- Combined Theoretical and Experimental Studies Unravel Multiple Pathways to Convergent Asymmetric Hydrogenation of Enamides
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We present a highly efficient convergent asymmetric hydrogenation of E/Z mixtures of enamides catalyzed by N,P-iridium complexes supported by mechanistic studies. It was found that reduction of the olefinic isomers (E and Z geometries) produces chiral amides with the same absolute configuration (enantioconvergent hydrogenation). This allowed the hydrogenation of a wide range of E/Z mixtures of trisubstituted enamides with excellent enantioselectivity (up to 99% ee). A detailed mechanistic study using deuterium labeling and kinetic experiments revealed two different pathways for the observed enantioconvergence. For α-aryl enamides, fast isomerization of the double bond takes place, and the overall process results in kinetic resolution of the two isomers. For α-alkyl enamides, no double bond isomerization is detected, and competition experiments suggested that substrate chelation is responsible for the enantioconvergent stereochemical outcome. DFT calculations were performed to predict the correct absolute configuration of the products and strengthen the proposed mechanism of the iridium-catalyzed isomerization pathway.
- Yang, Jianping,Massaro, Luca,Krajangsri, Suppachai,Singh, Thishana,Su, Hao,Silvi, Emanuele,Ponra, Sudipta,Eriksson, Lars,Ahlquist, M?rten S. G.,Andersson, Pher G.
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supporting information
p. 21594 - 21603
(2021/12/27)
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- Deoxygenative α-alkylation and α-arylation of 1,2-dicarbonyls
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Construction of C-C bonds at the α-carbon is a challenging but synthetically indispensable approach to α-branched carbonyl motifs that are widely represented among drugs, natural products, and synthetic intermediates. Here, we describe a simple approach to generation of boron enolates in the absence of strong bases that allows for introduction of both α-alkyl and α-aryl groups in a reaction of readily accessible 1,2-dicarbonyls and organoboranes. Obviation of unselective, strongly basic and nucleophilic reagents permits carrying out the reaction in the presence of electrophiles that intercept the intermediate boron enolates, resulting in two new α-C-C bonds in a tricomponent process. This journal is
- Arman, Hadi D.,Dang, Hang T.,Haug, Graham C.,Jin, Shengfei,Larionov, Oleg V.,Nguyen, Viet D.
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p. 9101 - 9108
(2020/09/17)
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- Modeling and optimization of lipase-catalyzed hydrolysis for production of (S)-2-phenylbutyric acid enhanced by hydroxyethyl-β-cyclodextrin
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An efficient reactive system was established to produce (S)-2-phenylbutyric acid (2-PBA) through the enzymatic enantioselective hydrolysis of 2-phenylbutyrate ester (2-PBAE) in aqueous medium. Lipase CALA from Canadian antarctica and hexyl 2-phenylbutyrate (2-PBAHE) were identified upon screening as the best enzyme and substrate, respectively. Adding hydroxyethyl-β-cyclodextrin (HE-β-CD) to improve the solubility of the substrate resulted in a 1.5 times increase in substrate conversion while retaining a high enantioselectivity compared with that when HE-β-CD was not added. The effects of lipase concentration, substrate concentration and HE-β-CD concentration, temperature, pH, and reaction time on enantiomeric excess and conversion rate were investigated, and the optimal conditions were identified using response surface methodology (RSM). Under the optimal conditions, namely 50 mg/mL lipase CALA, 30 mmol/L substrate, 60 mmol/L HE-β-CD, pH of 6.5, temperature of 83 °C and reaction time of 18 h, the enantiomeric excess and overall conversion rate were 96.05% and 27.28%, respectively. This work provides an efficient alternative method for improving the conversion of aromatic ester substrates by including β-cyclodextrin in an aqueous hydrolysis reaction system.
- Zhang, Panliang,Cheng, Qing,Xu, Weifeng,Tang, Kewen
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- Enantioselective Hydrogen Atom Transfer: Discovery of Catalytic Promiscuity in Flavin-Dependent 'Ene'-Reductases
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Flavin has long been known to function as a single electron reductant in biological settings, but this reactivity has rarely been observed with flavoproteins used in organic synthesis. Here we describe the discovery of an enantioselective radical dehalogenation pathway for α-bromoesters using flavin-dependent 'ene'-reductases. Mechanistic experiments support the role of flavin hydroquinone as a single electron reductant, flavin semiquinone as the hydrogen atom source, and the enzyme as the source of chirality.
- Sandoval, Braddock A.,Meichan, Andrew J.,Hyster, Todd K.
