- Sustainable electrochemical decarboxylative acetoxylation of aminoacids in batch and continuous flow
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Introduction of acetoxy groups to organic molecules is important for the preparation of many active ingredients and synthetic intermediates. A commonly used and attractive strategy is the oxidative decarboxylation of aliphatic carboxylic acids, which entails the generation of a new C(sp3)-O bond. This reaction has been traditionally carried out using excess amounts of harmful lead(iv) acetate. A sustainable alternative to stoichiometric oxidants is the Hofer-Moest reaction, which relies on the 2-electron anodic oxidation of the carboxylic acid. However, examples showing electrochemical acetoxylation of amino acids are scarce. Herein we present a general and scalable procedure for the anodic decarboxylative acetoxylation of amino acids in batch and continuous flow mode. The procedure has been applied to the derivatization of several natural and synthetic amino acids, including key intermediates for the synthesis of active pharmaceutical ingredients. Good to excellent yields were obtained in all cases. Transfer of the process from batch to a continuous flow cell signficantly increased the reaction throughput and space-time yield, with excellent product yields obtained even in a single-pass. The sustainability of the electrochemical protocol has been examined by evaluating its green metrics. Comparison with the conventional method demonstrates that an electrochemical approach has a significant positive effect on the greenness of the process.
- K?ckinger, Manuel,Hanselmann, Paul,Roberge, Dominique M.,Geotti-Bianchini, Piero,Kappe, C. Oliver,Cantillo, David
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supporting information
p. 2382 - 2390
(2021/04/12)
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- Base-catalyzed selective esterification of alcohols with unactivated esters
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A practical and efficient base-catalyzed esterification has been developed for the facile synthesis of a broad range of esters from simple alcohols with unactivated tert-butyl esters. This protocol could be conducted at mild conditions, providing esters in high to excellent yields with good functional tolerance. Mechanistic studies provided evidence of an exchange of the tert-butyl alkoxide metal with the alcohol, producing a new alkoxide to participate in the transesterification reaction.
- Zhang, Chunyan,Zhang, Guoying,Luo, Shizhong,Wang, Chunfu,Li, Huiping
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supporting information
p. 8467 - 8471
(2018/12/01)
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- Selectivity Enhancement in Dynamic Kinetic Resolution of Secondary Alcohols through Adjusting the Micro-Environment of Metal Complex Confined in Nanochannels: A Promising Strategy for Tandem Reactions
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(Figure Presented). Dichloro(η6-p-cymene) (1-butyl-3-cyclohexyl-imidazolin-2-ylidene) ruthenium(II) (RuL) was synthesized and confirmed. Five heterogeneous catalysts with similar ruthenium cores were prepared by chemical immobilization method using various silica-based supports, including mesoporous silica SBA-15 of different pore sizes (Ru/Si-9, Ru/Si-8, and Ru/Si-7), nonporous silica particles (Ru/SiO2), and surface trimethylsilylated SBA-15 (Ru/SiMe). The dynamic kinetic resolution (DKR) of 1-phenylethanol, which includes metal-enzyme bicatalytic racemization in tandem with stereoselective acylation, gave product in 99% yield and 0% ee with homogeneous catalyst RuL, whereas the heterogeneous Ru/Si-8 exhibited high catalytic activity and enantioselectivity (up to 96% yield and 99% ee). The racemization and acylation abilities of different catalysts were analyzed. The influences of pore size and surface properties for heterogeneous catalysts were investigated, and the nanocage effect was found to be the key factor in stereoselectivity. The catalyst Ru/Si-8 performed well in reactions with various substrates and can be reused for at least seven times.
- Cao, Hui,Zhu, Xiao-Han,Wang, Dong,Sun, Zhenkun,Deng, Yonghui,Hou, Xiu-Feng,Zhao, Dongyuan
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- Enhancement of (stereo)selectivity in dynamic kinetic resolution using a core-shell nanozeolite@enzyme as a bi-functional catalyst
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A core-shell nanozeolite@enzyme bi-functional catalyst is constructed, which greatly improves selectivity and stereoselectivity of products in dynamic kinetic resolution of aromatic secondary alcohols compared with mixed catalysts, especially those involving small acyl donors. This journal is the Partner Organisations 2014.
