( E,Z) Equilibria, 12. Differential NMR Shielding by Phenyl, Assigned from Chemical Labelling and ( Z,E ) Equilibration

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.