87901-56-2Relevant academic research and scientific papers
The nature of the transition state in diarylmethyl cation - Nucleophile combination reactions as probed by secondary α-deuterium isotope effects
Van Pham, Thuy,McClelland, Robert A.
, p. 1887 - 1897 (2007/10/03)
Transition-state structures for the carbocation-nucleophile combination reactions of (4-substituted-4′-methoxydiphenyl)methyl cations with water, chloride, and bromide ions in acetonitrile-water mixtures have been investigated by measuring the secondary α-deuterium kinetic and equilibrium isotope effects. Rate constants in the combination direction were measured with laser flash photolysis. Equilibrium constants were measured for the water reaction by a comparison method in moderately concentrated sulfuric acid solutions, for the bromide reaction via the observation of reversible combination, and for the chloride reaction from the ratio of the combination rate constant and the rate constant for the ionization of the diarylmethyl chloride product. The fraction of bond making in the transition state has been calculated as the ratio log (kinetic isotope effect):log (equilibrium isotope effect). For the water reaction, there is 50-65% bond making in the transition state; this is also true for cations that are many orders of magnitude less reactive. The same conclusions, 50-65% bond formation in the transition state independent of reactivity, have previously been made in corre-lations of log kw vs. log KR. Thus, two quite different measures of transition structure provide the same result. The kH:kD values for the halide combinations in 100% acetonitrile are within experimental error of unity. This is consistent with suggestions that these reactions are occurring with diffusional encounter as the rate-limiting step. Addition of water has a dramatic retarding effect on the halide reactions, with rate constants decreasing steadily with increased water content. Small inverse kinetic isotope effects are observed (in 20% acetonitrile:80% water) indicating that carbon-halogen bond formation is rate-limiting. Comparison of the kinetic and equilibrium isotope effects shows ～25 and ～40% bond formation in the transition states for the reactions with bromide and chloride, respectively.
Flash-Photolysis Generation and Reactivities of Triarylmethyl and Diarylmethyl Cations in Aqueous Solutions
McClelland, Robert A.,Kanagasabapathy, V. M.,Banait, Narinder S.,Steenken, Steen
, p. 3966 - 3972 (2007/10/02)
A series of 18 triarylmethyl cations and 10 diarylmethyl cations have been generated by nanosecond laser flash photolysis of cyanide, 4-cyanophenyl ether, and acetate precursors in acetonitrile/water (AN/W) solutions and first-order rate constants for their reaction with the solvent (ks) have been directly measured following the decay in their optical density.In the standard solvent employed, 1:2 AN/W, the triarylmethyl cations which were studied had ks values at 20 deg C ranging from 1E1 s-1 (for the 4,4',4''-(MeO)3-substituted ion) to 9*1E6 s-1 (4,4'-(CF3)2), while diarylmethyl cations had ks values ranging from 1E5 s-1 (4,4'-(MeO)2) to 3*1E7 s-1 (4,4'-Me2).The parent diphenylmethyl cation and its derivative with one 4-methyl substituent were too short-lived (s of varying the amounts of acetonitrile were investigated for several cations.As water is added to 100percent acetonitrile, ks increases significantly, but at around 15percent by volume water, there is a leveling and from that point to 100percent water, ks is almost unchanged, decreasing by about 20percent.A plot of log ks versus ?+ constructed for the triarylmethyl cations shows significant deviations from linearity for the points for the ? donors, in the direction which indicates that ?+ is underestimating the stabilizing effect of these substituents for a fully formed cation.A plot versus ?C+, a parameter obtained from the analysis of NMR spectra of solutions of carbocations, is reasonably linear.A two-parameter correlation indicates that polar andresonance interactions of substituents do not proceed in parallel along the reaction coordinate, the addition of water to cation resulting at the transition state in the loss of 73percent of the equilibrium resonance effect but only a 33percent loss of the polar effect.A rate-equilibrium plot (log ks versus pKR) was constructed which covers 23 pKR units.A single line of slope 0.64 can be drawn to include the entire set of data for both triarylmethyl and diarylmethyl cations.From a small extrapolation the ks value for the tert-butyl cation in water is obtained as 1E10.5 s-1.
Carbon-13 Nuclear Magnetic Resonance Studies of Carbocations. 10. Variation of Cationic Carbon Chemical Shifts with Increasing Electron Demand in 1,1-Diaryl-1-methyl (Benzhydryl) Carbocations
Kelly, David P.,Jenkins, Margot J.
, p. 409 - 413 (2007/10/02)
A series of 28 1-X-phenyl-1-Z-phenyl-1-methyl (benzhydryl) carbocations, where the X and Z substituents have been varied over the range of electron demand (3,4-CH2CH2O, 11; 4-OCH3, 12; 4-CH3, 13; 4-F, 14; 4-H, 1; 4-CF3, 15; 3,5-(CF3)2, 16), have been prepared from the corresponding alcohols by ionization in superacids and their 13C NMR spectra recorded at low temperatures (-70 to -10 deg C).Plots of the substituent chemical shifts of the cationic carbons (ΔδC+) at -70 deg C against ?C+ are linear for the electron donors (Z = 3,4-CH2CH2O to Z = H) of 13 - 16 and 1 but deviate upward from this correlation line (relative shielding) for the electron acceptors (Z = H to Z = 3,5-(CF3)2).The plots of the highly stabilized cations 11 and 12 approximate shallow curves where groups more electron demanding than 4-CH3 cause relative shielding of the cationic carbon.All these plots are interpreted in terms of competing resonance and localized inductive ?-polarization effects.
