- 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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- 2,2,6,6-Tetramethylpiperidin-1-yloxycarbonyl: A Protecting Group for Primary, Secondary, and Heterocyclic Amines
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The 2,2,6,6-tetramethylpiperidin-1-yloxycarbonyl (Tempoc) protecting group is readily introduced by the reaction of amines with a new acyl transfer reagent, 4-nitrophenyl (2,2,6,6-tetramethylpiperidin-1-yl) carbonate (NPTC). Tempoc has a reactivity profile that complements the commonly used t-butyloxycarbonyl (Boc) and benzyloxycarbonyl (Cbz) protecting groups. Deprotection can be achieved under mild reductive conditions with in situ generated Cu(I) species or by thermolytic cleavage at 135 °C. Mechanistic studies on the deprotection of Tempoc-indole suggest a combination of ionic and radical fragmentation pathways under thermal conditions.
- Lizza, Joseph R.,Bremerich, Maximilian,McCabe, Stephanie R.,Wipf, Peter
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p. 6760 - 6764
(2018/10/25)
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- ADDITION 1-3 D'HYDROSILANES ET STANNANES SUR DIVERSES NITRONES : RADICAUX NITROXYDES α-METALLES R3M-C-N.-O ET O- ET C-METALLA-HYDROXYLAMINES R3M-O-N-C-H ET R3M-C-N-OH
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Free radical 1,3-addition of silanes or stannanes to various nitrones generally leads to corresponding O-organometallohydroxylamines.The α-metallonitroxide radicals =-M-C-N.-O, obtained by trapping silyl or stannyl-centred radicals with an appropriate nitrone, have been characterised by e.s.r. spectroscopy.The O-stannylhydroxylamines are generally sufficiently stable to be isolated by distillation under vacuum.The less stable O-germyl or O-silylhydroxylamines decompose readily upon heating or by photolysis by competing intra- and intermolecular processes.Under special conditions favouring the formation of silylanions (MeNO2, NEt3), a competitive 1,3-dipolar addition of phenyldichloro- or trichlorosilane with the nitrone leads to the unstable C-metallohydroxylamine =-M-C-N-OH .
- Riviere, Pierre,Richelme, Suzanne,Riviere-Baudet, Monique,Satge, Jacques,Riley, Paul I.,et al.
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p. 1663 - 1683
(2007/10/02)
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