¹³ C‐NMR ortho ‐disubstituierter Isopropylbenzole: Chemische Verschiebung und Konformation bezüglich der (H ₃ C) ₂ CH — Aryl‐Bindung

Abstract

Hindered rotation about the C(sp2) -C(sp3) bond in 2-isopropylmesitylene (3), 1 -isopropyl-2-methylnaphthalene (4), and 9-isopropylanthracene (5) is investigated by 13C NMR spectroscopy. At low temperatures, 4 exists as two rotamers 4 a and 4 b of different populations. Since their conformations are known from 'H NMR spectra, assignment of the 13C signals to 4a and 4b is possible. The knowledge thus obtained, of the influence of the blocked isopropyl group on the &values of other carbon atoms, allows signal assignments to be made in the low temperature spectra of 3 and 5. The unusual long-range effect of the isopropyl group on 13C chemical shifts is explained by non-bonded interactions.Die gleichzeitige Anwesenheit von zwei 3-(R) und zwei ortho-Substituenten (X, Y) im ToluolGerust (I) fuhrt zu einer recht hohen Barriere fur die Rotation um die C'(sp2)-Ca(sp3)-Bindung2. ' ).

Structural and Variable-Temperature NMR Studies of 9-Diisopropylphosphanylanthracenes and 9,10-Bis(diisopropylphosphanyl)anthracenes and Their Oxidation Products

“…However, at 248 K in solution the molecule adopts the geometry shown in Figure (right), with both isopropyl substituents arranged staggered with respect to the anthracene unit. This conformation explains the nonequivalency of all aromatic hydrogen atoms as well as the through-space coupling by an overlap of the lone electron pair on the phosphorus atom as already suggested for 1,8-phosphanylnaphthalene derivatives. , The rotational barrier was determined to be 56 kJ mol −1 (Δ G 298K ), which is in very good agreement with the corresponding value found for 9-isopropylanthracene …”

Internal Rotation and Butterfly-like Ring Inversion in 9-tert-Butylanthracene

“…The solubility of 2 is poor in most solvents but a 5:2 mixture of CS 2 /acetone- d 6 allowed us to bring a dilute solution down to about 150 K without significant precipitation or freezing of the solvent . Even at 150 K there was no observable splitting or any significant line shape changes in the 13 C spectrum.…”

“…The spectral peaks were somewhat broadened due to the increase in viscosity of the sample at lower temperatures. In contrast, for 9-isopropylanthracene, where the anthracene ring bisects the isopropyl group in the lowest energy structure, the internal rotation of the isopropyl group can be frozen out (7:2 CS 2 :acetone- d 6 ) below 250 K.…”

Cooperative Use of VCD and XRD for the Determination of Tetrahydrobenzoisoquinolines Absolute Configuration: A Reliable Proof of Memory of Chirality and Retention of Configuration in Enediyne Rearrangements

“…This implies that k α is far superior to k β at least by two orders of magnitude. Typical rotational barriers around C‐C bonds like bond α are close to 7.5 kcal/mol . Conversely, rotation around bond β should be much slower, as is it is restricted by the extended delocalization of the single electron, the cyclic frame, and steric hindrance.…”