Conformations of highly hindered aryl ethers—XVII. Hindered rotation and carbon–halogen bond magnetic anisotropy in benzyl phenyl ethers

The PMR spectrum of 3,5-dibromo-4-methoxybenzyl 2′,6′-dibromo-4′-methylphenyl ether shows two aromatic proton signals at δ = 7·33 and 7·71. Comparison with related benzyl ethers and anisoles led to assignment of the upfield signal to the phenyl-ring protons (H3′ and H5′) and the downfield signal to the benzylic aromatic protons (H2 and H6). In view of the nearly identical inductive effects to which they are subjected, their large shift difference must be due to long-range effects in preferred conformations. Analysis showed that the anomalous shift observed for the benzylic aromatic protons is due to deshielding by both the other aromatic ring and by the carbon-halogen bonds. The relation of this work to related biphenyls and diphenyl ethers is discussed.