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Bulletin of the Chemical Society of Japan 01/2013; 86(5):557-568. · 1.44 Impact Factor
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ABSTRACT: The mechanism of asymmetric hydrogenation catalyzed by [Rh(NBD)((R)-PhenylBinepine)(2)]SbF(6)1 has been studied by NMR experiments and DFT computations. Either the low-temperature hydrogenation of the catalyst-substrate adduct 4 or the reaction of solvate dihydride 6 with MAC produced the hydrogenation product with over 99% ee (S).
Chemical Communications 02/2012; 48(16):2186-8. · 6.17 Impact Factor
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Tetrahedron Letters 01/2012; 53(27):3452-3455. · 2.68 Impact Factor
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ABSTRACT: Both enantiomers of 2,3-bis(tert-butylmethylphosphino)quinoxaline (QuinoxP*), 1,2-bis(tert-butylmethylphosphino)benzene (BenzP*), and 1,2-bis(tert-butylmethylphosphino)-4,5-(methylenedioxy)benzene (DioxyBenzP*) were prepared in short steps from enantiopure (S)- and (R)-tert-butylmethylphosphine-boranes as the key intermediates. All of these ligands were crystalline solids and were not readily oxidized on exposure to air. Their rhodium complexes exhibited excellent enantioselectivities and high catalytic activities in the asymmetric hydrogenation of functionalized alkenes, such as dehydroamino acid derivatives and enamides. The practical utility of these catalysts was demonstrated by the efficient preparation of several chiral pharmaceutical ingredients having an amino acid or a secondary amine component. A rhodium complex of the structurally simple ligand BenzP* was used for the mechanistic study of asymmetric hydrogenation. Low-temperature NMR studies together with DFT calculations using methyl α-acetamidocinnamate as the standard model substrate revealed new aspects of the reaction pathways and the enantioselection mechanism.
Journal of the American Chemical Society 12/2011; 134(3):1754-69. · 9.91 Impact Factor
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Angewandte Chemie International Edition 10/2010; 49(44):8157-60. · 13.45 Impact Factor
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ABSTRACT: The mechanism of Michael addition reactions of 1,3-dicarbonyl compounds to cyclic enones catalyzed by bifunctional Ru catalysts bearing N-sulfonylated (R,R)-DPEN ligands (DPEN = (R,R)-1,2-diphenylethylenediamine) was studied by NMR and DFT computational analyses. NMR investigation of the stoichiometric reactions of chiral amido Ru complexes, Ru(N-sulfonylated dpen)(η(6)-arene) 1a-c, with dimethyl malonate 2 and β-keto ester 3 revealed that at decreased temperatures deprotonation proceeds in a stereoselective manner to provide amine complexes. The reaction with malonic ester 2 provided exclusively C-bound amino Ru complexes 6a,c, while the reaction of β-keto ester 3 gave an equilibrium mixture of rapidly interconverting C- and O-bound complexes. The structures of C-bound Ru complex 6c and O-bound Ru complex 9c were determined by single crystal X-ray analysis. A computational study showed that the enatioselective C-C bond formation proceeds through intermediate formation of chelating ion pairs that coordinate a molecule of enone via the Ru metal center producing a highly organized environment for the C-C bond formation, yielding selectively only one enantiomer of the product. Systematic study of a series of the catalyst-substrate combinations revealed that the experimentally observed sense of enantioselection was consistently explained by computational analysis. The tendency of increasing ee with the bulk of the coordinated arene in Ru complex is reproduced computationally by changes in the difference of either ZPPE-corrected energies or Gibbs free energies for S- and R-pathways.
Journal of the American Chemical Society 10/2010; 132(46):16637-50. · 9.91 Impact Factor
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ABSTRACT: A new and efficient strategy for the synthesis of substituted quinolines via electrophilic cyclization is developed. The intramolecular cyclization of 1-azido-2-(2-propynyl)benzene 1 proceeds smoothly in the presence of electrophilic reagents (I(2), Br(2), ICl, NBS, NIS, and HNTf(2)) in CH(3)NO(2) at room temperature or in the presence of catalytic amounts of AuCl(3)/AgNTf(2) in THF at 100 degrees C to afford the corresponding quinolines 2 in good to high yields. In the case of the electrophilic reagents, E of 2 is either I, Br, or H, depending on the reagent type, while E of 2 is H in the case of the electrophilic catalyst.
