Nucleophilic Addition to Olefins. 15. Solvent and Substituent Effects on the Hydrolysis of Benzylidenemalononitriles in Basic Me2SO-Water Solutions

Article abstract of DOI:10.1021/ja00269a039

Benzylidenemalononitriles hydrolyze to ArCHO and CH(CN)2^- in basic solution.A kinetic study of this reaction with the parent compound and with the 4-NMe2, 4-OMe, 3-CL, 4-CN, and 4-NO2 derivatives in water and in 30percent, 50percent, 60percent, and 70percent aqueous Me2SO is reported.In water nucleophilic addition of OH^- to the olefin (k₁^OH, Scheme I), to form ArCH(OH)C(CN)2^- (T_OH^-) is rate limiting with all substrates.In the Me2SO-containing solvents, the formation of T_OH^- at high becomes faster than its conversion to products.This allows separate measurement of the rate of product formation.The major pathways for this latter process involve rate-limiting intramolecular proton switch (k_i, Scheme II) and rate-limiting carbon protonation of the dianonic intermediate ArCH(O^-)C(CN)2^- (T_O^2-) by water (k₆^H2O, Scheme II); the most "obvious" pathway through rate-limiting carbon protonation of T_OH^- by water (k₂^H2O) is insignificant, but carbon protonation of T_OH^- by piperidinium ion (k₂^BH) is important.At very high base concentration, there is a further change to rate-limiting breakdown of ArCH(O^-)CH(CN)2 (T_O^-( (k₄) for the 3-Cl and 4-NO2 derivatives but not for the other substrates.The intramolecular proton switch (k_i) is best understood as a concerted proton transfer with a transition state that includes a water bridge.Two kinetically equivalent alternatives involve carbon protonation of T_O^2- by H3O⁺, with transition stabilisation by hydrogen bonding between H3O⁺ and O^-, and carbon protonation T_OH^- by water, with transition-state stabilization by hydrogen bonding between the hydroxy group of T_OH^- and the incipient hydroxide ion.Both alternatives can be excluded because they cannot account for the high k_i values.On the basis of the substituent and solvent effects on the rate (k₁^OH) and equilibrium constants (k₁^OH) of OH^- addition, the following conclusions are drawn: (1) There is an imbalance between the polar substituent effect (ρ_nor) and the response of k₁^OH to the decreased solvation of OH^- in the Me2SO-containing solvens (β_nuc,sol).This may be attributed to a combination of factors, such as a lag in the resonance stabilization of the C(CN)2^- moiety in T_OH^-, an early desolvation of the OH^- ion, and an electrostatic interaction in the transition state between the residual negative charge on the attacking OH^- and the aryl substituent. (2) The ρ(k₁^OH) values depend very little on solvent effect on k₁^OH (β_nuc,sol) depends very little on the aryl substituent.This indicates that the cross-correlation coefficient p_xs = δρ(k₁^OH)/δ log γ_OH- = δβ_nuc,sol/δ? ca. 0, with γ_OH- being the solvent activity coefficient for the transfer of OH^- from water to Me2SO-water mixtures.