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( E,Z) Equilibria, 12. Differential NMR Shielding by Phenyl, Assigned from Chemical Labelling and ( Z,E ) Equilibration
Author(s) -
Knorr Rudolf,
HintermeyerHilpert Monika,
Böhrer Petra
Publication year - 1990
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19901230529
Subject(s) - chemistry , double bond , methylene , deprotonation , derivative (finance) , nitrile , medicinal chemistry , electromagnetic shielding , labelling , carbon 13 nmr , stereochemistry , geminal , organic chemistry , ion , financial economics , economics , biochemistry , electrical engineering , engineering
Unequivocal NMR assignments of the ( E,Z ) isomeric pairs 3 and 8 are possible by ( Z,E ) equilibration. Deprotonation of 2‐methyl‐1‐phenylpropene ( 1 ) with n ‐BuLi under any conditions gives only a single allyl‐metal derivative 2 . Methylation of the latter gives the isomer (Z)‐ 3 , while deuteriolysis leads to the isotopically labelled derivative 6 of the olefin 1 ; this Proves the NMR assignments for 1 as well as the endo configuration of 2 . The phenyl group gives rise to a differential shielding effect on the γ‐carbon nuclei in methyl and methylene gropus along a CC double bond (ca, 7 ppm), and this effect is comparable to that along a CN double bond as in 8 . The nitrile function in 5c is much less effective.