Conformations of highly hindered aryl ethers—IV PMR studies of 2,4- and 2,6- dinitroaryl ethers

PMR studies of a series of 2,4- and 2,6- dinitrophenyl- and 2,4-dinitronaphthyl- aryl ethers strongly indicate the existence of preferred conformations. Extensive conjugation of the ether linkage with the dinitro ring, resulting in their coplanarity, is an important stereochemical influence. In the ensuing skew conformation the dinitro ring is positioned in such a way that the 6-substituent is proximal to the other ring. In the case of the 2,4-dinitrophenyl ethers ( 1 ) this is the 6-proton, which shows the expected shielding due to the ring current in the other ring. In the 2,6-dinitrophenyl-( 2 ) and 2,4-dinitronaphthyl-( 3 ) ethers, this is a nitro group, twisted out of coplanarity to fit over the other ring, whose magnetic anisotropy is reflected in the shielding experienced by the 6′-protons. When there is a 2′-substituent present the ring which bears it twists in such a way as to bring the 6′-proton closer to the proximal nitro-group, explaining the greater shielding observed in these cases. Variable temperature studies provided further evidence in favor of these conformations since no significant changes were observed from ?55° to + 155°, even for compounds with four substituents flanking the ether linkage. Nevertheless, the compounds studied here are not frozen in these conformations, but probably inter-convert via concerted rotation with others.