Nucleophilic Addition to Olefins. 20. Large Transition-State Imbalances in the Reaction of α-Nitrostilbenes with Amines

Article abstract of DOI:10.1021/jo00390a015

A kinetic study of the reversible piperidine and morpholine addition to α-nitrostilbene and to α-phenyl-substituted α-nitrostilbenes (4-Me, 4-Br, 3-NO2, 4-NO2) in 50percent Me2SO-50percent water is reported.The reaction yield a zwitterionic adducts, PhCH(R2NH⁺)C(Ar)NO2^- (T_A^<*>), which is in rapid equilibrium with the anion, PhCH(R2N)C(Ar)NO2^- (T_A^-).Rate constants for addition (k₁) and its reverse (k_-1) as well as K₁=k₁/k_-1 and the acidity constant of the zwitterion (pK_a^<*>) could be determined.The Hammett ρ values for all parameters are nearly the same for the piperidine and morpholine reactions, and β_nuc is virtually independent of the aryl substituent, indicating p_yy'=δβ_nuc/δα = δρ/δpK_nuc = ca. 0.The average ρ values are (pk₁) = 0.90, ρ(k_-1)=0.02, p(K₁)=0.88, ρ(pK_a^<*>)=-1.02, while the average β values are β_nuc=0.34, β_1g=-0.58, β_eq=0.93, β_nucⁿ=β_nuc/β_eq=0.37, and β_1gⁿ=β_1g/β_eq=-0.63.The almost equal magnitude of ρ(k₁) and ρ(K₁)(α_nucⁿ=ρ(k₁)/ρ(K₁)=1.02) is reminiscent of Bordwell's observation of very large α_CH=ρ(k₁)/ρ(K₁) values in the deprotonation of phenylnitroalkanes and suggests an imbalanced transition state, i.e., one in which the delocalization of the negative charge into the nitro group has made little progress relative to bond formation.This lag in charge delocalization is not as pronounced as the α_nucⁿ=ρ(k₁)/ρ(K₁)=1.02 ratio might imply, though, because it is distorted by the effect of the positive charge on the amino nitrogen.After correcting for this effect one obtains α_nucⁿ(corr)=0.67 and an imbalance of α_nucⁿ(corr) - β_nucⁿ = 0.30.The factors which render this imbalance smaller than that found in the deprotonation of the phenylnitroalkanes are discussed.Comparison with the reaction of amines with β-nitrostyrenes reveals a similarity in the imbalances but a substantial difference in the intrinsic rate constants for the two systems.Possible interpretations of these findings are offered