Substituent-induced chemical shifts (SCS) by the phenyl group in sterically congested styrene derivatives

The two 13C NMR methyl signals of the sterically congested 9‐benzylidene‐1,8‐dimethylfluorene (1a) have the surprisingly small separation of only 0.37 ppm; for 9‐benzylidene‐2,7‐dimethylfluorene the corresponding methyl separation is 0.04 ppm. An alternative analysis of 1a with respect to 1,8‐dimethylfluorene shows that the perturbing syn‐phenyl substituent has caused a downfield ( + 0.47 ppm) 13C shift but an upfield 1H shift ( − 0.97 ppm) of the compressed 1‐CH3 group. For further comparisons, NMR assignments were also made for 2,7‐dimethylfluorene, 1,8‐dimethylfluoren‐9‐one and 2‐(1,1‐dimethylethyl)‐3,3‐dimethyl‐1‐phenylbut‐1‐ene.