Influence of substituent and push‐pull effect on the chemical shifts of the carbon in bridging bond of 1‐furyl/thienyl‐2‐arylethylene. (15th November 2020) - Cao, Chao‐tun
Abstract: 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 δ C (X) and δ 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 δ C (X) and δ C (Y) was studied, meanwhile, the push‐pull effect of substituent on Δ δ C = δ C (Y) − δ C (X) and δ C (Y/X) = δ C (Y)/ δ C (X) were discussed. The results show the following: (a) Both the δ C (X) and δ C (Y) are affected by excited‐state substituent constant σ CC ex X of group X, Hammett constant σ (Y), and excited‐state substituent constant σ CC ex Y of group Y. The effect of σ (Y) on the δ C (X) is positive, whereas that on the δ C (Y) is negative. That is to say, the σ (Y) of group Y has an alternative influence on the δ C (X) and δ 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 π 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 σ parameter but also related to the σ CC ex parameter, which results from the combined effect of the two parameters σ and σ CC ex . The observation of this paper provides a new perspective in understanding the push‐pull effect of substituent. Abstract : In the compounds 1‐furyl/thienyl‐2‐arylethylene (XCHCHArY), the nuclear magnetic resonance spectra (NMR) chemical shift δ C (X) and δ C (Y) of the bridging bond carbon atoms CHCH are influenced by the groups X and Y. The push‐pull effect of group X versus Y can be expressed as δ C (Y/X) = δ C (Y)/ δ C (X), which is not only affected by the σ parameter but also related to the σ CC ex parameter, that is, δ C (Y/X) = f ( σ, σ CC ex ).