- Wang, Wanlu,Li, Xiang,Wang, Zhoujun,Tang, Yi,Zhang, Yahong
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supporting information
p. 9501 - 9504
(2014/08/18)
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- Core-Shell Composite as the Racemization Catalyst in the Dynamic Kinetic Resolution of Secondary Alcohols
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Beta-Silicalite-1 core-shell microcomposites with controllable shell thickness were synthesized and used as racemization catalysts in the one-pot dynamic kinetic resolution (DKR) of secondary alcohols by using lipase-catalyzed transesterification. The inert Silicalite-1 shell covered the external acidic sites of the Beta zeolite core, suppressing dehydration and non-enantioselective transesterification of the alcohol. The alcohols could penetrate the Silicalite-1 shell to access the acidic sites at the core Beta for racemization, however, the enzymatically formed (R)-esters were excluded owing to their larger size. As a result, the high ee of the (R)-ester products was conserved and dehydration side products were minimized. Owing to the shape selective nature of the composite racemization catalyst, small and readily available acyl donors could be used in the enzyme-catalyzed transesterification to obtain the esters with high enantiopurity. The DKR of 1-phenylethanol with isopropenyl acetate using an optimized core-shell catalyst, CS-60, gave 92% selectivity to ester formation and the desired (R)-1-phenylethyl acetate was formed with 94% ee.
- Wang, Jie,Do, Dong-Minh,Chuah, Gaik-Khuan,Jaenicke, Stephan
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p. 247 - 254
(2013/03/13)
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- Ceria nanoparticles as an efficient catalyst for oxidation of benzylic CH bonds
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Catalytic oxidation of benzylic CH bonds with potassium bromate to carbonyl compounds was studied in the presence of ceria nanoparticles (NPs). Aldehydes and ketones in high yields were obtained when the oxidation was conducted in water/1,4-dioxane/acetic acid (AcOH) by ratio 5/1/1 (v/v/v). Benzyl esters were also yielded as the main products from the oxidation of benzylic CH bonds with potassium bromate in the presence of ceria NPs in glacial acetic acid. In comparison with other methods reported in the literature, ceria NPs as an efficient catalyst in oxidation of benzylic CH bonds have advantageous such as selectivity, recyclability, high reaction rate, and high yield of product because of their large specific surface area to volume ratio.
- Akhlaghinia, Batool,Ebrahimabadi, Hossein,Goharshadi, Elaheh K.,Samiee, Sara,Rezazadeh, Soodabeh
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experimental part
p. 67 - 72
(2012/05/20)
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- Investigation of steapsin lipase for kinetic resolution of secondary alcohols and synthesis of valuable acetates in non-aqueous reaction medium
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In present study, the application of steapsin lipase (as a biocatalyst) was investigated for kinetic resolution of secondary alcohols (1-phenyl ethanol and their derivatives) using vinyl acetate as an activated acyl donor. The enzymatic protocol was optimized for various reaction parameters such as effect of the molar ratio, solvent, temperature, time and biocatalyst loading to obtain best reaction conditions. On optimization, developed enzymatic methodology provided considerable enantiomeric excess of the product (up to 92% ee) at 55 °C in n-hexane as a solvent. Furthermore using the developed protocol, synthesis of several industrially important acetates was successfully achieved with excellent yield (up to 99%). During acetate synthesis, the biocatalyst was remarkably reused for eight consecutive recycles without any significant loss in its catalytic activity. This revealed the good potential of steapsin lipase for application in organic solvents.
- Dhake, Kishor P.,Deshmukh, Krishna M.,Wagh, Yogesh S.,Singhal, Rekha S.,Bhanage, Bhalchandra M.
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experimental part
p. 15 - 23
(2012/06/29)
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- Highly efficient dynamic kinetic resolution of secondary aromatic alcohols with low-cost and easily available acid resins as racemization catalysts
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A new and efficient dynamic kinetic resolution (DKR) process of secondary aromatic alcohols was developed with acid resins as racemization catalysts. Acid resin CD8604 was shown to have excellent racemization activity and good biocompatibility. When employing CD8604 and complex acyl donors as racemization catalyst and acyl donor, respectively, enantiomerically pure aromatic acetate was obtained with excellent yield and ee values through the DKR process. It is noteworthy that the system could be reused more than 10 times with little loss of yield and ee value.
