This reductive amination.aldehyd reacts with amine to form imine which gets reduced to amine: https://www.organic-chemistry.org/synthesis/C1N/amines/reductiveamination.shtm

Dear Dr Wintner,

Thank you very much for your answer. Would you please elaborate the addition mechanism? As I have already done the procedure. But unfortunately the reaction is not happening fruitfully. Please add a reference if possible.

Well yes, the reaction is not optimal. Usually you would want to use sodium cyanoborohydride (Borch reagent) which is a little more stable and selective. The tetrahydrido borate most likely reduces you carbonyl compound to the alcohol and thus stops/ foils your reaction to yield the reductive amination compound. (The cyanoborohydride is more selective when it comes to reducing the imine / iminium ion)

Dear Fabian,

Thanks for your answer. My question is if it is possible to selectively reduce the aldehyde part of 2-aminoterepthalic acid without hampering the acid parts? Fabian Rüger

I don't know where to find an aldehyde in this molecule.

Dear Fabian,

Extremely sorry! In my previous question, I missed one point. I meant if I want to condense 2-aminoterepthalic acid and 2-pyridine carboxyaldehyde, then is it possible to condence with help of sodium borohydrate? Fabian Rüger . NaBH4 may reduce carboxylic group also.

Under normal conditions the carboxylic acids should be safe, but ! you will most likely reduce the carboxyaldehyde to the corresponding methylalcohol and what is more the 2-aminoterepthalic should be a zwitterionic species (and electron poor aromatic) so the amine will not react as fast as a normal (aliphatic) amine.

i would do it in 2 steps:

1. first make the imine

2. then reduce it with the tetrahydro borate .

NaBH4, NaHB(OAc)3 neither NaCNBH3 reduce carboxylic acid.

Yes you should do Indirect reductive amination (First prepare imine and afterwards reduction)

https://pubs.acs.org/doi/abs/10.1021/jo960057x?src=recsys&journalCode=joceah

For examples see:

http://commonorganicchemistry.com/Rxn_Pages/Reductive_Amination/Reductive_Amination_Index.htm