[science123]

Which is the correct way of writing reagents for the reaction ? I have seen problems that write like the third example without H+. Isn't H+ required for imine formation ?
Also, in the first 3 examples, is H3O+ after NaBH3CN necessary? I think it is necessary since primary amine will make a neutral imine. The nitrogen of the imine will become negative after hydride ion reacts and the double bond becomes the second lone pair of Nitrogen. In case of secondary amine with ketone/aldehyde, the imine generated has positive which will become neutral after hydride delivery so H3O+ wouldn't be required.

Any comments?

[frisbee]

Some acid is needed for imine formation which is why it is often
times written in for your step 1. The proton source after hydride adds using borohydride reagents is usually the solvent which is why step three is not always written (usually aqueous acid is added to form and remove the borate). Depending on the reducing reagent used, the entire conversion can be carried out in one step.

[BobfromNC]

I prefer to write the actual reagent, such as AcOH rather than H+, since it is difficult to buy H+, but easy to get AcOH.   It is always better to specify exactly what reagent you used, as many acids would destroy a reductive amination, but acetic acid is fine.  I have tried to follow many experiments where the author did not clearly show what they did, so the more detail the better.  Option 2 is fine, if you replace H+ with AcOH, as you would mix all of the reagents at once.   If you actually made the imine first and isolated it, then 1 or 4 would be appropriate, but that is normally not necessary for reductive aminations.

[Vidya]

last step is better and it also gives the clear indication of all steps of mechanism .

[AWK]

NaBH₃CN reductive amination needs protonated imines