin fragments that don't sink (repeat as necessary until there is no more froth or very fine resin fragments). Load the suspension into a column and run off the water to the level of the packed resin.

ii) Carefully add more water with minimum disturbance of the resin and run it down to the level of the resin again. Repeat this until the eluent emerging at the end of the column is neutral. Your resin is now cleaned and ready to use.

iii) Load your mixture onto the column, and run the solvent down to the level of the resin. Now repeat step (ii) until the eluent emerges neutral again.

iv) Carefully add NH₃(aq) and run it down to the level of the resin again, repeat. On Dowex, I use 1-2 M  NH₃(aq). Your amine products will be somewhere in the ammoniacal fractions you obtain in this process. I have no experience with the resin you mentioned, but on Dowex the amine is liberated very quickly and is typically completely removed from the resin after elution with 2-4 column volumes* of NH₃(aq). This method of purification separates amines from everything else, but does not separate amines from each other. I suspect the procedure you mention is similar in this respect (but I can't be certain).

*column volume = volume of resin, i.e. if your resin is 5 cm deep, one column volume of eluent is 5 cm deep. Run through one column volume at a time because there will be some diffusion of your amine back into the eluent.

[Babcock_Hall]

I believe that AG just stands for analytical grade.  Sometimes when I work with Dowex, I prerinse with water or ethanol to remove an orange dye.

[meshohel]

Ashton, W. T.; Brown, R. D.; Jacobsen, F. E.; Walsh, C. T. J. Am. Chem. Soc. 1979, 101, 4419–4420.

This is the reference.

Could you please suggest me what is the best way to regenerate the resin after eluting with NH3 and and my sample. The resin turns dark brown.

[Dan]

Could you please suggest me what is the best way to regenerate the resin after eluting with NH3 and and my sample.

Check the manufacturer's website, they should have the optimised procedure there. Probably involves a few rounds of HCl(aq) soaking and washing.

[camptzak]

What compounds does this form? like you said its a reductive amination but Im having a hard time following.

The sugar converts to an aldose and the 3-aminophenol reacts with the aldehyde functionality to form an imine.

The sodium cyanide attacks the BH₃ and the resulting hydride attacks the lumo of the imine placing the negative charge on the nitrogen.

The negatively charged nitrogen is stabilized by the Boron cyanide, leaving the negative charge on the boron.

Also It seems like you would need an acid to quench this reaction.
What is the purpose of the exchange resin? does an H⁺ exchange for sodium in this case?
This is confusing to me.

[meshohel]

The product is highly polar and only resin can separate efficiently. These resins are good for separating amines from other products.

[Babcock_Hall]

Ashton, W. T.; Brown, R. D.; Jacobsen, F. E.; Walsh, C. T. J. Am. Chem. Soc. 1979, 101, 4419–4420.

This is the reference.

Could you please suggest me what is the best way to regenerate the resin after eluting with NH3 and and my sample. The resin turns dark brown.

One manufacturer suggests 4 volumes of 1 M HCl (pH < 2) followed by 4 volumes of water rinse (pH > 4), to covert from the sodium ion form to the protonated form.  Because ammonium ions (relative selectivity 1.95) are a little bit more tightly bound than sodium ions (1.5) which are slightly more tightly bound than protons (1.0), one might want to use more column volumes of HCl than one uses to convert the Na+ form of Dowex-50 to the H+ form, possibly six.  A fastidious researcher could also test for the absence of ammonium ions.