- Cheng, Yongmei,Xu, Gang,Wu, Jianping,Zhang, Chensheng,Yang, Lirong
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experimental part
p. 2366 - 2369
(2010/06/13)
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- Enantioselective transesterification catalysis by Candida antarctica lipase immobilized on superparamagnetic nanoparticles
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Lipase B from Candida antarctica can be directly immobilized onto functionalized superparamagnetic nanoparticles, preserving its enzymatic activity in the enantioselective transesterification of secondary alcohols, with excellent results in terms of enantiomeric discrimination. The immobilized enzyme can be easily recovered with a magnet, allowing its reuse with negligible loss of activity.
- Netto, Caterina G.C.M.,Andrade, Leandro H.,Toma, Henrique E.
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experimental part
p. 2299 - 2304
(2010/03/04)
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- Synthesis of chiral aromatic alcohols: Use of new C2-symmetric RhIIICp*, RuII(cymene), or RuII(benzene) complexes containing chiral diaminocyclohexane ligand as asymmetric transfer hydrogenation catalyst
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Twelve chiral secondary alcohols were synthesized by asymmetric transfer hydrogenation (ATH) using C2-symmetric bis(sulfonamide) ligand (2) derived from (1R,2R)-cyclohexane-1,2-diamine and complexed with [RhCl 2CP*]2, [RuCl2(cymene)] 2, or [RuCl2(benzene)]2 and then used in situ in the reduction of prochiral ketones. The alcohols were obtained in 85-99% yield and 90-99% enantioselectivity with isopropanol as the hydrogen source. Two-fold rate enhancement and better yields were achieved (88-99%) with 80-99% enantioselectivity using the complex [RhCl2CP*] 2 and aqueous sodium formate as the hydrogen source.
- Montalvo-Gonzalez, Ruben,Chavez, Daniel,Aguirre, Gerardo,Parra-Hake, Miguel,Somanathan, Ratnasamy
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experimental part
p. 2737 - 2746
(2009/12/06)
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- Lipase-catalyzed Irreversible Transesterification of Secondary Alcohols Using Isopropenyl Acetate
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Asymmetric acetylation of a set of secondary alcohols with the innocuous acyl donor isopropenyl acetate catalyzed by a lipase from Pseudomonas cepacia immobilized on ceramic particles (PSL-C) in toluene as organic medium afforded the chiral alcohols and the corresponding acetates in high enantiomeric excess (up to 99%). An effective baseline separation of the enantiomers of both substrate and product was performed in one analysis without derivatization using gas chromatography on a new chiral stationary phase (CSP) Chirasil-β-Dex containing an undecamethylene spacer (C11-Chirasil-Dex).
- Ghanem, Ashraf,Schurig, Volker
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p. 1151 - 1157
(2007/10/03)
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- Entrapment of Pseudomonas cepacia lipase with peracetylated β-cyclodextrin in sol-gel: Application to the kinetic resolution of secondary alcohols
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Co-lyophilized Pseudomonas cepacia lipase with peracetylated β-cyclodextrin was immobilized by the sol-gel process. The gel-entrapped lipase/cyclodextrin was prepared by the hydrolysis of methyltrimethoxysilane (MTMS) in the presence of the co-lyophilized lipase with peracetylated β-cyclodextrin prepared with different weight ratios (enzyme to CD). This type of enzyme preparation was subsequently used in the kinetic resolution of a set of secondary alcohols using isopropenyl acetate as an innocuous acyl donor in toluene as the organic medium. The resulting chiral alcohols (substrate) and the corresponding acetates (product) were baseline separated in one analysis without derivatization using gas chromatography on a new chiral stationary phase (CSP) Chirasil-β-Dex containing an undecamethylene spacer (C11-Chirasil-Dex).
- Ghanem, Ashraf,Schurig, Volker
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p. 2547 - 2555
(2007/10/03)
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- Resolution of racemic 1-arylethyl acetates by Pseudomonas fluorescens in the presence of a surfactant
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The Pseudomonas fluorescens RRLJ 134 strain efficiently resolved (R/S)-1-arylethyl acetates into S-(-)-1-arylethyl acetates and R-(+)-1-arylethanol in the presence of the surfactant Tween-8. The surfactant reverses the enantioselectivity compared to the surfactant free medium.
- Bora, U.,Saikia, C. J.,Chetia, A.,Mishra, A. K.,Kumar, B. S. D.,Boruah, R. C.
