Enantiotopic and diastereotipic protons

Hi Guys,

I have a question asking me to explain the NMR spectrum shown for the structure given,

So I know (pretty sure) that from the shape of the spectrum the protons in each pair are diastereotopic to each other, but then each pair of protons are enantiotopic to each other. So the pairs of protons dont couple to each other but the protons in each respective pair will couple to each other, giving 2 doublets showing roofing due to similar environments.

However, when I did the substitution test to see if the protons in each pair are diastereotopic, it seems to me that they are enantiotopic! Where have I gone wrong?



For the second part of the question, assuming the pairs of protons are enantiotopic to each other but the protons in each pair are diastereotopic to each other, I know that enantiotopic protons will behave identically with achiral reagents but differently with chiral, and diastereotopic protons will behave differently with chiral and achiral reagents, but how do we predict the rates????

Thanks!

This is a nightmare that's come back to haunt me

I'm not sure what trouble you're having with the first part. The hydrogens attached to a single carbon are diasteretopic and the pairs on either side are enantiotopic.

The second part of the question is annoying. I think all four are equivalent and that the chiral base has no effect (it's an achiral molecule). I swapped one set of hydrogens for D and considering that 2's acetal stereocentre is undefined you can drawn hydrogens that are cis and trans to the Ph group, both in equatorial positions. Ignore that

This is a nightmare that's come back to haunt me

I'm not sure what trouble you're having with the first part. The hydrogens attached to a single carbon are diasteretopic and the pairs on either side are enantiotopic.

The second part of the question is annoying. I think all four are equivalent and that the chiral base has no effect (it's an achiral molecule). I swapped one set of hydrogens for D and considering that 2's acetal stereocentre is undefined you can drawn hydrogens that are cis and trans to the Ph group, both in equatorial positions. Ignore that

If the ring doesn't ring flip faster than the NMR timescale.... Which may well be the case with a large Ph group.... then the H's in question will be not be equivalent.

Ha! Not a fan?

But doesn't the drawing that I did show that the protons on the same carbon are enantiotopic? The relationship between the 2 structures (with one proton being H* in each) is enantiomeric isnt it?

Draw the molecule in 3D, remembering the stereocentre of the acetal. Change one of the hydrogens to X, then draw it's mirror. These are enantiomers. Finally replace the 'other' hydrogen with X and see that it's not the same as the mirror of the previous substitution.

Second part, you should have two different rates, based on what JMaydom said above with 3.

OK thanks, initially the carbon with the Ph attached isnt a stereocentre right? Because the molecule is symmetrical, but when you change one of the H to X you generate 2 stereocentres. Correct?

Thanks