The Styrene Probe Applied to 15N and 77Se NMR

Abstract : Previous work had shown that the Fluorine 19 NMR chemical shift (delta/Hammett Substituent Constant (sigma) correlation of ethyl alpha- fluorocinnamates gave a positive slope (greater deshielding with increasing sigma value), opposite to that predicted by the pi-polarization mechanism. To ascertain if this was a general phenomenon in alpha-substituted cinnamate esters for atoms having unshared pairs of electrons, Nitrogen 15 chemical shift correlations were obtained for the Z and E isomers of ethyl alpha- diethylaminocinnamates (Series A, 13 compounds). A positive slope was found for both isomers, with the sigma(-) scale giving the best single substituent parameter result compared to sigma, sigma(+), and sigma 13, indicating through- resonance to be predominant in these compounds. This correlation study was extended to a series of benzylidene azlactones (Series B, 10 compounds) and ethyl alpha-cyanocinnamates (Series C, 7 compounds). Selenium 77 NMR of the E and Z isomers of ethyl alpha-(phenylseleno)-cinnamates (Series D) also revealed a positive slope, although there was a large isomer disparity in correlative behavior. Theses. (aw)