The rate constants for reactions of a family of 19 carbanions, ArCHSO2Ph− (derived from benzyl phenyl sulfones) with n-butyl chloride have been measured in Me2SO solution. A plot of log k vs. the pKa of the conjugate acids for 12 of these carbanions give a linear plot (R2 = 0.999) with a Brønsted coefficient of βNu = 0.402. Points for para electron-withdrawing substituents, SPh, SOPh, SO2Ph. COPh, CN and NO2, deviate substantially from the plot. The deviations are attributed to the enhanced solvation of these remote substituents in the anion which leads to rate retardation. A curved Brønsted plot can be drawn through all the points, which would be consistent with the predictions of the Hammond–Leffler postulate (HLP) and the reactivity–selectivity postulate (RSP), but this interpretation is rejected. Instead, it is suggested that the apparent curvature in Brønsted plots for acid–base reactions – upon which HLP and RSP are based – is also caused by deviations due to solvent effects, donor atom effects in the bases, mechanistic changes and/or the failure to keep electronic and steric effects constant. A reactivity–selectivity plot for reactions of 9 ArCHSO2Ph− ions with n-butyl bromide and n-butyl chloride indicated constant selectivity. A similar plot for 4 carbanions derived from α-methylbenzyl phenyl sulfones reacting with n-butyl bromide and n-butyl chloride also showed constant selectivity. Based on these results and an examination of the literature, it is concluded that there is no firm experimental basis for HLP and RSP. 相似文献
Rate constants for the reversible deprotonation of acetylacetone were measured in carboxylate and amine buffers in water and in 50%, 90% and 95% Me2SO at 20°C. The Brønsted plot for the carboxylate ions is curved in the Me2SO—water mixtures, but straight in water. The curvature is in the direction predicted by the Reactivity—Selectivity Principle (RSP). However, the Brønsted plot for the reaction with primary amines is straight in all solvents. This suggests that the curvature observerd with the carboxylate ions is caused by loss of solvation of the base; this loss of solvation is ahead of bond formation in the transition state rather than being a manifestation of the RSP. (Note that all Brønsted plots are based on pKa values measured in the respective solvents.) The intrinsic rate constant (k0) for proton transfer increases with the addition of Me2SO, and more so with the carboxylate buffers than with the amines. This increase in k0 is attributed to delayed solvation of the developing enolate ion in the transition state; with the carboxylate buffers, an additional factor is the early loss of solvation of the base. The various solvation effects observed in this study can all be understood in the context of the Principle of Imperfect Synchronization (PIS). 相似文献
The addition reaction of dibutylphosphane oxide [Bu2P(O)H] with alkynes proceeds efficiently in the presence of palladium‐chelating phosphane–Brønsted acid catalyst systems. Terminal alkynes afford branched‐structured products selectively. On the other hand, the same reaction using monodentate phosphane ligands or the reaction run in the absence of a Brønsted acid affords a much lower yield. A mechanistic study has revealed that Brønsted acids (XOH) interact with oxygen in M P(O)R2 species (M=Pd, Pt) through hydrogen bonding to transform them to ionic M+←PR2(OH⋅⋅⋅O−X) species, which was confirmed by NMR spectroscopy and X‐ray crystallography. The phosphane‐like PR2(OH⋅⋅⋅O−X) moiety is coordinatively labile, as substantiated by the ligand exchange reaction with tert‐butyl isocyanide. A new mechanism that accommodates these observations has been proposed to rationalize the enhancement of catalytic activity and the regioselectivity induced by the Brønsted acid. 相似文献
1‐Hexene metathesis was performed over standard and potassium‐doped WO3/SiO2 catalysts. The samples were tested at various reaction temperatures, molar feed compositions, and space times. Under the applied reaction conditions, doping with potassium reduced the isomerization and cracking activity of the catalyst by at least half and improved the yield of detergent‐range alkenes twofold. However, increasing the potassium loading to a higher amount resulted in a significant reduction in the metathesis activity as both Brønsted and Lewis acid sites were affected. Optimum operating conditions for the yield of detergent‐range alkenes were identified using response surface methodology. 相似文献
