Stereochemical relationship of molecules including double bond configuration and absolute configuration

Question

Are (S,E)-4-isopropoxypent-3-en-2-ol and (R,Z)-4-isopropoxypent-3-en-2-ol both enantiomers and diastereomers? I am a bit confused because they would be diastereomers when only looking at the configuration of the double bond or enantiomers when just looking at the chiral center. However, it doesn't make sense that molecules can be diastereomers and enantiomers at the same time. Could someone explain? Thanks.

Answer 1

Diastereomers, by definition, are stereoisomers that are not enantiomers, so you're right they can't be both.

I think you might be confused because you are trying to apply a simplified heuristic (opposite configuration of double bond = diastereomer, opposite configuration at chiral center = enantiomer) outside of its proper scope. This works well for molecules with a single stereocenter, but for slightly more complex cases you have to step back and think about the meaning of the terms for a moment.

Two molecules are enantiomeric when they are mirror images of one another, that is, if they are identical in every respect, except the configuration around their chiral center(s). (R)-4-isopropoxypentan-2-ol and (S)-4-isopropoxypentan-2-ol are enantiomers, because their single chiral center contains the exact same set of four substituents (H, OH, methyl and 2-isopropoxypropyl), just in reverse orders. However, your pair of molecules have only three substituents (H, OH and methyl) in common; the fourth substituent at C₂ is (E)-2-isopropoxypropen-1-yl in the first compound and (Z)-2-isopropoxypropen-1-yl in the second, which are isomeric but clearly distinguishable groups. (R)-4-isopropoxypentan-2-ol and (S)-4-ethoxypentan-2-ol aren't enantiomers, and neither are these molecules.

They are, however, still stereoisomers of one another, so they must be diastereomers.