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supporting information
p. 11313 - 11316
(2017/08/30)
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- Preparation, structure, and reactivity of nonstabilized organoiron compounds. Implications for iron-catalyzed cross coupling reactions
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A series of unprecedented organoiron complexes of the formal oxidation states -2, 0, +1, +2, and +3 is presented, which are largely devoid of stabilizing ligands and, in part, also electronically unsaturated (14-, 16-, 17- and 18-electron counts). Specifically, it is shown that nucleophiles unable to undergo β-hydride elimination, such as MeLi, PhLi, or PhMgBr, rapidly reduce Fe(3+) to Fe(2+) and then exhaustively alkylate the metal center. The resulting homoleptic organoferrate complexes [(Me4Fe)(MeLi)] [Li(OEt2)]2 (3) and [Ph4Fe][Li(Et 2O)2][Li(1,4-dioxane)] (5) could be characterized by X-ray crystal structure analysis. However, these exceptionally sensitive compounds turned out to be only moderately nucleophilic, transferring their organic ligands to activated electrophiles only, while being unable to alkylate (hetero)aryl halides unless they are very electron deficient. In striking contrast, Grignard reagents bearing alkyl residues amenable to β-hydride elimination reduce FeXn (n = 2, 3) to clusters of the formal composition [Fe(MgX)2]n. The behavior of these intermetallic species can be emulated by structurally well-defined lithium ferrate complexes of the type [Fe(C2H4) 4][Li(tmeda)]2 (8), [Fe(cod)2][Li(dme)] 2 (9), [CpFe(C2H4)2][Li(tmeda)] (7), [CpFe(cod)][Li(dme)] (11), or [Cp*Fe(C2H4) 2][Li(tmeda)] (14). Such electron-rich complexes, which are distinguished by short intermetallic Fe-Li bonds, were shown to react with aryl chlorides and allyl halides; the structures and reactivity patterns of the resulting organoiron compounds provide first insights into the elementary steps of low valent iron-catalyzed cross coupling reactions of aryl, alkyl, allyl, benzyl, and propargyl halides with organomagnesium reagents. However, the acquired data suggest that such C-C bond formations can occur, a priori, along different catalytic cycles shuttling between metal centers of the formal oxidation states Fe(+1)/Fe(+3), Fe(0)/Fe(+2), and Fe(-2)/Fe(0). Since these different manifolds are likely interconnected, an unambiguous decision as to which redox cycle dominates in solution remains difficult, even though iron complexes of the lowest accessible formal oxidation states promote the reactions most effectively.
- Fuerstner, Alois,Martin, Ruben,Krause, Helga,Seidel, Guenter,Goddard, Richard,Lehmann, Christian W.
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p. 8773 - 8787
(2008/12/23)
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- Multicomponent, one-pot sequential synthesis of 1,3,5- and 1,3,5,5-substituted barbiturates
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(Chemical Equation Presented) Carbodiimides and malonic acid monoethylesters readily react to afford N-acylurea derivatives that could be cyclized in situ by addition of a suitable base. This process represents a general and straightforward one-pot sequential synthesis of 1,3,5-trisubstituted barbiturates in very mild conditions (organic solvent/2 N NaOH aqueous solution, 20°C). Performing the reaction in the presence of an electrophile resulted in the formation of fully substituted (namely, 1,3,5,5- tetrasubstituted) barbiturates through a three-component one-pot sequential process. The latter, however, occurred only with highly reactive electrophiles, such as benzyl and, in some instances, allyl halides. In order to expand the scope of the process, we sought to develop a general method for the C-alkylation of 1,3,5-trisubstituted barbiturates. We found that C-alkylation occurred upon treatment of 1,3,5-trisubstituted barbiturates with an alkyl halide in CH 3CN at 120°C in the presence of anhydrous K2CO 3 affording the target 1,3,5,5-tetrasubstituted barbiturates in good yields. The multicomponent process was accomplished by combining the three steps in a one-pot sequential fashion, i.e., the condensation of carbodiimides with malonic acid monoethylesters, the cyclization of the resulting N-acylureas, and the C-alkylation of the resulting 1,3,5-substituted barbiturates. A detailed study of the influence of the structure of the reactants on the reaction outcome and mechanism is presented. By selective N′-deprotection of 1,3,5,5-tetrasubstituted barbiturates, the corresponding 1,5,5-trisubstituted barbiturates were also prepared.
- Volonterio, Alessandro,Zanda, Matteo
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p. 7486 - 7497
(2008/12/22)
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- An efficient laboratory synthesis of α-deuteriated profens
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An efficient and practical laboratory synthesis of a series of 2-deuterio-2-arylpropionic acids (α-deuterioprofens) is described. The levels of deuterium incorporation are high and the products are synthetically useful. Copyright
- Coumbarides, Gregory S.,Dingjan, Marco,Eames, Jason,Flinn, Anthony,Northen, Julian
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p. 903 - 914
(2008/02/09)
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- CHIRAL ORGANOSILICON HYDRIDES
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The invention provides a method for enantioselectively reducing a prochiral carbon centred radical having one or more electron donor groups attached directly to the central prochiral carbon atom of the radical, and/or attached to a carbon atom within 1 to 4 atoms of the central prochiral carbon atom, comprising treating said radical with an activated chiral non-racemic organosilicon hydride in the presence of a Lewis acid. The invention also provides a novel class of activated chiral non-racemic organosilicon hydrides.
