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, p trimethylsilyltrifluorostyrene (1), p ‐acetyltrifluorostyrene ethylene ketal (2), p ‐acetyltrifluorostyrene (3) and p ‐phenoxytrifluorostyrene (4), have been synthesized. The rate constants ( k 2 ) of the thermal cyclodimerization of 1 and 3 have been measured in the temperature range 120—160°C. The polar parameters σ P of the four TFSs calculated from 19 F 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.