[Machuu]

Hello, my question is about optical isomers. Is there a possibility to exist a molecule with 2 chiral centers and only R,R and S,S optical isomers? If, yes could you give me and example and explain why? Or maybe every molecule with 2 chiral centers must have 4 isomers?

[Babcock_Hall]

It is a forum rule that you must provide your answer or your thoughts before we can help you.

[Machuu]

Okay, so I looked through a few books and old notebooks and found information that molecules with n chiral centers have 2^n stereoisomers.
I'm asking here because I can not find information about R,S and S,R isomers of N,N′-Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine (CAS  135616-40-9 R,R and 135616-36-3  S,S)

[OrganicDan96]

look up meso isomers

[Babcock_Hall]

The number of stereoisomers ≤ 2^N.

[kriggy]

Okay, so I looked through a few books and old notebooks and found information that molecules with n chiral centers have 2^n stereoisomers.
I'm asking here because I can not find information about R,S and S,R isomers of N,N′-Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine (CAS  135616-40-9 R,R and 135616-36-3  S,S)

You are partial right about the number of stereoisomers. However, in case of some molecules, the number of stereoisomers is lower than 2ⁿ. THat is due to the fact, that some of the stereoisomers are in fact same compounds (= meso compounds: stereoisomers that can be superimposed with their mirror image). The best example is tartaric acid which exist as pair of enantiomers (D / L tartaric acid) and one meso compound.
In case of your compounds, the reason (IMO) why you cant find information about the other two stereoisomers is that they are not chiral and therefore of little interest as a ligands. For more info, see attached picture that might give you better idea.
As you can see, the two compounds on right side are identical and not chiral (they have plane of symetry).