- 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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- Michael additions in water of ketones to nitroolefins catalyzed by readily tunable and bifunctional pyrrolidine-thiourea organocatalysts
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An operationally trivial and environmentally benign procedure for direct Michael addition has been developed. The reaction of various ketones with nitroolefins can be performed in water to afford the corresponding nitro compounds in high yields in the pre
- Cao, Yi-Ju,Lai, Yuan-Yuan,Wang, Xiang,Li, Yong-Jian,Xiao, Wen-Jing
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- An effective bifunctional thiourea catalyst for highly enantio- and diastereoselective Michael addition of cyclohexanone to nitroolefins
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A series of new tunable and bifunctional thiourea catalysts have been synthesized and their catalytic activities are evaluated in the direct Michael addition of cyclohexanone to nitroolefins. High isolated yields (up to 97%), enantioselectivities (up to 9
- Cao, Yi-Ju,Lu, Hai-Hua,Lai, Yuan-Yuan,Lu, Liang-Qiu,Xiao, Wen-Jing
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p. 3795 - 3800
(2008/02/09)
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- Pyrrolidine-thiourea as a bifunctional organocatalyst: Highly enantioselective Michael addition of cyclohexanone to nitroolefins
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Asymmetric Michael additions of cyclohexanone to both aryl and alkyl nitroolefins in the presence of 20 mol % of organocatalyst 2b and 10 mol % of n-butyric acid afford adducts 5 with high diastereoselectivities and enantioselectivities.
- Cao, Chun-Li,Ye, Meng-Chun,Sun, Xiu-Li,Tang, Yong
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p. 2901 - 2904
(2007/10/03)
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