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p. 9099 - 9102
(2007/10/03)
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- Synthesis of chiral hydroxyl phospholanes from D-mannitol and their use in asymmetric catalytic reactions
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Chiral hydroxyl monophosphane 3 [(2S,3S,4S,5S)-3,4-dihydroxy-2,5-dimethyl-1-phenylphospholane] and bisphospholanes 5a [1,2-bis[(2S,3S,4S,5S)-3,4-dihydroxy-2,5-dimethylphospholanyl]benzene] and 5b [1,2-bis[(2S,3S,4S,5S)-2,5-diethyl-3,4-dihydroxyphospholanyl]benzene] were synthesized from readily available D-mannitol in high yields. Strategies for protection and deprotection of OH-groups in the presence of phosphines have been explored. Rate acceleration in the Baylis - Hillman reaction was observed when a hydroxyl phosphine was used as the catalyst. Rhodium complexes with chiral bisphospholanes are highly enantioselective catalysts for the asymmetric hydrogenation of various kinds of functionalized olefins such as dehydroamino acid derivatives, itaconic acid derivatives, and enamides. An interesting feature of the hydroxyl phospholane system is that hydrogenation of some substrates can be carried out in water with >99% ee and 100% conversion (e.g., itaconic acid).
- Li, Wenge,Zhang, Zhaoguo,Xiao, Dengming,Zhang, Xumu
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p. 3489 - 3496
(2007/10/03)
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- The chemistry of acylals. Part I. The reactivity of acylals towards Grignard and organolithium reagents
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Aldehyde acylals have been prepared and reacted with Grignard and alkyllithium reagents. Acylals from formaldehyde furnished complex reaction mixtures when reacted with both reagents. Acylals of other aldehydes gave reaction mixtures that consisted mainly of an ester, generated by replacing one of the carboxy groups with the organic part of the organometallic reagent, and regenerated aldehyde. The esters were formed in the highest yields. Yields above 90% were experienced when the acylals were reacted with Grignard reagents under Barbier conditions.
- Sydnes, Leiv K.,Sandberg, Marcel
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p. 12679 - 12690
(2007/10/03)
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- Enantioselective acylation of alcohols catalyzed by lipase QL from Alcaligenes sp.: A predictive active site model for lipase QL to identify the faster reacting enantiomer of an alcohol in this acylation
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Lipase QL-catalyzed acylation of secondary alcohols using isopropenyl acetate as the acylating agent in diisopropyl ether gave preferentially the corresponding acetate with an R configuration. On the basis of the results, a predictive active site model for lipase QL is proposed for identifying which enantiomer of a secondary alcohol reacts faster in this reaction.
- Naemura,Murata,Tanaka,Yano,Hirose,Tobe
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p. 1581 - 1584
(2007/10/03)
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- Enantioselective acylation of primary and secondary alcohols catalyzed by lipase QL from Alcaligenes sp.: A predictive active site model for lipase QL to identify which enantiomer of an alcohol reacts faster in this acylation
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Lipase QL (from Alcaligenes sp.)-catalyzed acylation of alcohols using isopropenyl acetate as the acylating agent in diisopropyl ether converted preferentially primary alcohols with an S configuration and secondary alcohols with an R configuration into the corresponding homochiral acetates. On the basis of observed enantiomer selectivities, a predictive active site model for lipase QL is proposed for identifying which enantiomer of a primary or a secondary alcohol reacts faster in this acylation. Copyright (C) 1996 Published by Elsevier Science Ltd.
- Naemura, Koichiro,Murata, Masaki,Tanaka, Rie,Yano, Masashi,Hirose, Keiji,Tobe, Yoshito
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p. 3285 - 3294
(2007/10/03)
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- ESTERIFICACION DE DERIVADOS DE ESTIRENO CON ACIDO ACETICO EN PRESENCIA DE ACIDO PERCLORICO COMO CATALIZADOR
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The present work is focused to develop the production of esters from derivatives of styrene by acetic acid using perchloric acid as a catalyst.The kinetics of the reaction was studied and analysis of the variables was carried out.Key words: Esterification, styrene, perchloric, acid, acetic acid.
