A large nuclear magnetic resonance shielding involving a carbon–carbon bond
Abstract
The resonances of the H(3-endo) protons in the Diels–Alder adducts of allyl compounds (CH2CH·CH2Y, where Y = Cl, Br, OH, or SPh) with cyclopentadiene and hexachlorocyclopentadiene are at considerably higher field than the corresponding proton resonances in norbornene and hexachloronorbornene. This is thought to be due to the diamagnetic anisotropy of the carbon–carbon bond joining C(2) to the –CH2Y group. The extent of the diamagnetic shielding is modified by the electronegativity of the group Y.