ORGN 467 |
| Vicinal coupling constants 3JHH have been shown to follow a cos(2τ)-type function of the dihedral angle (τ) and their magnitude therefore provides an estimate of molecular conformation. Over the last decades this functional dependence has been extended to many other coupling constants. One-bond carbon-carbon coupling constants (1JCC) in stereochemical assignments have been studied and reviewed extensively. The hybridization of the carbons, the inductive effect of attached substituents, the steric effects, and the antiperiplanarity of the carbon-carbon bonding orbital to a heteroatom lone pair were described as the factors responsible for determining the magnitude of 1JCC in these compounds. Here, we report that in a series of ethers the 1JCC of the C-C-O-C fragment can be best described using a cos(τ) term, rather than the cos(2τ) characteristic of n-σ* delocalization. We attribute the observed cos(τ) variation to dipolar interactions between the oxygen lone pair and the bonding orbitals of the neighboring carbons that affect the electron density of these carbons and their hybridization. The proposed equation, showing a cos(τ) dependence, is based on the 13C NMR investigation of twelve sterically well-defined ethers. The results and their interpretation are in good agreement with the computational analysis of 1JCH in ethers as well as with the Perlin effect observed for oxygen containing six-membered rings. |
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New Reactions, Methodology, Total Synthesis, Physical Organic Chemistry
8:00 PM-10:00 PM, Tuesday, 15 March 2005 Convention Center -- Sails Pavilion, Poster
Sci-Mix
Division of Organic Chemistry |