- Iridium-catalyzed reduction of secondary amides to secondary amines and imines by diethylsilane
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Catalytic reduction of secondary amides to imines and secondary amines has been achieved using readily available iridium catalysts such as [Ir(COE) 2Cl]2 with diethylsilane as reductant. The stepwise reduction to secondary amine proceeds through an imine intermediate that can be isolated when only 2 equiv of silane is used. This system requires low catalyst loading and shows high efficiency (up to 1000 turnovers at room temperature with 99% conversion have been attained) and an appreciable level of functional group tolerance.
- Cheng, Chen,Brookhart, Maurice
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supporting information; experimental part
p. 11304 - 11307
(2012/09/05)
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- Synthesis and Thermal Reorganisations of Iminoaziridines. Ab Initioo Calculations of Their Transition States, of Diazatrimethylenemethanes, and Some Cyclic Members of the C2H4N2 Potential-Energy Hypersurface
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2-Iminoaziridine 19 and the pairs of isomers 22/23 and 32/33 can be obtained in high yields by base-induced 1,3-dehydrohalogenation of the corresponding α-halo amidines.Regioselectivity of the ring closure reaction of 21 is achieved by treatment with potassium hydride as base in the presence of 18-crown-6 to afford almost exclusively 23.At low temperatures, the cyclisation of 21a under the influence of potassium tert-butoxide occurs diastereoselectively yielding (Z)-22 and (Z)-23 (87:13).Resolution of racemic 21a can be attained by single recrystallisation of the nicely crystallised mandelates of the like configuration, thus providing an efficient route to (R)- and (S)-23 of high enantiomeric purity.Chiroptical data are reported for 21a and 23. - (E)- and (Z)-22 equilibrate faster by more than one order of magnitude than (E)- and (Z)-23 but almost the same preference for the E diastereomer is found in both cases. - Only first-order decomposition into isocyanides 6 and imines is observed on thermolysis of 19, 22, 23, and 32 with activation parameters depending on the substitution pattern.In contrast, the thermal valence isomerisation 35 -> 36 is fast enough to complete with the cycloreversion of 35.Thermal racemisation of (R)-23 does not occur.Quantum-chemical calculations were performed on the parent iminoaziridines (E)- and (Z)-38, 3-amino-2H-azirine (39), the methylenediaziridines cis- and trans-40, and diastereomeric closed-shell and open-shell planar (41) and (open-shell) "mono-orthogonal" diazatrimethylenemethanes 42, 43.Complete geometry optimisations were appropriately performed with the RHF/6-31+G** and the UHF/6-31+G** basis sets.Energies of the closed-shell states were calculated on the RHF, MP2, MP4SDTQ, and CCSD(T) levels, those of open-shell states on the UHF, UMP2, UMP4SDTQ, and CCSD(T)/UHF levels. - (E)-38, being lowest in energy of the parent iminoaziridines (Erel = 0.0), equilibrates with (Z)-38 via an almost linear transition state (Erel = 113.4 kJ mol-1) and decomposes into hydrogen isocyanide and formaldimine in a one-step cheletropic process rel = 179.8 kJ mol-1, CCSD(T)/RHF> with a highly unsymmetrical transition state in which the N1-C2 bond is almost completely broken whereas the C2-C3 bond is still strong.The same is true for the cheletropic decomposition of (Z)-38 the transition state of which (Erel = 168.2 kJ mol-1) is even lower by 11.6 kJ mol-1. - Energy-rich zwitterionic transition states are found with the RHF method among which 1A'-(E,Z)-41 is lowest in energy rel = 203.0 ... - Keywords: Aziridines, 2-imino-, chiral, non-racemic; Amidines, N,N'-dialkyl-2-halo-; 1,3-Dehydrohalogenation, regioselectivity and diastereoselectivity of; E/Z Diastereomerisation; Cycloreversion of isocyanides and imines; Valence isomerisation; Calculations, high-level ab initio, RHF, UHF, MPn, CCSD(T); 2H-Azirine, 3-amino; Diaziridine, 3-methylene; Diazatrimethylenemethanes, diastereomeric, closed-shell and open-shell; Reorganisations, transition states of thermal
- Quast, Helmut,Aldenkortt, Sven,Schaefer, Peter,Schmitt, Edeltraud,Wuerthwein, Ernst-Ulrich
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p. 2171 - 2188
(2007/10/03)
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- Synthesis and Thermolysis of a Chiral, Non-Racemic Iminoaziridine
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The 2-halo imidoyl chlorides 7 are obtained from the amide 5 and the 2-halo amides 6 by the action of phosphorus pentachloride and thionyl chloride, respectively.Non-racemic (S)-6a is converted into 7a which is racemic, however.The reaction of Lawesson's reagent with 6a furnishes the diasteromeric 1,3,2-thiazaphospholidine derivatives 15.Treatment of (S)-6a (98percent ee) with methyl triflate affords 2-chloro imidate 8 (95percent ee) which reacts with methanamine in the presence of methanammonium chloride to yield the 2-chloro amidine (S)-9a (90percent ee).The 2-halo imidoyl halides 7a and b react with methanamine to produce the 2-halo amidines 9a and b. - Strong bases, e.g. potassium tert-butoxide or sodium hydride in the presence of catalytic amounts of tert-butyl alcohol, eliminate hydrogen chloride or bromide from the 2-halo amidines 9a and b and (S)-9a to yield mixtures of the 2-iminoaziridines (E)- and (Z)-4, and (E,R)- and (Z,R)-4 (83percent ee), respectively.The 1,3-elimination of hydrogen bromide from 9b is diastereoselective at -30 to -40 deg C 90>.The diastereomers equilibrate at 36 deg C with (kEZ + kZE) = (5.92+/-0.08)*1E-5 s-1 (K = kEZ/kZE = 0.428 +/- 0.013). - The thermolysis of (E)- and (Z)-4 in 6>benzene solution yields the imine 16 and methyl isocyanide (17).The decomposition follows the first-order rate law.The following Arrhenius and Eyring parameters are calculated from five rate constants obtained in the temperature range of 70-110 deg C.Ea = (115.2 +/-0.4) kJmol-1, lgA = (12.06 +/- 0.28), ΔH(excit.) = (112.1 +/- 0.4) kJmol-1, ΔS(excit.) = (-23.9 +/- 0.7) JK-1 mol-1, ΔG(excit.) 373K = 121 kJmol-1.The enantiomeric excess of the surviving fraction of (E,R)- and (Z,R)-4 is unchanged after two half-lives at 80 deg C. - Key Words: Aziridines, 2-imino-, chiral, non-racemic / Imidoyl halides, 2-halo / Imidates, 2-chloro / 1,3,2-Thiazaphospholidines / Amidines, 2-halo / 1,3-Elimination, inversion of configuration in / E/Z Diastereomerization / Cycloreversion, kinetics of thermal
- Quast, Helmut,Aldenkortt, Sven,Heller, Eberhard,Schaefer, Peter,Schmitt, Edeltraud
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p. 1699 - 1706
(2007/10/02)
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