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- Cross-coupling of alkyl halides with aryl Grignard reagents catalyzed by a low-valent iron complex
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A striking reversal of the usual reactivity pattern of aryl Grignard reagents is observed for reactions in the presence of catalytic amounts of the "bare" ferrate complex [Li(tmeda)]2[Fe(C2H 4)4] (1). Highly reduced iron-magnesium clusters may play a decisive role in the exceptionally facile and chemoselective cross-coupling reaction with alkyl halides (see scheme).
- Martin, Ruben,Fuerstner, Alois
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p. 3955 - 3957
(2007/10/03)
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- ORGANOGERMANIUM COMPOUNDS AND METHODS FOR THEIR USE
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The invention provides a method for enantioselectively reducing a prochiral carbon centred radical having one or more electron donor groups attached directly to the central prochiral carbon atom of the radical, and/or attached to a carbon atom within 1 to 4 atoms of the central prochiral carbon atom, comprising treating said radical with a chiral non-racemic organogermanium hydride in the presence of a Lewis acid. The invention also provides a novel class of chiral non-racemic organogermanium hydrides and a method of preparing chiral non-racemic organogermanium compounds.
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Page/Page column 28; 30
(2010/02/06)
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- The use of dialkyl carbonates for safe and highly selective alkylations of methylene-active compounds. A process without waste production
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The non-toxic compound dimethyl carbonate (DMC) can be used as a methylating and a methoxycarbonylating agent in place of methyl chloride and phosgene, respectively. We report here that DMC and other dialkyl carbonates (DAlkCs: dimethyl, diethyl and dibenzyl carbonates) allow very selective alkylations of a variety of CH2-acidic compounds. Both arylacetonitriles and alkyl arylacetates react with DAIkCs to yield the mono-C-alkylated derivatives (α-alkyl-α-arylacetonitriles and alkyl α-alkyl-α-arylacetates) with a selectivity of up to 99%, at complete conversion. Likewise, the mono-C-methylation by DMC proceeds selectively also on (aryloxy)acetonitriles and methyl (aryloxy)acetates. The reactions are carried out at temperature of 180-220°C in the presence of weak bases (usually K2CO3); under such conditions, DAlkCs efficiently replace the common and very toxic alkylating agents (dialkyl sulfates and alkyl halides). In addition to the high selectivity obtained and the intrinsic safety of the dialkyl carbonates, the reported reactions give rise to neither organic nor inorganic waste products.
- Bomben, Andrea,Selva, Maurizio,Tundo, Pietro
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p. 256 - 260
(2007/10/03)
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- Selective Mono-methylation of Arylacetonitriles and Methyl Arylacetates by Dimethyl Carbonate
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Both arylacetonitriles and methyl arylacetates react with dimethyl carbonate (DMC) (20 molar excess) at 180 - 200 deg C in the presence of K2CO3 to produce monomethylated 2-arylpropionitriles and methyl 2-arylpropionates, respectively, with a selectivity >99.5percent.The reaction, with wide application, proceeds by DMC acting as a methoxycarbonylating agent towards the ArCH-X anion (X = CN, CO2Me) and as a methylating agent to ArC-(CO2Me)X.DMC also proved to be the best solvent for such reactions.
- Selva, Maurizio,Marques, Carlos Alberto,Tundo, Pietro
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p. 1323 - 1328
(2007/10/02)
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- Process for the alpha-monoalkylation of arylacetonitriles, arylacetoesters and arylacetic acids
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A process for the α-monoalkylation of arylacetonitriles, arylacetoesters and arylacetic acids with dialkyl carbonates in liquid phase, in the presence of bases at temperatures ranging from about 100° C. to about 270° C.
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- A process for the alpha-monoalkylation of arylacetonitriles, arylacetoesters and arylacetic acids
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A process for the α-monoalkylation of arylacetonitriles, arylacetoesters and arylacetic acids with dialkyl carbonates in liquid phase, in the presence of bases at temperatures ranging from about 100°C to about 270°C.
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- THE ADDITION REACTION OF DIALKYL CARBONATES TO KETONES
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The reaction of benzyl phenyl ketone with dimethyl carbonate gives methyl benzoate and methyl phenylacetate; diethyl carbonate gives the corresponding ethyl esters.The reaction takes place at high temperature (about 200 deg C) and in the presence of potassium carbonate as a catalyst.Other benzyl ketones react similarly.Aliphatic ketones yield a less selective addition.Accordingly, cyclohexanone with dimethyl carbonate gives dimethyl pimelate.The reaction is a retro-Claisen condensation, which occurs through the intermediate formation of the alkoxycarbonyl derivative.
- Selva, Maurizio,Marques, Carlos Alberto,Tundo, Pietro
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p. 515 - 518
(2007/10/02)
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