- Reexamination of diisobutylaluminum hydride as a stereoselective reducing agent for reduction of cyclic ketones to thermodynamically more stable alcohols
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The reducing property of diisobutylaluminum hydride (DIBAH) has been reexamined as a stereoselective reducing agent for reduction of representative cyclic ketones. When the reduction of excess cyclic ketone with DIBAH was carried out at 0°C in ethyl ether, only 1 equiv of the free hydride was involved to show a low stereoselectivity. However, when performed at 25°C or under reflux in ethyl ether, one isobutyl group as well as the free hydride was also involved in this reduction: the first equiv of ketone was reduced rapidly and the second one reduced, in a relatively slow rate. In addition, the stereoselectivity increases consistently with increase of reaction time to afford the thermodynamically more stable isomer alcohols exclusively.
- Cha, Jin Soon,Kwon, Oh Oun,Kim, Jong Mi,Cho, Sung Dong
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- RHODIUM(I)-CATALYZED ASYMMETRIC HYDROBORATION OF ALKENES WITH 1,3,2-BENZODIOXABOROLE
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Several rhodium(I) complexes containing chiral phosphine, (+)DIOP, (+)BINAP, (S,S) CHIRAPHOS, and (S)(R)BPPFA, have been found to be effective as catalyst for the asymmetric hydroboration of prochiral alkenes with catecholborane (1,3,2-benzodioxaborole) to give optically active 2-alkyl-1,3,2-benzodioxaboroles. among the ligands examined, DIOP has been recognized to be most effective to give high asymmetric induction.
- Sato, Makoto,Miyaura, Norio,Suzuki, Akira
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- Use of achiral (diphosphine)RuCl2(diamine) precatalysts as a practical alternative to sodium borohydride for ketone reduction
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Stoichiometric sodium borohydride is frequently used in the chemoselective reduction of ketones to racemic secondary alcohols. Catalytic homogeneous hydrogenation using (diphosphine)RuCl2(diamine) complexes provides a practical and economic alternative. A range of substrates were investigated and the optimum precatalyst identified in each case. Norcamphor was reduced with high diastereoselectivity using (Ph3P)2-RuCl 2(en); (E)-4-phenylbut-3-en-2-one was reduced with good chemoselectivity, and acetophenone was hydrogenated very efficiently using the same precatalyst. Isophorone and 3-dimethylaminopropiophenone were effectively hydrogenated using (dppf)RuCl2(en).
- De Koning, Pieter D.,Jackson, Mark,Lennon, Ian C.
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- A short asymmetric synthesis of methyl 2-((1S,3R)-3-((tert-butyldiphenylsilyl)oxy)cyclopentyl)acetate from norbornene
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Methyl 2-((1S,3R)-3-((tert-butyldiphenylsilyl)oxy)cyclopentyl)acetate has been synthesized from norbornene using Hayashi's (S)-MOP Pd-catalyzed asymmetric hydrosilation. On a 1 mol scale, asymmetric hydrosilation of norbornene afforded an 84:16 exo- to endo-norborneol mixture but with exclusive 1R,4S-stereochemistry at the bridgehead carbons. The norborneol mixture was converted to an optically pure chiral bicyclic lactone via a high-yielding tandem oxidation/Baeyer-Villiger reaction. Acid-promoted ring-opening of the lactone followed by immediate silyl protection afforded the chiral cis-1,3-cyclopentane intermediate in five steps with an overall yield of 41%.
- Huang, Buwen,Elleraas, Jeff,Ewanicki, Jason,Sutton, Scott C.
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- Asymmetric Functionalization of Bicycloalkenes by Catalytic Enantioposition-Selective Hydrosilylation
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Hydrosilylation of norbornene with trichlorosilane in the presence of palladium catalyst (0.01-0.1 mol percent) coordinated with (R)-MOP ligand gave a quantitative yield of exo-2-trichlorosilylnorbornane, which was oxidized with hydrogen peroxide to give (1S,2S,4R)-exo-2-norbornanol in 96percent ee.The similar hydrosilylation and oxidation of endo-5,6-dicarbomethoxy-2-norbornene, bicyclooctene, and norbornadiene gave the corresponding bicyclic alcohols of 94percent ee, 92percent ee, and 95percent ee, respectively.
- Uozumi, Yasuhiro,Lee, Sang-Yong,Hayashi, Tamio
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- Chemoselective Cleavage of Si-C(sp3) Bonds in Unactivated Tetraalkylsilanes Using Iodine Tris(trifluoroacetate)
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Organosilanes are synthetically useful reagents and precursors in organic chemistry. However, the typical inertness of unactivated Si-C(sp3) bonds under conventional reaction conditions has hampered the application of simple tetraalkylsilanes in organic synthesis. Herein we report the chemoselective cleavage of Si-C(sp3) bonds of unactivated tetraalkylsilanes using iodine tris(trifluoroacetate). The reaction proceeds smoothly under mild conditions (-50 °C to room temperature) and tolerates various polar functional groups, thus enabling subsequent Tamao-Fleming oxidation to provide the corresponding alcohols. NMR experiments and density functional theory calculations on the reaction indicate that the transfer of alkyl groups from Si to the I(III) center and the formation of the Si-O bond proceed concertedly to afford an alkyl-λ3-iodane and silyl trifluoroacetate. The developed method enables the use of unactivated tetraalkylsilanes as highly stable synthetic precursors.
