THE EFFECT OF BETA-FLUORINE SUBSTITUENTS ON THE RATE AND EQUILIBRIUM-CONSTANTS FOR THE REACTIONS OF ALPHA-SUBSTITUTED 4-METHOXYBENZYL CARBOCATIONS AND ON THE REACTIVITY OF A SIMPLE QUINONE METHIDE

The reactions of 4-MeOArCH(R)Cl (R = CH2F, CHF2, CF3) and 4-MeOArC(CF3)2Y (Y = OTs and Br) with nucleophilic reagents were studied in 50:50 (v/v) trifluoroethanol/water. There is chloride common ion rate depression of the solvolysis of 4-MeOArCH(R)Cl. The large leaving group effect, k(OTs)/k(Br) = 9 X 10(4), for the solvolysis of 4-MeOArC(CF3)2Y suggests that the reaction of the tosylate is accelerated by the relief of ground-state strain. The solvolysis of 4-MeOArC(CF3)2OTs gives a 33% yield of 4-[bis(trifluoremethyl)methylene]cyclohexa-2,5-dienone which forms by the aromatic substitution reaction of water at 4-MeOArC(CF3)2+. This quinone methide reacts slowly with solvent (t1/2 approximately 11 h) and rapidly with azide ion. All of these compounds react with azide ion by an S(N)1 mechanism. Values of k(az)/k(s) and k(Cl)/k(s) (M-1) for the partitioning of the carbocation intermediates between capture by azide or chloride ion and solvent are reported. Absolute rate constants k(s)(s-1) and k(Y) (M-1s-1, Y- = Cl-,Br-) for the capture of 4-MeOArCR1(R2)+ were calculated from the values of k(az)/k(s) (M-1) or k(az)/k(Y) and an estimated value of k(az) = 5 X 10(9) M-1 s-1 for the diffusion-limited reaction of N3-. Values of K(eq) for the ionization of 4-MeOArCR1(R2)Y to give 4-MeOArCR1(R2)+ were calculated as the ratio of the microscopic rate constants k(solv)/k(Y). A single beta-fluorine at 4-MeOArCH(CH3)+ causes a 2-fold increase in k(s), due to a small inductive substituent effect. Additional beta-fluorine substituents cause k(s) to decrease due to a steric effect; the value of k(s) for the capture of 4-MeOArC(CF3)2+ is 40-fold smaller than k(s) for 4-MeOArCH2+. The effect of beta-fluorine substituents on log K(eq) are consistent with steric congestion at the more highly substituted substrates that destabilized 4-MeOArCR1(R2)Y relative to 4-MeOArCR1(R2)+. These results support the proposal that the low reactivity of 4-MeOArCH(CF3)+ toward solvent is due, in part, to a resonance effect.