Effect of substituents on the chemical shift of benzylic protons: evidence for a conformational dependence

Abstract

The benzyl proton shifts in 11 series of para-substituted toluenes bearing different cc substituents have been determined. I n each series, a plot of the benzylic shift against the Hanimett o values of the parasubstituent is linear. The slope of these plots varies from 0.2 to 0.0 p.p.m./o. The variation is found to be conformationally dependent, a fact which is not explicable by current theory. A hyperconjugative interaction is proposed to account for these results and for several anomalies in the literature.Canadian Journal of Chemistry, 46. 1595 (1968) In the last decade a large number of authors have presented data showing a correlation between the effect of substituents on the chemical shifts (SCS) of protons on a benzene ring, and the Hammett o values of the substituents (1-3). More recently, correlations between o values and the SCS of protons in the side chain of aromatics have been reported (4, 5, and references therein). It is usually assumed that the SCS results from the inductive and resonance effects of the substituent, which alter the electron density at the ring carbon atoms. This change is then transmitted through the n electrons to the proton in question. An alternative explanation involving field and mesomeric effects (the F-M correlation) has recently been put forward by Dewar and Takeuchi (6).We wish to report the determination of the para-substituent effect on the benzylic proton shifts, in eleven series of compounds of the formula X-C6H5-CH,-Z.For each Z, at least five X substituents were studied whose range of o values was 0.95 or greater.' An example of the Hammett correlation for the benzylic protons of eight tetrahydropyranyl ethers is shown in Fig. 1. This plot is typical of 'All chemical shifts were measured on a Varian DA-60 nuclear magnetic resonance spectrometer using dilute solutions (0.2 M ) in CDCI,, except in the case of compounds whose methylene protons were magnetically non-equivalent. In these cases 0.3 M solutions were used. In each series, 0.1 M solutions were also studied and dilution shifts beyond the experimental error of 0.2 c.p.s. could not be detected. Satisfactory elemental analyses were obtained for all new compounds included in this study.

Molecular Architectures for Trimetallic d/f/d Complexes: Structural and Magnetic Properties of a LnNi2 Core

“…The diasterotopic hydrogens are further distinguished by assuming the proton in the pseudoaxial position, H2B, is more shielded than the benzylic proton in the pseudoequatorial environment, H2A, because of the proximity of the collinear p-orbital from the neighboring benzene ring. The excess shielding causes the pseudoaxial hydrogen to resonate at lower frequencies (3.39 vs 4.23 ppm of the pseudoequatorial proton). …”

Effect of substituents on the chemical shift of benzylic protons

“…The extent to which the chemical shifts are correlated by o is well illustrated by a graph given in our earlier paper (16). Almost all points except that for chlorine fall within 1 Hz of the best straight line.'…”