Substituent Conformational effects in vicinal 13C13C spin–spin coupling constants

A series of 7‐13C‐labeled o‐substituted toluene derivatives and carboxyl‐13C‐isocrotonic and crotonic acid were synthesized and studied by 13C NMR spectroscopy to obtain 13C13C spin‐spin coupling constants involving the labeled carbon. The cis 3J(CC) values were different from those in previous studies in that these J(CC) values were relatively small and the usual dependence of 3J(CC) on the s‐character of the terminal carbon was reversed. Further, a strong dependence of 3J(CC) on the conformational orientation of a terminal carbonyl group was shown to exist. Through‐space interactions of the two coupling carbons were shown to contribute to these ‘anomalous’ results, and thus it was shown the cis carbon‐carbon couplings may not be directly related to geometrically equivalent proton‐proton couplings, as are other carbon‐carbon couplings.