Kinetics of Reversible Thiolate Ion Addition to Substituted β-Nitrostyrenes in Water. Radicaloid Transition State or Principle of Nonperfect Synchronization?

Article abstract of DOI:10.1021/jo00034a032

The kinetics of reversible nucleophilic thiolate ion (RS^- with R = n-Bu, HOCH2CH2, MeO2CCH2CH2 and MeO2CCH2) addition to Z-substituted β-nitrostyrenes (Z = 4-Me2N, 4-MeO, 4-MeS, 4-Me, H, 3-Cl, 4-Cl, 3-CN, 4-CN, and 3-NO2), to form ArCH(RS)CH=NO2^-, have been measured in water at 20 deg C.Rate constants in the forward (k₁) and reverse direction (k_-1) and equilibrium constants (K₁) correlate reasonably well with Hammett ?-constants for the non-?-donor substituents but show deviations for the ?-donors 4-Me2N, 4-MeO, and 4-MeS.These deviations are negativefor K₁ but positive for k₁ and k_-1; the positive deviations for the ?-donor substituents are also observed when plotting log k₁ vs log K₁ (Broensted plots).The negative deviations of K₁ are a consequence of resonance stabilization of the olefin.The positive deviations are attributed to a transition-state stabilization stemming from a preorganization created by the ?-donor which leads to a better delocalization of the negative charge into the nitro group.An alternative interpretation of the rate acceleration in terms of a radicaloid transition state (Gross, Z.; Hoz, S.J.Am.Chem.Soc. 1988, 110, 7489) cannot be ruled out but is shown to be less attractive and unnecessary.Broensted parameters such as β_nuc, β_eq, βⁿ_nuc, and βⁿ_lg, and intrinsic rate constants (k₀ = k₁ = k_-1 when K₁ = 1) were determined from the dependence on the RS^- basicity for β-nitrostyrene and 3-cyano-β-nitrostyrene. β_eq is low (0.5), indicating that the carbon basicity of RS^- is less sensitive to electronic effects in R than its proton basicity. β_nuc (βⁿ_nuc) is very low, suggesting a transition state withvery little C-S bond formation.The low βⁿ_nuc (0.22) contrasts with a large αⁿ_nuc = d log k₁/d log K₁ = 0.74 (variation of Z), indicating a large transition-state imbalance (αⁿ_nuc - βⁿ_nuc), as previously observed in the reaction of RS^- with α-nitrostilbenes.The intrinsic rate constant (log k₀ = 3.5) is also similar to that for the reaction of RS^- with α-nitrostilbenes and significantly higher than for the reaction of amines with β-nitrostyrenes.Most of these features can, at least in part, be attributed to the soft acid-soft base interactions of RS^- with the nitroolefins.Rate constants for carbon protonation of several of the ArCH(RS)CH=NO2^- adducts by acetic acid (k^HA_p were also determined.They display the unusual, but for nitronate ions typical, acceleration when Ar and/or R is made more electron withdrawing.