108695-61-0Relevant articles and documents
Nucleophilic Addition to Olefins. 20. Large Transition-State Imbalances in the Reaction of α-Nitrostilbenes with Amines
Bernasconi, Claude F.,Renfrow, Richard A.
, p. 3035 - 3041 (1987)
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- (TA), which is in rapid equilibrium with the anion, PhCH(R2N)C(Ar)NO2- (TA-).Rate constants for addition (k1) and its reverse (k-1) as well as K1=k1/k-1 and the acidity constant of the zwitterion (pKa) could be determined.The Hammett ρ values for all parameters are nearly the same for the piperidine and morpholine reactions, and βnuc is virtually independent of the aryl substituent, indicating pyy'=δβnuc/δα = δρ/δpKnuc = ca. 0.The average ρ values are (pk1) = 0.90, ρ(k-1)=0.02, p(K1)=0.88, ρ(pKa)=-1.02, while the average β values are βnuc=0.34, β1g=-0.58, βeq=0.93, βnucn=βnuc/βeq=0.37, and β1gn=β1g/βeq=-0.63.The almost equal magnitude of ρ(k1) and ρ(K1)(αnucn=ρ(k1)/ρ(K1)=1.02) is reminiscent of Bordwell's observation of very large αCH=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 αnucn=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 αnucn(corr)=0.67 and an imbalance of αnucn(corr) - βnucn = 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
Hydrolysis of substituted α-nitrostilbenes: Dissection of rate coefficients for individual steps in the four-step mechanism. Estimates of intrinsic rate constants and transition-state imbalances
Bernasconi, Claude F.,Fassberg, Julianne
, p. 514 - 522 (2007/10/02)
A kinetic study of the hydrolysis of substituted α-nitrostilbenes (NS-Z with Z = 4-Me, H, 4-Br, 3-NO2, and 4-NO2) in 50% Me2SO-50% water (v/v) at 20 °C is reported. The mechanism consists of four steps: nucleophilic addition to NS-Z of water (k1H2O) and OH- (k1OH) to form PhCH(OH)C(Ar)NO2- (TOH-); carbon protonation of TOH- by water (k2H2O), H3O+ (k2H), and buffer acids (k2BH) to form PhCH(OH)CH(Ar)NO2 (TOH0); rapid oxygen deprotonation of TOH0 (KaOH) to form PhCH(O-)CH(Ar)NO2 (TO-); collapse of TO- (k4) into benzaldehyde and arylnitromethane anion. The aci-form of TOH0, PhCH(OH)CH(Ar)NO2H (TOH,aci0), can also be generated as a transient by reaction of TOH- with strong acid. A combination of kinetic experiments involving the reaction of the olefin with water and OH-, the reaction of TOH- with acid, and the reaction of independently synthesized TOH0 with base allowed the dissection of the rate coefficients of most of the individual steps. From the substituent dependence of these rate coefficients, it is concluded that water and OH- addition to NS-Z (k1H2O and k1OH) as well as the collapse of TO- to benzaldehyde and ArCH=NO2- (k4) is characterized by substantial transition-state imbalances, reminiscent of the imbalance observed in the deprotonation of ArCH2NO2. It is also shown that the intrinsic rate constants for the k1OH and k4 steps are low compared to those for the corresponding steps in the hydrolysis of other olefins of the type PhCH=CXY (XY = (H, NO2), (CO)2C6H4, (COO)2C(CHj)2, and (CN)2). This again parallels the behavior in the deprotonation of ArCH2NO2.
