17O NMR investigation of p,π‐interactions in α,β‐unsaturated and aromatic ethers

Abstract

AbstractThe 17O chemical shifts have been measured for 51 α,β‐unsaturated and aromatic ethers. A good linear relationship is found between the 17O chemical shifts in a series of dialkyl and the corresponding alkyl vinyl ethers. Hence, the extent of p,π‐interaction, between the oxygen atom and the vinyl group in the latter series does not, apparently, depend upon branching at the α‐carbon atom in the alkyl moiety of these ethers. The PhOBut ether, however, as compared to the other alkyl phenyl ethers, shows significantly weakened p,π‐interaction, which is apparently related to the steric hindrance of this interaction. The effects of two unsaturated groups upon the 17O chemical shifts in the corresponding ethers are non‐additive. This is undoubtedly a result of ‘rivalry’ between these groups for conjugation with the lone electron pairs on the ethereal oxygen. The 17O chemical shift ranges of substituted methyl and vinyl phenyl ethers are nearly equal (≈30 ppm). An analysis of the 17O shielding for cyclopropyl ethers shows no observable p,σ‐conjugation in these compounds. Excellent correlation (r>0.99) between the values of 17O chemical shifts and the calculated (MO LCAO SCF, CNDO/2) π‐electron charges on the corresponding oxygen atoms look promising for experimental estimations of π‐electron densities on the ethereal oxygen.