- Molecular Insights into the Ligand-Reactant Interactions of Pt Nanoparticles Functionalized with α-Amino Acids as Asymmetric Catalysts for β-Keto Esters
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The asymmetric hydrogenation of ?-keto esters over α-amino acid-functionalized Pt nanoparticles was explored in order to expand our understanding of ligand-reactant interactions underlying the chiral induction. A comprehensive investigation aimed at the quantification of the nonlinear effects demonstrated that for most of the ligands and reactants enantiodifferentiation is determined by 1 : 1 ligand-reactant interaction. However, attachment of phenyl substituents to the ligands or reactants likely involves the formation of more intricate intermediate complexes. We have shown that the asymmetric bias is sensitive to even small changes in the geometry of the ligand. Additionally, we have found that alkali metal cations, which balance the negative charge of the ligand's carboxyl group and originate from the metal hydroxide used for ligand functionalization, play a key role in the process of chiral induction. As the nature of the cation can be varied by simply changing the metal hydroxide used during functionalization, this finding opens an additional possibility to control the stereoselectivity by tuning the ligand-reactant interaction.
- ?ulce, Anda,Mitschke, Nico,Azov, Vladimir,Kunz, Sebastian
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- Development of an axially chiral sp3P/sp3NH/sp2N-combined linear tridentate ligand - Fac-selective formation of Ru(II) complexes and application to ketone hydrogenation
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A newly developed chiral linear tridentate ligand, R-PN(H)N (R=H or Ph), possesses Ph2P and PyCH2NH groups at C(2) and C(2′) positions of the 1,1′-binaphthyl skeleton without or with a C(3)-Ph substituent. The steric effect of C(3)-Ph and the electronic effect of the DMSO co-ligand realize the facial selective generation of fac-RuCl2(Ph-PN(H)N)(dmso) and fac-[Ru(H-PN(H)N)(dmso)3](BF4)2, respectively. Both an H-Ru?sp3N-H reaction site responsible for the donor-acceptor bifunctional catalyst (DACat) and a fence/plane chiral context were constructed by means of the following advantageous points: i) the sp3P, sp3N, and sp2N ligating atoms have different electronic properties; ii) DMSO trans to sp3N strongly coordinates to Ru and is fixed by a PyC(6)H?O=S hydrogen bond; and iii) the single NH function simplifies the DACat reaction site. The synergistic effect has led to success in the asymmetric hydrogenation of sterically demanding ketones. Structural characteristics of first-row transition metal complexes of R-PN(H)N have been also investigated.
- Yamamura, Tomoya,Nakane, Satoshi,Nomura, Yuko,Tanaka, Shinji,Kitamura, Masato
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- Hydroxy-Directed Amidation of Carboxylic Acid Esters Using a Tantalum Alkoxide Catalyst
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We describe herein a new strategy for the chemoselective synthesis of amides by using a metal-catalyzed hydroxy-directed reaction. A hydroxy group located at the β-position of an ester group promoted the activation of a carbonyl group with a tantalum alkoxide catalyst followed by amidation reactions, leading to a wide variety of β-hydroxyamides with excellent chemeselectivity. The chemoselective amidation strategy can be extended to the catalytic synthesis of dipeptide derivatives, which remains challenging research subjects in modern organic synthesis.
- Tsuji, Hiroaki,Yamamoto, Hisashi
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supporting information
p. 14218 - 14221
(2016/11/13)
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- Asymmetric hydrogenation of tert-alkyl ketones: DMSO effect in unification of stereoisomeric ruthenium complexes
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Face off: The ruthenium complexes of a new axially chiral PNNligand (L) are highly efficient in the presence of dimethylsulfoxide (DMSO) for hydrogenation of both functionalized and unfunctionalized tert-alkyl ketones. DMSO is thought to narrow down the many possible complex stereoisomers into a single facial L/Ru complex, thus enhancing the reactivity, selectivity, and productivity. Copyright
- Yamamura, Tomoya,Nakatsuka, Hiroshi,Tanaka, Shinji,Kitamura, Masato
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supporting information
p. 9313 - 9315
(2013/09/12)
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- Reduction of aromatic and aliphatic keto esters using sodium borohydride/MeOH at room temperature: a thorough investigation
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Reduction of keto esters is a valuable alternative to produce diols. Sodium borohydride/MeOH system at room temperature and short reaction time efficiently reduced α, β, γ, and δ-keto esters having α-keto esters as the most reactive. The ester functionality was reduced effectively due to the presence of oxo group that somehow facilitates the formation of ring intermediate. As expected, the chemoselective experiments showed that ester functionality was not reduced using this system. This study presents a simple, easy, and benign reduction process of various keto esters to its corresponding diols.
- Kim, Juryoung,De Castro, Kathlia A.,Lim, Minkyung,Rhee, Hakjune
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supporting information; experimental part
p. 3995 - 4001
(2010/07/05)
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- Exploring the synthetic applicability of a cyanobacterium nitrilase as catalyst for nitrile hydrolysis
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The substrate specificity and synthetic applicability of the nitrilase from cyanobacterium Synechocystis sp. strain PCC 6803 have been examined. This nitrilase catalyzed the hydrolysis of both aromatic and aliphatic nitriles to the corresponding acids in high yields. Furthermore, the stereoselective hydrolysis of phenyl-substituted β-hydroxy nitriles to (S)-enriched β-hydroxy carboxylic acids and selective hydrolysis of α,ω- dinitriles with five or less methylene groups to ω-cyano carboxylic acids have been achieved. This suggested that nitrilase from Synechocystis sp. PCC 6803 could be a useful enzyme catalyst for the "green" nitrile hydrolysis. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.
- Mukherjee, Chandrani,Zhu, Dunming,Biehl, Edward R.,Hua, Ling
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p. 5238 - 5242
(2007/10/03)
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- Synthesis of isotetronic acids by cyclization of 1,3-bis(trimethylsilyloxy) alk-1-enes with oxalyl chloride
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Isotetronic acids were regioselectively prepared by cyclization of 1,3-bis(trimethylsilyloxy)alk-1-enes with oxalyl chloride.
- Dede, Rüdiger,Michaelis, Lars,Langer, Peter
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p. 8129 - 8131
(2007/10/03)
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- Enantioselective synthesis of α-amino acids from N-tosyloxy β-lactams derived from β-keto esters
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A novel synthetic sequence has been developed to convert simple β-keto esters into enantiomerically enriched α-amino acids. The key features of this sequence include the addition of azide to the C3 position of β-keto ester derived N-tosyloxy-β-lactams through a concomitant nucleophilic addition/N-O bond reduction reaction, a mild CsF-induced N1 benzylation of α-azido monocyclic β-lactams, the preparation of α-keto-β-lactams through a novel four-step sequence from the corresponding 3-azido-1-benzyl-β-lactams, and TEMPO-mediated ring expansion of these compounds to the corresponding N-carboxy anhydrides (NCAs). In addition, the synthesis, isolation, and characterization of unusual 3-imino and 3-chloramino-β-lactams is reported.
- Durham, Timothy B.,Miller, Marvin J.
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