1H splitting pattern of benzyl CH2 protons During my PhD, I prepared the two compounds below (there was actually a slightly longer chain but it's not inherently relevant).

When looking at the 1H NMR spectra of these compounds, something slightly weird occurred. 

In one of them, the CH2 group of the benzyl appeared as a singlet (s, 2H). In the other, the CH2 group of the benzyl was observed as a multiplet (an AB quartet like structure, to be specific).

Is there any rationale for why the two cases would be different, given the distal nature of the closest stereocentre (i.e. why are the two CH2 protons diastereotopic to begin with given how distal they are)?
This is a bit speculative. I'm still thinking about it, and maybe this shouldn't even been an answer. I drew out the left structure and the enantiomer of the right structure because I want the configuration at the carbon with the OMOM group to be the same.



I think you're calling these the 1,2-anti-1,4-anti and 1,2-syn-1,4-syn, respectively. The main focus here was on the conformation about the 1,2-carbon-carbon axis. I put the two hydrogens anti to each other. In the anti-anti case, the alkyl chain extends off away from the benzyl moiety while in the syn-syn case, the alkyl chain is pointed more towrads the benzyl moiety.

I'm not in love with this explanation. It's a bit speculative, but I used the tools I have and at least, it's consistent with your observation about splitting.