- Cuesta, Pedro J. Martinez de la,Martinez, Eloisa Rus,Sosa, Rafael Palomino,Perez, Fuensanta I. Palomino
-
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- Spectroscopic and Molecular Mechanics Calculations of Discrimination between Enantiomers Possessing an Electron Rich Aromatic Group Directly Attached to the Chiral Carbon Atom with Optically Pure Benzoyl Derivatives
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The discrimination between enantiomers that have a chiral centre directly attached to the electron rich aromatic ring by interactions with optically pure electron deficient aromatic compounds was studied by 1H NMR and molecular mechanics calculations.The enantiomeric spectroscopic discrimination is due to formation of non-bonding interactions which lead to the formation of diastereoisomeric complexes.The calculated HOMO energy values for electron rich racemic compounds and LUMO values for electron poor resolving agents are in good agreement with the observed chemicalshift differences between enantiomers by 1H NMR.Exceptions to these observations are seen for compounds which possess groups that are in a position to sterically push apart the complexation components and thus decrease the binding energy and diminish the enantiomeric discrimination.The discrimination strongly depends on concentration of the components in the solution, their ratio, temperature and the polarity of the media.Polar racemic compounds show the smallest enantiomeric discrimination by increasing the polarity of the media.For good enantiomeric discrimination both chiral centres must be rigidly bound to the aromatic rings.If the chiral centre is separated from the aromatic ring by flexible bonds the enantiomeric recognition fails despite the formation of strong non-bonding interactions.The binding energies calculated by AMBER and MM + force fields are relatively weak (ca. 2 kcal mol-1) suggesting that only a small portion of the molecules in solution are incorporated into the complexes.Our attempt to provide evidence for selective binding of only one enantiomer to the template molecule was unsuccessful.Neverthelessour results support the formation of inter-exchangeable diastereomeric complexes.Although it might be argued that two sets of signals should also be obtained for the template molecule, all our 1H NMR spectra show only one set of signals for the optically pure template and two sets of signals for the racemic component in their chloroform solutions.Despite low binding energies enantiomeric discrimination with strong electron accepting resolving agents can be achieved in as low as 0.01 mol dm-3 chloroform solution.
- Jursic, Branko S.
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p. 961 - 970
(2007/10/02)
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- Anodic Oxidation of α-Substituted p-Xylenes. Electronic and Stereoelectronic Effects of α-Substituents in the Deprotonation of Alkylaromatic Radical Cations
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The effect of α-substituents on the deprotonation rate from the benzylic position of alkylaromatic radical cations, k(CH2Z)/k(CH3), has been investigated by determining the intramolecular selectivity in the anodic oxidation in acetic acid of a α-Z-substituted p-xylenes 1 (Z = H, OMe, OH, Me, tert-butyl, OAc, COOMe, CN), 5,6-dimethylindan 4 (R = H), and 2,2,5,6-tetramethylindan 4 (R = Me).Some oxidations induced by CAN have also been carried out.It has been found that, with the exception of when Z = tert-butyl, the deprotonation rate of 1.+ is always faster from the CH2Z group than from the CH3 group, independently of the electron-donating or electron-withdrawing nature of Z.The electron-donating groups (OH, OMe, Me), however, exert a larger effect than the electron-withdrawing ones (COOMe, CN).The negligible deprotonation rate from CH2-t-Bu has been ascribed to stereoelectronic effects (the bulky tert-butyl group does not allow the C-H bonds to be collinear with the ?-system), the suggestion being nicely confirmed by the observation that the deprotonation rate from the position 1(3), relative to that from the 5(6)-methyl group, is almost identical in the radical cations of 4 (R = H and Me).The effect of the other α-substituents is mainly of electronic nature and has been rationalized on the basis of a variable transition-state structure for the deprotonation process.It is suggested that with +R groups most of the charge, in the transition state, has been transferred to the Cα-H bond where it can be stabilized by the α-substituent.With electron-withdrawing groups less charge transfer has taken place and the rate-enhancing effects of these groups is ascribed to their capability to significantly decrease the strength of the Cα-H bond.
- Baciocchi, Enrico,Mattioli, Mario,Romano, Roberta,Ruzziconi, Renzo
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p. 7154 - 7160
(2007/10/02)
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- A Highly Selective Ester Hydrolase from Pseudomonas Sp. for the Enzymatic Preparation of Enantiomerically Pure Secondary Alcohols; Chiral Auxiliaries in Organic Synthesis
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A series of valuable chiral auxiliaries, (R)- and (S)-(1)-(11), have been prepared in high chemical and optical yields by enzymatic hydrolysis of their esters in the presence of a lipase from Pseudomonas sp.