- Matsuoka, Keitaro,Komami, Narumi,Kojima, Masahiro,Mita, Tsuyoshi,Suzuki, Kimichi,Maeda, Satoshi,Yoshino, Tatsuhiko,Matsunaga, Shigeki
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supporting information
p. 103 - 108
(2021/01/13)
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- Generalized Chemoselective Transfer Hydrogenation/Hydrodeuteration
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A generalized, simple and efficient transfer hydrogenation of unsaturated bonds has been developed using HBPin and various proton reagents as hydrogen sources. The substrates, including alkenes, alkynes, aromatic heterocycles, aldehydes, ketones, imines, azo, nitro, epoxy and nitrile compounds, are all applied to this catalytic system. Various groups, which cannot survive under the Pd/C/H2 combination, are tolerated. The activity of the reactants was studied and the trends are as follows: styrene'diphenylmethanimine'benzaldehyde'azobenzene'nitrobenzene'quinoline'acetophenone'benzonitrile. Substrates bearing two or more different unsaturated bonds were also investigated and transfer hydrogenation occurred with excellent chemoselectivity. Nano-palladium catalyst in situ generated from Pd(OAc)2 and HBPin extremely improved the TH efficiency. Furthermore, chemoselective anti-Markovnikov hydrodeuteration of terminal aromatic olefins was achieved using D2O and HBPin via in situ HD generation and discrimination. (Figure presented.).
- Wang, Yong,Cao, Xinyi,Zhao, Leyao,Pi, Chao,Ji, Jingfei,Cui, Xiuling,Wu, Yangjie
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supporting information
p. 4119 - 4129
(2020/08/10)
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- Erbium-Catalyzed Regioselective Isomerization-Cobalt-Catalyzed Transfer Hydrogenation Sequence for the Synthesis of Anti-Markovnikov Alcohols from Epoxides under Mild Conditions
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Herein, we report an efficient isomerization-transfer hydrogenation reaction sequence based on a cobalt pincer catalyst (1 mol %), which allows the synthesis of a series of anti-Markovnikov alcohols from terminal and internal epoxides under mild reaction conditions (≤55 °C, 8 h) at low catalyst loading. The reaction proceeds by Lewis acid (3 mol % Er(OTf)3)-catalyzed epoxide isomerization and subsequent cobalt-catalyzed transfer hydrogenation using ammonia borane as the hydrogen source. The general applicability of this methodology is highlighted by the synthesis of 43 alcohols from epoxides. A variety of terminal (23 examples) and 1,2-disubstituted internal epoxides (14 examples) bearing different functional groups are converted to the desired anti-Markovnikov alcohols in excellent selectivity and yields of up to 98%. For selected examples, it is shown that the reaction can be performed on a preparative scale up to 50 mmol. Notably, the isomerization step proceeds via the most stable carbocation. Thus, the regiochemistry is controlled by stereoelectronic effects. As a result, in some cases, rearrangement of the carbon framework is observed when tri-and tetra-substituted epoxides (6 examples) are converted. A variety of functional groups are tolerated under the reaction conditions even though aldehydes and ketones are also reduced to the respective alcohols under the reaction conditions. Mechanistic studies and control experiments were used to investigate the role of the Lewis acid in the reaction. Besides acting as the catalyst for the epoxide isomerization, the Lewis acid was found to facilitate the dehydrogenation of the hydrogen donor, which enhances the rate of the transfer hydrogenation step. These experiments additionally indicate the direct transfer of hydrogen from the amine borane in the reduction step.
- Liu, Xin,Longwitz, Lars,Spiegelberg, Brian,T?njes, Jan,Beweries, Torsten,Werner, Thomas
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p. 13659 - 13667
(2020/11/30)
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- Potential Synthetic Adaptogens: V. Synthesis of Cage Monoamines by the Schwenk–Papa Reaction
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The reduction of cage ketoximes under Schwenk–Papa reaction conditions was studied to establish that the d,l, d- and l-camphor oximes are smoothly reduced to the corresponding amines in high yields. At the same time, d,l-norcamphor and adamantan-2-one oximes undergo partial catalytic deoximation to form a mixture of the corresponding amines and alcohols.
- Brunilin, R. V.,Mkrtchyan, A. S.,Nawrozkij, M. B.,Novakov, I. A.,Vernigora, A. A.,Voloboev, S. N.,Vostrikova, O. V.
