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Applications of NMR spectroscopy of chiral association complexes. 6. rotation about the C(sp 2 )—C(aryl) bond in 2,6‐disubstituted pivalophenones
Author(s) -
Holík Miroslav,
Mannschreck Albrecht
Publication year - 1979
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270120107
Subject(s) - chemistry , substituent , aryl , chemical shift , nuclear magnetic resonance spectroscopy , ring (chemistry) , bromine , nmr spectra database , spectroscopy , methyl group , crystallography , carbon 13 nmr , molecular geometry , bond length , reagent , spectral line , stereochemistry , molecule , alkyl , organic chemistry , crystal structure , physics , quantum mechanics , astronomy
For the investigation of the barrier to rotation about the C(sp 2 )—C(aryl) bond in non‐planar pivalophenones five derivatives were prepared and their 1 H and 13 C NMR spectra assigned. Methyl and bromine groups in the 3‐position have opposite substituent effects on the chemical shifts of the 1 H and 13 C signals of Me 2 and Me 4 . The Δ G c ≠values were determined from the coalescence temperatures of the signal splittings generated by the addition of optically active shift reagents. The accuracy of this method was estimated by using different signals of 3‐bromo‐2,4,6‐trimethylpivalophenone and by computer simulation of the line shape. A buttressing effect of substituents in the aromatic ring was observed. A change of the twist angle by the substitution of methyl by bromine in the tert ‐butyl group was suggested in order to explain the changes in Δ G c ≠and the chemical shifts.