- Basicities and Nucleophilicities of Pyrrolidines and Imidazolidinones Used as Organocatalysts
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The Br?nsted basicities pKaH (i.e., pKa of the conjugate acids) of 32 pyrrolidines and imidazolidinones, commonly used in organocatalytic reactions, have been determined photometrically in acetonitrile solution using CH acids as indicators. Most investigated pyrrolidines have basicities in the range 16 aH aH aH 12.6) and the 2-imidazoliummethyl-substituted pyrrolidine A21 (pKaH 11.1) are outside the typical range for pyrrolidines with basicities comparable to those of imidazolidinones. Kinetics of the reactions of these 32 organocatalysts with benzhydrylium ions (Ar2CH+) and structurally related quinone methides, common reference electrophiles for quantifying nucleophilic reactivities, have been measured photometrically. Most reactions followed second-order kinetics, first order in amine and first order in electrophile. More complex kinetics were observed for the reactions of imidazolidinones and several pyrrolidines carrying bulky 2-substituents, due to reversibility of the initial attack of the amines at the electrophiles followed by rate-determining deprotonation of the intermediate ammonium ions. In the presence of 2,4,6-collidine or 2,6-di-tert-butyl-4-methyl-pyridine, the deprotonation of the initial adducts became faster, which allowed the rate of the attack of the amines at the electrophiles to be determined. The resulting second-order rate constants k2 followed the correlation log?k2(20 °C) = sN(N + E), where electrophiles are characterized by one parameter (E) and nucleophiles are characterized by the two solvent-dependent parameters N and sN. In this way, the organocatalysts A1-A32 were integrated in our comprehensive nucleophilicity scale, which compares n-, -, and σ-nucleophiles. The nucleophilic reactivities of the title compounds correlate only poorly with their Br?nsted basicities.
- An, Feng,Maji, Biplab,Min, Elizabeth,Ofial, Armin R.,Mayr, Herbert
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supporting information
p. 1526 - 1547
(2020/02/04)
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- Single enantiomer, chiral donor-acceptor metal complexes from bisoxazoline pseudoracemates
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Single enantiomer, chiral donor-acceptor metal complexes were synthesized via the self-discriminating zinc(II) complexation of a pseudoracemic mixture of donor/acceptor-substituted bisoxazoline derivatives.
- Atkins, Jeffery M.,Moteki, Shin A.,DiMagno, Stephen G.,Takacs, James M.
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p. 2759 - 2762
(2007/10/03)
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- Anomalous Reaction of Arylmalononitriles with Nitric Acid. Para-Directing Nature of Dicyanomethyl Group and a Through-Ring Nitro/aci-Nitro Tautomerism of 4-Nitrophenylmalononitrile
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Phenylmalononitrile reacts with nitric acid in dichloromethane at room temperature to afford 1,2-bis(4-nitrophenyl)-1,1,2,2-tetracyanoethane as an initial product, which readily suffers oxidative cleavage to give 4-nitrobenzoyl cyanide.Contrary to common
- Suzuki, Hitomi,Koide, Hideki,Ogawa, Takuji
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p. 501 - 504
(2007/10/02)
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- Termodynamics and Kinetics of Carbon-Carbon Bond Formation and Heterolysis Through Reactions of Carbocations with Carbanions in Solution
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Rate data are presented for the heterolysis of carbon-carbon bonds and their formation through coordination of resonance-stabilized carbocations and carbanions in acetonitrile solution at 25 deg C.These rates were determined by NMR line broadening and by the T jump technique (in a solution containing 0.48 M supporting electrolyte).Preliminary results are given for a "master equation" to predict some heterolysis energies in solution as a complement to Benson's method for homolysis energies.The results provide for the first time an opportunity to compare the effects of structure variation on the kinetic and thermodynamic properties for such an ostensibly simple reaction in solution.All evidence so far accumulated indicates that these reactions are dominated by ion-solvation factors so that they have little bearing on the gas-phase heterolysis energies.Ionic strength effects and substituent variation suggest that charge development is about half developed at the transition state, but we argue that this cannot be translated simply into pictures of transition-state structure.The results provide a flagrant reversal of the frequently invoked "reactivity selectivity principle" since the most reactive cation is also most selective.On the basis of these results and many others which have appeared recently it may be appropriate to discard the reactivity selectivity principle as a useful principle for either prediction or interpretation.
- Arnett, Edward M.,Molter, Kent
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p. 383 - 389
(2007/10/02)
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