- Laumen, Kurt,Schneider, Manfred P.
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p. 598 - 600
(2007/10/02)
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- ASYMMETRIC ACYLATION OF ALCOHOLS IN THE PRESENCE OF CHIRAL TERTIARY AMINES
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The asymmetric acylation of secondary alcohols with acetic anhydride in the presence of chiral tertiary amines, synthesized from S-(-)-α-phenylethylamine, was investigated.The optical yield of the obtained alcohols was not greater than 2.4percent.It was established that in the presence of S-(-)-N,N-dimethyl-α-phenylethylamine the unreacted alcohol is rich in the S isomer.On the basis of this relationship the S configuration was proposed for (-)-4-iodo-, (-)-4-chloro-, and (-)-3-nitrophenylehanols.
- Potapov, V. M.,Dem'yanovich, V. M.,Khlebnikov, V. A.,Koval'skaya, S. S.
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p. 1602 - 1606
(2007/10/02)
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- Concerted Bimolecular Substitution Reactions of 1-Phenylethyl Derivatives
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Substituted 1-phenylethyl derivatives with ?+ > -0.08 exhibit bimolecular substitution reactions with azide ion in 20percent acetonitrile in water.The reactions with 1-phenylethyl chlorides follow a Hammett correlation with ρ = -2.9, compared with ρ = -5.6 (r+ = 1.15) for solvolysis.Swain-Scott correlations give values of s = 0.46 and 0.22 for 1-(4-nitrophenyl)ethyl chloride and tosylate, respectively; there are large positive deviations for azide ion and water and negative deviations for cyanide ion.The value of βnuc is 0.09 for reactions of substituted acetates with the chloride.The reactions exhibit ''synergism'' between the nucleophile and leaving group that favors the bimolecular reaction with Me2S, Br- > Cl- > OTs- leaving groups.The bimolecular reaction with azide follows the Grunwald-Winstein Y correlation with m = 0.8 in methanol-water mixtures.Bimolecular reactions with less reactive nucleophiles in the series N3-, CN-, AcO-, and ROH appear at progressively larger ? values, as the carbocation becomes less stable.It is concluded that these reactions are SN2 displacements that proceed through an open, ''exploded'' transition state that closely resembles a carbocation.Specific salt effects are small in water but are significant in acetonitrile-water mixtures and could be mistaken for normal or induced common ion rate depressions.No evidence was obtained for nucleophilic assistance to the formation of a carbocation intermediate.Concurrent SN1 and SN2 pathways occur in the reactions with solvent and azide of dimethylsulfonium ion, 1-(4-fluorophenyl)ethyl chloride, 1-(3-methoxyphenyl)ethyl chloride, and, probably, 1-(3-nitro-4-methoxyphenyl)ethyl chloride.Crude estimates of the lifetime of the carbocation intermediate in the presence of the nucleophile are consistent with the hypothesis that the concerted reactions are enforced by the absence of a significant lifetime of the carbocation in the presence of the nucleophile and that stepwise mechanisms are followed when the intermediate has a significant lifetime; the change from a stepwise to a concerted mechanism occurs when the intermediate ceases to have a lifetime in the presence of a nucleophile.
- Richard, John P.,Jencks, William P.
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p. 1383 - 1396
(2007/10/02)
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- Reactions of Substituted 1-Phenylethyl Carbocations with Alcohols and Other Nucleophilic Reagents
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Selectivities of a series of substituted 1-phenylethyl carbocations toward alcohols and other nucleophiles have been determined by product analysis.The 1-(4-dimethylamino)phenyl)ethyl carbocation exhibits a high selectivity in its reactions with alcohols , with KEtOH/KTFE = 140 and βnuc = 0.5.The selectivity for activation-limited reactions with alcohols decreases progressively with increasing reactivity of the carbocation, in contrast to the behavior expected from the N+ scale of reactivity.A sharper drop in selectivity for carbocations that react faster than ca. 109 S-1 is attributed to an approach to limiting rate constants for the more reactive alcohol.The limiting selectivity of kEtOH/kTFE = 2 for carbocations with ks ca. 1011 S-1 may represent reaction from a pool of solvent molecules in which there is a modest charge-dipole interaction between the alcohol and carbocation.The relatively low reactivity of water corresponds to that expected for an alcohol of pKa ca. 13.This is ascribed to an imbalance between charge development and solvation of the transition state compared with H3O+.Substituted acetate anions react with the 1-(4-methoxyphenyl)ethyl carbocation with βnuc = 0.13.The selectivity decreases with increasing cation reactivity as the carboxylate ions approach limiting rate constants of ca. 5 * 108 M-1 s-1.This relatively low limit is attributed to a requirement for desolvation of basic oxygen anions before reaction.A dependence of solvent selectivity on the leaving group shows that the 1-(4-methylphenyl)ethyl carbocation reacts with solvent, in part, through an ion pair.Azide ion reacts from a pool that can be described by an equilibrium constant of Kas = 0.3 M-1.Styrene formation from this carbocation is catalyzed by a leaving carboxylate ion and by added buffers, wih β = 0.14.The equilibrium constant for the formation of a reactive base-cation pair is ca. 0.04 M-1.Rate constants for collapse of the ion pair, to form ester, and for proton removal, to form 4-methylstyrene, were estimated to be approximately 1.6 * 1010 s-1 and 6 * 107 s-1, respectively.The rate constants for deprotonation and for hydration of the styrene give the acid dissociation constant of the carbocation to form 4-methylstyrene, pKA = -11.2.
