STRUCTURE-PROPERTY RELATIONSHIPS OF FLUOROOLEFINS 5. SYNTHESIS OF SUBSTITUTED α, β,β-TRIFLUOROSTYRENES, α,β, β-TRIFLUOROETHENYLNAPHTHALENES HAMMETT CORRELATIONS OF THEIR ~(19)F NMR PARAMETERS AND THE CONCEPT OF "DISTORTED π-ELECTRON CLOUDS" (1983)

An improved new method for the synthesis of α, β, β-trifluorostyrene(TFS) was used in the synthesis of six substituted TFS, together with α- and β-trifluoroethenylnaphthal enes. It involves the use of aromatic Grignard reagents with tetrafluoroethylene in THF, as shown in eq. 1(cf. Chinese text for equations, tables and figures). The yield of TFS in THF is the highest among the four solvents tried(i. e. THF, ether, DME and dioxane, Table 1). The structure of all products were establsihed by elemental analysis, IR and ~(19)F NMR, and their yields are listed in Table 2.The UV λ_(max) values of the substituted TFS and styrenes are given in Table 3. The substituent effects on the wave-length shifts of the λ_(max) values are expressed in terms of λ_Y-λ_H values(see Table 3). Obviously, this effect is smaller in the TFS series. The smaller effect is probably caused by the steric interference of the coplanarity of the molecule by the cis-F atom and the phenyl ring.According to the simple model ofdistorted π-electron clouds, in compounds of type 3, mutually dependent p-π repulsive effects and the electronic effects of the substituents combine to polarize a double bond. The p-π repulsive forces of F~2 atom and F~3 atom cancell out. If the repulsive force of the substituent Z is smaller than that of the F~1 atom, the π-electron will be pushed to the right(C~1 to C~2), as well as downward from F~1 to F~2. This crude model is useful in the discussion of structure-property relationships of fluoroolefins, e. g., it easily predicts that δ(F~2) be larger than δ(F~1). This model also anticipates that there may be a linear relationship between the substituent parameters of Z and the chemical shift differences of F~1 atom and F~3 atom, △δ_(3-1)(or △δ_(3-2)). To test these ideas, we selected substituted TFS for our study, because with these compounds straight forward comparisons of electronic effects can be made without complications from other factors. The chemical shifts δ(F), spin-spin coupling constants J_(12), and the chemical shift differences △δ of substituted TFS are presented in Table 4. Indeed as expected, four linear relationships were obtained between σ and the following parameters, viz. δ(F~1), δ(F~2), J_(12) and △δ_(3-1)(or △δ_(3-1)).Karplus and Das stated that fluorine chemical shifts were dominated by the second order paramagnetic term and hence by the nature of the fluorine bonding. In Fig. 1 the chemical shifts of F~1 and F~2 move regularly to lower fields in the series p-CH_3O, p-CH_3, m-CH_3, H, p-Cl, m-Cl and m-CF_3, implicating the decrease of electron density of the C~1-F~1 bond and C~1-F~2 bond in the series.As shown in Fig. 3, the J_(12) value decreases with an increase of σ value. This probably can be ascribed to the change of electron density in the region of F~1-C~1-F~2. A reduction of electron density in this region corresponds to a reduction of the availability of electrons which transmits coupling information.A comparison of the π-bond polarity in trifluoroolefins is necessary in the discussion of their properties. We suggest that △δ_(3-1)(or △δ_(3-2)) value be used as a quantitative indicator of the polarity of π-bond in the trifluoroolefin molecule. A higher △δ(3-1) value implies a higher degree of polarization of the π-bond in question.With this criterion the chemical behavior manifested as the yields in the synthesis of TFEN can be rationalized. As shown in Table 5, △δ_(3-1) value of α-TFEN is by 20ppm smaller than that of β-TFEN, the yield of α-TFEN is by 30% larger than that of β-TFEN, whilst the yield of α-TFEN disubstituted product, C_(10)H_7CF:CFC_(10)H_7, is smaller. The rationale is: as indicated by its larger △δ_(3-1) value, the more polarized double bond of β-TFEN is more easily further attacked by another nucleophilic Grignard reagent, thus the monosubstituted product is consumed, in the meantime of course, more of the disubstituted product is formed.The difference of the π-bond polarity between α-TFEN and β-TFEN is probably a consequence of steric factors. In α-TFEN molecule, steric interaction of the F~3 atom and peri-hydrogen atom must have rendered the maintenance of the coplanarity of naphthalene ring and the olefin double-bond difficult, this in turn reduces conjugation of the system and the polarization of the double bond. In short, in the absence of other complicating factors, the larger the △δ_(3-1) value, the more easily is the π-bond attacked by a nucleophilic reagent. Thus by means of arranging a △δ_(3-1) value scale of trifluoroolefins, a quantitative comparision of their reactivity is now possible.