The reaction rates of CO2 with an innovative CO2‐capturing organic solvent (CO2COS), consisting of blends of 2‐tert‐butyl‐1,1,3,3‐tetramethylguanidine (BTMG) and 1‐propanol, were obtained as function of BTMG concentration and temperature. A stopped‐flow apparatus with conductivity detection was used. The reaction was modeled by means of a modified termolecular reaction mechanism which resulted in a second‐order rate constant, and activation energies were calculated for a defined temperature range. Quantum chemical calculations at the B3LYP/6‐31G(d) level also produced the activation energy of this reaction system which strongly supports the experimental findings. 相似文献
Lewis base–Brønsted base bifunctional catalysis is a novel and practical strategy for the asymmetric Michael addition. The addition of malonates to a series of α,β‐unsaturated aldehydes can take place under base–base bifunctional catalytic conditions using 0.5–5 mol% of (S)‐2‐[diphenyl(trimethylsilyloxy)methyl]pyrrolidine as catalyst and 5–30 mol% of lithium 4‐fluorobenzoate as additive base with up to 99% ee. 相似文献
A new sequential two‐step multicatalytic strategy is presented consisting in the efficient DBU‐catalysed trichloroacetimidation of an alcohol followed by a ditriflylamine (Tf2NH)‐catalysed intermolecular alkylation by silicon‐based nucleophiles and C H nucleophiles. The distinct feature of the trichloroacetimidate group allows use of weaker acid catalysts such as 1,1′‐bi‐2‐naphthol (BINOL)‐derived phosphoric acid, pointing out the possible development of an enantioselective variant. This unprecedented sequential one‐pot Brønsted base‐Brønsted acid catalysis further expands the synthetic scope of the trichloroacetimidate group. 相似文献
A direct enantioselective reaction of cyclopent‐2‐enone‐derived Morita–Baylis–Hillman alcohols with 4‐hydroxycoumarins has been developed under the catalysis of a chiral primary amine derived from cinchonine in combination with a Brønsted acid. The reaction provides pyranocoumarin products with three vicinal chiral carbon centers in highly regio‐, diastereo‐ and enantioselectivities through a tandem allylic alkylation/intramolecular oxa‐Michael addition.
Benzoic acid catalysts bearing two amide groups that increase the Brønsted acidity of the carboxylic acid moiety by internal hydrogen‐bonding interactions were designed as a novel class of carboxylic acid catalysts for the Friedel–Crafts reaction of indoles with β‐nitrostyrenes and 3,3‐disubstituted 3H‐indoles to obtain the corresponding Friedel–Crafts adducts in high yields. The internal hydrogen‐bonding benzoic acid catalysts have a relatively high Brønsted acidity compared with benzoic acid based on the pKa measurements in DMSO by UV spectrophotometric titration.
A new enantioselective Brønsted acid‐catalyzed Friedel–Crafts reaction of indole with cyclic imines has been develeoped. This organocatalytic reaction provides for the first time optically active indolindolinone derivatives in high yields and with excellent enantioselectivities (up to 91% ee) under mild reaction conditions. 相似文献
Brønsted–Lewis acidic ionic liquids (IL) were used in the esterification of glycerol and acetic acid to produce glycerol triacetate. The results show that the IL (3–sulfonic acid)–propyltriethylammonium chloroironinate [HO3S–(CH2)3–NEt3]Cl–[FeCl3]x (molar fraction of FeCl3, x = 0.67) was an efficient catalyst for the esterification reaction. The yield of glycerol triacetate and its content were greater than 98 % when reacted under reflux for 4 h. It was observed that a synergistic effect of Brønsted and Lewis acid sites enhanced the catalytic performance of IL. The reusability of IL was good. After six reaction cycles, the glycerol triacetate yield and concentration were still greater than 98 %. Likewise, the Brønsted–Lewis acidic IL was an efficient catalyst for esterification reactions of high boiling points alcohols with acetic acid. 相似文献