- Richard, John P.,Jencks, William P.
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p. 1373 - 1383
(2007/10/02)
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- ipso Nitration. XXIII. Reactions of cyclohexadiene adducts from nitration of 4-ethyltoluene in acetic anhydride
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The diastereoisomers of 4-ethyl-1-methyl-4-nitrocyclohexa-2,5-dienyl acetate (1) and 1-ethyl-4-methyl-4-nitrocyclohexa-2,5-dienyl acetate (2) are stereospecifically reduced to the corresponding nitrocyclohexadienols with aluminum hydride.Each dienol is stereospecifically methylated to the corresponding methyl ether with methyl iodide, silver oxide, and potassium hydroxide.Acid-catalysed solvolysis of the acetates 1 and 2 results in the substitution of the acetate moiety by other nucleophiles and these reactions are not stereospecific.The products of rearomatization of dienyl acetates, dienols and dienyl methyl ethers depend on the acidity and ionizing power of the solvents and are readily explained in terms of reactions involving a nitrocyclohexadienyl cation or acetoxy- (hydroxy-, methoxy-)cyclohexadienyl cation as key intermediates.In the 4-acetoxy-4-alkylcyclohexadienyl cation 1,2-migration of the acetoxyl group is more rapid then alkyl migration, but 1,2-alkyl migration is faster then migration of the hydroxyl or methoxyl groups in the corresponding cations. 1-Ethyl-4-methoxy-4-methylcyclohexa-2,5-dien-1-ol and 4-ethyl-3-nitrotoluene are significant minor products in the solvolysis of 1-ethyl-4-methyl-4-nitrocyclohexa-2,5-dien-1-ol in aqueous methanol.Nitration of p-ethyltoluene in the presence of sulfuric acid or in trifluoracetic anhydride gives a mixture of 4-ethyl-2-nitro- and 4-ethyl-3-nitrotoluene in a 2:1 ratio.
- Fischer, Alfred,Henderson, George N.
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p. 2314 - 2327
(2007/10/02)
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- INFLUENCE OF NITROGEN LIGANDS ON SYNERGIC EFFECT OF METAL CATALYSTS IN OXYDATION OF ALKYLAROMATIC COMPOUNDS
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Synergic effect of metal mixtures Co-Mn, Co-Ce and Co-U in oxidation of 1,2,4- and 1,3,5-trimethylbenzenes dissolved in acetic acid has been studied in dependence of the presence of nitrogen ligands 2,2',2''-nitrilotrisethanol, 1,6-hexanediamine and pyridine.The complex CoBr2Py2 and its equimolar mixture with the complex MnBr2Py2 are highly active in oxidation of alkylaromatic derivatives containing other alkyl than methyl.Their activity has been tested experimentally in oxidations of ethylbenzene, 1,3- and 1,2-dimethylbenzenes, 1-ethyl-4-methylbenzene, (1-methylethyl)benzene, propylbenzene, 1-methyl-4-(1-methylethyl)benzene, dodecylbenzene, 1,3-diethylbenzene and 4-octyl-1,1'-biphenyl
- Hronec, Milan,Holotik, Stefan,Ilavsky, Jan
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p. 880 - 887
(2007/10/02)
-