The effect of substituents on the Lewis acidity of α-cyanostilbenes

Abstract

The effect of substituents in the two aromatic rings of α-cyanostilbene on the Lewis acidities of these compounds towards alkoxide ions has been examined. Substituents in the β ring produce effects on the acidity which correlate with the σ values of the substituents and yield an average ρ value of + 2.22. Substituents in the α ring, however, are correlated by the σ− values and yield an average ρ value of + 4.62. A p-nitro group in the α ring exhibits a greatly enhanced σ− value (+ 1.73).

Kinetics and Mechanism of the Addition of Benzylamines to α-Thiophenyl-β-phenylacrylonitriles in Acetonitrile

“…So the exact correspondence of the reactivity with Σσ − cannot be expected, but it is certain that the reactivities of BMN and BID are larger than those of the rest: NS, TPA and CNS. 6 Previous works indicate that there is steric inhibition of resonance for the α-thiophenyl and phenyl rings in TPA and CNS so that the resonance effect of the α-ring in these compounds is nonexistent. Eliminating the resonance effect of the α-ring in these compounds changes the Σσ − values in the order CNS < TPA < NS as shown in the parentheses under the Σσ − columns in Table 2.…”

“…Thus for the reactions in acetonitrile the reacitivity depends primarily on the resonance electron-withdrawing effect of the Z,Z' groups. Since in such resonance stabilized TS the two large activating groups, e.g., Z,Z' = CN and thiophenyl groups, interfere sterically, 6 the resonance effect of the thiophenyl group becomes negligible since steric hindrance prevents π-overlap with the thiophenyl group. This is why the two compounds, TPA (Z,Z' = CN, C 4 H 3 S) and NS (Z,Z' = NO 2 , H), have almost the same reactivity (Table 2), i.e., the resonance effect of the C 4 H 3 S group becomes nearly zero.…”