The spin delocalization substituent parameters σ: 3. The spin-delocalizing abilities of the acetyl and trimethylsilyl groups. Synthesis of four substituted trifluorostyrenes

Four substituted α, β, β-trifluorostyrenes (TFSs) namely, ptrimethylsilyltrifluorostyrene (1), p-acetyltrifluorostyrene ethylene ketal (2), p-acetyltrifluorostyrene (3) and p-phenoxytrifluorostyrene (4), have been synthesized. The rate constants (k₂) of the thermal cyclodimerization of 1 and 3 have been measured in the temperature range 120—160°C. The polar parameters _σP of the four TFSs calculated from ¹⁹F NMR chemical shifts are: for p-trimethylsilyl-1, 0.21; 1-ethylenedioxy-ethyl-, —0.01; p-acetyl-, 0.61 and p-phenoxy-,—0.47. The spin de-localization substituent parameters σ?_T of p-trimethylsilyl and p-acetyl groups are 0.29 and 0.52 respectively. Thus both the p-trimethylsilyl and the p-acetyl groups act as electron-attracting groups which can also effectively stabilize a spin.