Aromatic rings, neighbouring group

An example is the study by Lambert9 of the influence of aromatic rings and neighbouring electron-withdrawing groups on Sn2 reactions. He needed the few-tosylate 50. This comes from the diol 51 and now he had a choice. He could epoxidise an -alkene or dihydroxylate a Z-alkene. He chose the latter as Z-52 could be made by a Wittig reaction. 

We have already seen that identical protons do not couple with each other. The three protons in a methyl group may couple to some other protons, but never couple with each other. They are an A3 system. Identical neighbours do not couple either. In the para-disubstituted benzenes we saw on pp. 253-254, all the protons on the aromatic rings were singlets. 

Anion B is protonated by water with preservation of the right hand aromatic ring. The final product is a chiral molecule having no plane of symmetry so the boxed CH2 group is diastereotopic with Jab 15.4 Hz. This is larger than usual because of the jt-contribution a neighbouring Jt-bond increases V by about 2 Hz. 

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