Mechanisms of long-range 13C, 13C spin–spin coupling in thioanisole and its derivatives. Conformational applications

The 13C nuclear magnetic resonance chemical shifts and the long-range 13C,13C spin–spin coupling constants are reported for 23 thioanisole derivatives enriched in 13C at the methyl position. For para and meta substituted thioanisole derivatives, nJ(C,C) (n being the formal number of bonds intervening between the coupled nuclei) can be related to functions of the angle by which the thiomethyl group twists out of the aromatic plane. For n = 3,4,5, the ensuing relationships yield estimates of the twofold barriers to rotation about the C(1)—S bond. The barrier is lower in ethyl phenyl sulfide than in thioanisole derivatives. Complications arise for ortho substituted thioanisole derivatives but estimates of the torsional motion about the C(1)—S bond can be obtained from the observed nJ(C,C). Among the complications is the expected fact that 3J(C,C), which is shown to be larger in the cis than the trans orientation of the intervening bonds ("anti-Karplus" behaviour), is perturbed by the substituent attached to ...