The Journal of Organic Chemistry 02/2010; 75(4):1266-70. · 4.45 Impact Factor
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ABSTRACT: This article describes recent developments in the understanding of the mechanism of enantioselection in one of the most efficient artificial catalytic reactions--Rh-catalyzed asymmetric hydrogenation. Numerous experimental and computational data are consistent with the idea that the enantioselection takes place through the reversible coordination of the double bond in octahedral dihydride complexes, which can happen only if the resulting chelate cycle is formed in the less hindered quadrant. In its main features, this mechanism of generating chirality resembles the recently uncovered three-stage recognition process occurring in enzymatic reactions.
Chemical Communications 12/2009; · 6.17 Impact Factor
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ABSTRACT: The reaction of 2-alkynyl-1-methylene azide aromatics 1 with iodine gives 1,3-disubstituted 4-iodoisoquinolines 3, and the treatment of 1 with a gold-silver combined catalyst affords isoquinolines 4. On the other hand, TfOH-catalyzed reaction of 1 produces 1,3-dipolar cycloadducts, triazoles 5. Computations reveal that non-symmetrical and slightly non-symmetrical coordinations between the triple bond and electrophiles (cationic Au and iodonium species) are prone to give the isoquinolines, while symmetrical coordination with electrophiles (a Brønsted acid and Au(I)) affords the triazoles. Keeping this background material in mind, the reactions through alkyne activation with electrophiles are surveyed. In most cases, products having similar structural frameworks were obtained through alkyne activation with Brønsted acids, iodine and gold complexes; the difference is whether H or I is incorporated in the final products. However, in a few cases, different reactivities and product structures were observed between those three reagents and catalysts.
Chemical Communications 09/2009; · 6.17 Impact Factor
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ABSTRACT: This paper describes the recent advances in the conceptually new bifunctional Ir and Ru catalysts for asymmetric catalytic reactions. These reactions include the enantioselective Michael addition of 1,3-dicarbonyl compounds to cyclic enones and nitroalkenes, and the enantioselective direct amination of alpha-cyanoacetates with diazoesters. The outcome of these reactions in terms of reactivity and selectivity was delicately influenced by the catalyst structures and the reaction conditions including the solvents used. Even with a 1 : 1 molar ratio of donors to acceptors, the reactions proceeded smoothly to give the corresponding chiral adducts with an excellent yield and enantiomeric excess (ee). Preliminary mechanistic studies showed that the key stage of the catalytic cycle is the interaction of the bifunctional catalyst with a pronucleophilic reagent that leads to stereoselective formation of C-, O-, or N-bound complexes. The resulting protonated catalyst bearing metal-bound nucleophiles readily reacts with electrophiles to provide C--C and C--N bond formation products in a highly stereoselective manner.
The Chemical Record 02/2009; 9(2):106-23. · 4.38 Impact Factor
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ABSTRACT: Deuterium isotope effects on the enantioselectivity in the rhodium-catalyzed asymmetric hydrogenation of enamides and related substrates have been studied. Distinct deuterium isotope effects were observed in the hydrogenation of aryl-substituted enamides having an ortho substituent that is capable of forming a hydrogen bond. The observed isotope effects are interpreted in terms of the competitive reactions of two dihydride intermediates and dideuteride intermediates that exist in equilibrium in the catalytic cycle.
