Influence of substituent and push-pull effect on the chemical shifts of the carbon in bridging bond of 1-furyl/thienyl-2-arylethylene

Sixty-six samples of 1-furyl/thienyl-2-arylethylene model compounds XCH=CHArY (abbreviated XEBY) were synthesized. The nuclear magnetic resonance spectra (NMR) of model compounds were determined. The chemical shift delta(C)(X) and delta(C)(Y) of the carbon atoms in bridging bond CH=CH which connected to the X group and the ArY group, respectively, were confirmed with the two-dimensional NMR method. The effect of substituent on the delta(C)(X) and delta(C)(Y) was studied, meanwhile, the push-pull effect of substituent on Delta delta(C) = delta(C)(Y) - delta(C)(X) and delta(C)(Y/X) = delta(C)(Y)/delta(C)(X) were discussed. The results show the following: (a) Both the delta(C)(X) and delta(C)(Y) are affected by excited-state substituent constant sigma(ex)(CC)(X) of group X, Hammett constant sigma(Y), and excited-state substituent constant sigma(ex)(CC)(Y) of group Y. The effect of sigma(Y) on the delta(C)(X) is positive, whereas that on the delta(C)(Y) is negative. That is to say, the sigma(Y) of group Y has an alternative influence on the delta(C)(X) and delta(C)(Y). (b) Compared with furyl, there is a p-d effect formed by the 3d orbit of S atom and the p electron of the pi system in thienyl, which increase the chemical shift of the carbon atoms in bridging bond. (c) The push-pull effect between substituent X and Y is not only affected by the sigma parameter but also related to the sigma(ex)(CC) parameter, which results from the combined effect of the two parameters sigma and sigma(ex)(CC). The observation of this paper provides a new perspective in understanding the push-pull effect of substituent.