Hi!
I'm wondering about the differences between all the available reducing agents - and more specifically, why special reducing agents are used in different reactions.

I mean, sometimes, reductions are done with LiAlH4, sometimes with HI/P, Al/Hg, NaCNBH3, etc...

Now, for example, why does epehdrine need to be reduced with HI/P, why can't you use LiALH4 for example? I understand that some reducing agents are stronger than others, but that can't explain the huge variety of agents used in different reactions.

they work differently.

for example you can't reduce an alcohol with LAH (usually).
you can't reduce a ketone with rP/I2.

and so on...

check your o-chem book. (if you don't have one, buy one   )

Different functional groups are usually reduced with different reducing agents. Check out Vogel's on Rhodium.ws or some other org chem book.

ok, that makes sense, thanks

as i don't want to ask so many stupid questions, can somebody recommend a book to me that would make me understand how KrZ's method from tryptamine to dmt works? i don't understand what the formaldehyde does, and i don't understand what's being redoced

hm, would a different reducing agent than NaCNBH3 work just as well... if yes, which?

I would reccommend this article:

https://www.thevespiary.org/rhodium/Rhodium/pdf/sodium.cyanoborohydride.pdf

On page 141, there is an example of tryptamine being reductively alkylated with an aldehyde and NaBH3CN, and in section 5 on page 137 it is explained why the reduction works so well with NaBH3CN.

One of the main reasons for other reducing agents not working as good is that some reduce the indole ring of tryptamine to an indoline, see

https://www.thevespiary.org/rhodium/Rhodium/pdf/nabh4.carboxylic.acids.pdf

for example. Another limitation is that tryptamine and aldehydes under acidic conditions undergoes a so-called pictet-spengler cyclization, forming a tetrahydro-beta-carboline instead of the desired product, therefore all common reductive aminations using acid conditions won't work.