Proton chemical shifts in NMR: Part 17. Chemical shifts in alkenes and anisotropic and steric effects of the double bond

Abstract

The 1 H NMR spectra of a number of alkenes of known geometry were recorded in CDCl 3 solution and assigned, namely ethylene, propene, 4-methylcyclohexene, 1,4-dimethylcyclohexene, methylene cyclohexane (in CFCl 3 -CD 2 Cl 2 at 153 K), 5-methylene-2-norbornene, camphene, bicyclopentadiene, styrene and 9-vinylanthracene. These results together with literature data for other alkenes, i.e. 1,3-and 1,4-cyclohexadiene, norbornene, norbornadiene, bicyclo[2.2.2]oct-2-ene and a-and b-pinene, and other data allowed the determination of the olefinic shielding in these molecules. The shielding was analysed in terms of the magnetic anisotropy and steric effects of the double bond together with a model (CHARGE7) for the calculation of the two-and three-bond electronic effects. For the aromatic alkenes ring current and p-electron effects were included. This analysis showed that the double bond shielding arises from both anisotropic and steric effects. The anisotropy is due to the perpendicular term only with a value of 1c.C C/ of −12.1 × 10 −6 cm 3 mol −1 . There is also a steric deshielding term of 82.5/r 6 (r inÅ). The shielding along the p-axis changes sign from shielding at long range (>2.5Å) to deshielding at short range (<2Å). The model gives the first comprehensive calculation of the shielding of the alkene group. For the data set considered (172 proton chemical shifts) ranging from d = 0.48 to 8.39, the r.m.s. error of observed vs calculated shifts was 0.11 ppm.

Assignment of the 1H and 13C NMR of tocotrienols

“…The carbon resonances of the methyl groups are clearly different from each other and we have assigned 12 a-CH 3 (cis) as the more downfield signal at 25.72 ppm and 12 b-CH 3 (trans) to the signal at 17.7 ppm. [29] This is supported by the presence of a correlation between 12 a-CH 3 and H10 and the lack of such for 12 b-CH 3 .…”

HiFSA Fingerprinting Applied to Isomers with Near-Identical NMR Spectra: The Silybin/Isosilybin Case

“…, regioisomers) frequently exhibit substantial differences in chemical shifts ( δ ) due to differences in the nuclei’s local electronic and, therefore, magnetic environments. 2–4 In addition, different spin-coupled systems commonly exhibit different signal splitting patterns. In the case of diastereomers, changes in relative configurations give rise to substituent-induced chemical shift effects.…”

The Truth about False Unicorn (Chamaelirium luteum): Total Synthesis of 23R,24S‐Chiograsterol B Defines the Structure and Stereochemistry of the Major Saponins from this Medicinal Herb

“…[46] At this stage, we anticipated that stereoselective dihydroxylation of the C-23, C-24 olefin could be achieved by utilising the well-known Sharpless asymmetric dihydroxylation (SAD) methodology, [47] despite the moderate enantioselectivities reported with cis alkenes. [48][49][50] However, when alkene 37 was subjected to typical dihydroxylation conditions, there was no detectable diol formation even after one week.…”