Chemistry - An Asian Journal 08/2008; 3(8-9):1636-41. · 4.50 Impact Factor
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ABSTRACT: The rhodium complex of (R)-(tert-butylmethylphosphino)(di-tert-butylphosphino)methane used in Rh-catalyzed asymmetric hydrogenation of representative substrates 3-14 demonstrated high catalytic activity coupled with wide scope and nearly perfect enantioselectivity. Mechanistic studies (NMR and DFT computations) were carried out in order to investigate the mechanism of the enantioselection in the asymmetric hydrogenation of (Z)-alpha-acetamidocinnamate (3). Although catalyst-substrate complexes 15a,b with the double bond coordinated near the non-"chiral" phosphorus atom were formed as kinetic products upon the addition of 3 to solvate complex 2 at -100 degrees C, they rapidly rearranged to more stable isomers 15c,d with the double bond coordinated near the "chiral" phosphorus atom. The thermodynamic and kinetic parameters of the interconversion between 15c and 15d were determined by NMR; mainly, the interconversion occurred intramolecularly via nonchelating catalyst-substrate complexes 16. The equilibrium between 15d and 16d was directly observed from NMR line shape changes at temperatures ranging from -100 to -40 degrees C, whereas no such equilibrium was observed for 15c. This result was accounted for computationally by determining the corresponding transition states for the methanol insertion into 15c,d. Three sets of experiments of the low-temperature hydrogenation of different catalyst-substrate complexes gave the same order and sense of enantioselectivity (97% ee (R)) even in the case when 15c, having Re-coordinated double bond, was hydrogenated under the conditions precluding its isomerization to 15d. It was concluded that the hydrogenation of 15c,d does not occur directly, but is preceded by the dissociation of the double bond to result in the more reactive species 16. This indicates that enantioselection must occur at a later step of the catalytic cycle. DFT computations of association and migratory insertion steps suggest that enantioselection takes place during the association step when chelating dihydride 19d.MeOH is formed from nonchelating dihydride 18d.
Journal of the American Chemical Society 03/2008; 130(8):2560-72. · 9.91 Impact Factor
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Journal of the American Chemical Society 03/2008; 130(7):2158-9. · 9.91 Impact Factor
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ABSTRACT: The MgBr2.OEt2-mediated intramolecular allylation of a 4:1 diastereoisomeric mixture of the alpha-chloroacetoxyl ether 1a bearing the A-G/JK ring system of brevetoxin B in CH2Cl2 gave a 1:1 diastereoisomeric mixture of the trans- and cis-cyclization products 4a and 5a having the A-G/I-K ring system, while that in CH3CN afforded the trans-isomer 4a nearly as the single product. To help clarify a reason for this marked solvent effect in the cyclization of the brevetoxin B precursor, DFT computations for the starting materials, intermediates, transition states, and products were carried out. The cyclization would proceed through a carbocation intermediate 3a having sp2 flat structure (SN1 type mechanism) in CH2Cl2, in which the activation energies leading to both diastereoisomers are approximately identical, while in CH3CN alkylnitrilium salts 6a would be formed through the coordination of CH3CN to the carbocation leading to an sp3-type intermediate in which sever steric hindrance takes place in the transition state leading to the undesired diastereoisomer. The scope of this novel solvent-controlled stereoselectivity was tested for simple compounds. In small model compounds the marked solvent dependence was absent, but the model bearing two consecutive cyclic ether rings 1b exhibited a remarkable solvent effect similar to that observed in the brevetoxin B system.
The Journal of Organic Chemistry 11/2007; 72(22):8371-5. · 4.45 Impact Factor
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ABSTRACT: The amplifying asymmetric autocatalysis discovered by Soai and co-workers is dependent on the unique steric properties of the isopropyl group.
Chemical Communications 09/2007; · 6.17 Impact Factor
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ABSTRACT: The solvento complex [Rh(L)2(S)2]+ where L=tBuP(R-binaphthoxo) is shown to be in equilibrium with an eta-arene dirhodium complex and only weak, monodentate binding of alkenes is observed; in addition, an intermediate Rh alkyl hydride complex containing two coordinated monophosphonites is unambiguously characterised by NMR.
