THE SPIN DELOCALIZATION SUBSTITUENT PARAMETER σ 10. SYNTHESIS OF SUBSTITUTED α, β, β-TRIFLUOROSTYRENES BY THE Pd-CATALYZED CROSS-COUPLING OF TRIFLUOROVINYLZINC BROMIDE WITH ARYL HALIDES

Palladium-catalyzed coupling of trifluorovinylzinc reagent with substituted aryl halides Y-C_6H_4-X (Y=p-NO_2, p-and m-COOMe, p-COOH, p-CONH_2, p- and m-CN; X=Br, I, Cl) provides a synthetic method for α, β, β-trifluorostyrenes(Y-TFS's). A series of previously unavailable Y-TFS's were thus obtained.

Effects of substituent and solvent on the UV absorption energy of 4,4'-disubstituted stilbenes

Twenty five samples of 4,4'-disubstituted stilbene derivatives were synthesized, and their UV absorption max wavelengths were determined in over 10 kinds of solvents including cyclohexane, ether, chloroform, acetonitrile and ethanol, in which 242 experimental data were recorded. The effects of substituents and solvents on the energy of their UV absorption max wavelengths were discussed. The research results showed: the energy of UV absorption max wavelengths of 4,4'-disubstituted stilbenes was mainly affected by their intramolecular structure (substituent effect) in a given solvent, that is, the energy is dominated by both of excited-state substituent parameter o-~c and polar substituent constant crp. While their energy was dominated by the substituent effect and solvent effect in different kinds of solvents. An equation quantifying the energy of UV absorption max wavelengths of 4,4'-disubstituted stilbenes was developed. In addition, it is found that the n-octanol/water partition coefficient (logP) is more effective than the solvatochromic dye (ET(30)) in scaling the solvent effect. The equation employed the parameter logP has a better correlation and more specific physical meaning. Further, the energies of UV absorption max wavelengths of some reported compounds were predicted by the obtained equation, which are in agreement with their experimental values.

The spin delocalization substituent parameter σ——8. The spin-delocalizing abilities of the nitro, cyano, carbomethoxy, carboxy and carbamyl groups

The rate constants (k) of the homo-cycloaddition reactions of five substituted α, β, β-tri- fluorostyrenes (TFS’s), i.e. p-nitrotrifluorostyrene (4), p-cyanotrifluorostyrene (5), p-carbomethoxytri- fluorostyrene (6), p-carboxytrifluorostyrene (7) and p-carbamyltrifluorostyrene (8), have been measured in the temperature range of 110-160℃. The ~σmb polar substituent parameters of these TFS's cal- culated from ^(19) F NMR chemical shifts are: NO_2, 0.86; CN, 0.86; CO_2CH_2, 0.40; CO_2H, 0.31; CONH_2, 0.10. The spin delocalization substituent parameters ~σT of NO_2, CN, CO_2 CH_2, CO_2H and CONH_2 are 0.32, 0.38, 0.31, 0.37 and 0.37 respectively. Thus all these electron-pair attracting groups are also very effective spin-stabilizers.

The spin delocalization substituent parameter σ(?)——6.The spin-delocalizing abilities of the thiomethyl,cyclopropyl and t-butyl groups.Synthesis of three substituted trifluorostyrenes

Three substituted α,β,β-trifluorostyrenes(TFS's),i.e.,p-thiomethyltrifluorostyrene(4), p-cyclopropyltrifluorostyrene(5)and p-t-butyltrifluorostyrene(6),have been synthesized.The rate constants(k_2)of the thermal cyclodimerization of these compounds have been measured in the tem- perature range 120—160℃.The polar parameters σ_(mb) of these TFS's calculated from ^(19)F NMR chemi- cal shifts are:for p-thiomethyl,-0.18;p-cyclopropyl,-0.31 and p-t-butyl,-0.22.The spin delo- calization substituent parameters σ'_T(140°)of p-thiomethyl,cyclopropyl and t-butyl groups are 0.59, 0.27 and 0.30 respectively.Thus all of these groups act as electron-donating groups which can also effectively stabilize a spin.