Chemical Communications 05/2007; · 6.17 Impact Factor
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ABSTRACT: The PtBr2-catalyzed reaction of 1-ethynyl-2-(1-alkoxybut-3-enyl)benzenes at 120 degrees C in CH3CN gave functionalized indenes in good to allowable yields. Most probably, the hydrogen at the terminal alkyne is transferred to the adjacent internal beta-carbon to form an (eta2-vinylidene)platinum carbene intermediate, which activates the sp3 C-H bond at the benzylic carbon. This reaction pathway is supported by DFT calculations which suggest that the formation of the Pt-vinylidene complex is the rate-limiting stage for the whole transformation.
The Journal of Organic Chemistry 09/2006; 71(16):6204-10. · 4.45 Impact Factor
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ABSTRACT: A conceptually novel approach for asymmetric intramolecular hydroamination, hydroalkoxylation and hydrocarbonation of alkynes using chiral palladium catalysts are described. The reactions of the aminoalkynes 5, alkynols 7, and alkynylmethines 9 in the presence of Pd2(dba)3 x CHCl3/PhCOOH/renorphos 4 in benzene (or benzene-hexane) at 100 degrees C gave the corresponding cyclization products (nitrogen heterocycles 6, oxygen heterocycles 8, and carbocycles 10) in good yields with good enantioselectivities. The origins of enantioselectivities in the hydroamination reaction are discussed based on DFT computations.
The Journal of Organic Chemistry 06/2006; 71(11):4270-9. · 4.45 Impact Factor
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ABSTRACT: Optically active C2-symmetric tetraphosphine 4 was prepared via the phosphine-borane methodology. Its dirhodium complex 5 was structurally characterized and probed as a catalyst in asymmetric hydrogenations of representative prochiral substrates, demonstrating high activity and good to excellent enantioselectivities. A mechanistic study revealed that 5 can be cleanly and stereoselectively converted to the tetrahydride species 6a, which is stable up to 0 °C and at higher temperatures slowly decomposes without the loss of hydrogen. The low-temperature (−80 °C) reaction of 6a with methyl (Z)-α-acetamidocinnamate (MAC) cleanly gave the tetrahydride complex 7 containing one molecule of the substrate coordinated only via the amidocarbonyl group, whereas the double bond of the substrate remained noncoordinated. Raising the temperature to −40 °C resulted in irreversible isomerization of complex 7 to 8, which differs from 7 only by the spatial arrangement of ligands. Migratory insertion proceeding simultaneously with the isomerization of 7 to 8 yields the trihydride complex 9, which is an analogue of the monohydride intermediates described previously. When the reaction of 6a with MAC was carried out in the presence of an excess of MAC, the released dirhodium complex was captured by the substrate to give the catalyst−substrate complex 10, which was characterized by multinuclear NMR. Substrate MAC is much more loosely bound in octahedral complexes 7 and 8 than in the square planar catalyst−substrate complex 10. This finding provides experimental support for the stereoselection during the association step of the Rh-catalyzed asymmetric hydrogenation.
01/2006;
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ABSTRACT: Two new heterobimetallic complexes, (7-exo-triphenylstannyl)-η4-cycloheptatrienylirontricarbonyl (3) and (7-exo-triphenylstannyl)-η4-cycloheptatrienylrutheniumtricarbonyl (5), have been synthesized via the ligand exchange reactions from triphenyl(cycloheptatrienyl)tin (1). Both 3 and 5 are fluxional. Facile [1,3]-Fe and [1,3]-Ru haptotropic shifts are the fastest dynamic processes observed in 3 and 5, respectively. Slower diatropic rearrangements involving simultaneous [1,7]-Sn and [1,2]-M (or [1,4]-M) migrations (M = Fe, Ru) were observed in each case. Activation parameters of the fast rearrangements in 3 and 5 were determined via 2D EXSY NMR (1H and 13C). The mode of intramolecular oscillations was switched in solution by reacting 3 with bromodimethylborane, which yielded cleanly (7-exo-phenyl(methyl)boryl)-η4-cycloheptatrienylirontricarbonyl (14), which exhibited a single type of intramolecular dynamics, viz., [1,7]-B + [1,2]-Fe diatropic rearrangement.
08/2005;