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p. 1742 - 1748
(2020/01/11)
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- Scalable and safe synthetic organic electroreduction inspired by Li-ion battery chemistry
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Reductive electrosynthesis has faced long-standing challenges in applications to complex organic substrates at scale. Here, we show how decades of research in lithium-ion battery materials, electrolytes, and additives can serve as an inspiration for achieving practically scalable reductive electrosynthetic conditions for the Birch reduction. Specifically, we demonstrate that using a sacrificial anode material (magnesium or aluminum), combined with a cheap, nontoxic, and water-soluble proton source (dimethylurea), and an overcharge protectant inspired by battery technology [tris(pyrrolidino)phosphoramide] can allow for multigram-scale synthesis of pharmaceutically relevant building blocks. We show how these conditions have a very high level of functional-group tolerance relative to classical electrochemical and chemical dissolving-metal reductions. Finally, we demonstrate that the same electrochemical conditions can be applied to other dissolving metal-type reductive transformations, including McMurry couplings, reductive ketone deoxygenations, and epoxide openings.
- Peters, Byron K.,Rodriguez, Kevin X.,Reisberg, Solomon H.,Beil, Sebastian B.,Kawamata, Yu,Baran, Phil S.,Hickey, David P.,Klunder, Kevin,Gorey, Timothy J.,Anderson, Scott L.,Minteer, Shelley D.,Collins, Michael,Starr, Jeremy,Chen, Longrui,Udyavara, Sagar,Neurock, Matthew
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p. 838 - 845
(2019/04/30)
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- OPEN-FLASK HYDROBORATION AND THE USE THEREOF
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The present disclosure generally relates to a process for hydroboration of an alkene or alkyne using ammonia borane (AB). In particular, the present invention relates to hydroboration of an alkene or alkyne in the presence of air or moisture, and a clean process for facile preparation of an alcohol by oxidizing the organoborane so formed with hydrogen peroxide. The products, including aminodialkylboranes, ammonia trialkylborane complexes, as well as various alcohols so prepared, are within the scope of this disclosure.
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Paragraph 0046; 0049; 0059; 0060; 0061
(2018/03/25)
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- Microwave-assisted isomerizations of epoxides to allylic alcohols
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The present work reports a study on the isomerization reactions of several alkyl epoxides to the corresponding allylic alcohols or bicyclic alcohols under microwave irradiation. The reaction occurred in the presence of lithium diisopropylamide as a base and different experimental conditions in terms of solvent, amount of the base, times and temperatures. The traditional heating with an oil-bath and the use of alternative organometallic bases, as the Lochmann-Schlosser bases, have been furthermore compared with the microwave heating. The results obtained show that the use of microwave irradiations on promoting the isomerization of epoxides gives access to a series of synthetically useful products, among which allylic alcohols and bicyclic alcohols, depending on the starting substrate.
- Consiglio, Gabriella Barozzino,Mordini, Alessandro
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p. 447 - 454
(2018/05/22)
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- Lipophilic oligopeptides for chemo- and enantioselective acyl transfer reactions onto alcohols
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Inspired by the extraordinary selectivities of acylases, we envisioned the use of lipophilic oligopeptidic organocatalysts for the acylative kinetic resolution/desymmetrization of rac- and meso-cycloalkane-1,2-diols. Here we describe in a full account the discovery and development process from the theoretical concept to the final catalyst, including scope and limitations. Competition experiments with various alcohols and electrophiles show the full potential of the employed oligopeptides. Additionally, we utilized NMR and IR-spectroscopic methods as well as computations to shed light on the factors responsible for the selectivity. The catalyst system can be readily modified to a multicatalyst by adding other catalytically active amino acids to the peptide backbone, enabling the stereoselective one-pot synthesis of complex molecules from simple starting materials.
- Mueller, Christian E.,Zell, Daniela,Hrdina, Radim,Wende, Raffael C.,Wanka, Lukas,Schuler, Soeren M. M.,Schreiner, Peter R.
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p. 8465 - 8484
(2013/09/24)
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- Salt effects on the rates and mechanisms of solvolysis reaction of organic halides and water structure distortion in N,N-dimethylformamide-and N,N-dimethylacetamide-water mixed solvents
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In 50% (v/v) DMFand DMAH2O mixed solvents, salt effects on the solvolysis reaction rates of aliphatic halides and related compounds (RX) have been examined for further exploring. Compared with previous results obtained in other organic solvent systems, such as 50% (v/v) acetone, 1,4-dioxane, or sulfolaneH2O, no exceptional behavior was observed in the "pseudo" first-order rate constants (k/s-1) of all the typical SN1 and SN2 substrates by the addition of alkali metal and alkaline earth metal perchlorates or many kinds of tetraalkylammonium salts in 50% (v/v) DMFand DMA-H2O mixed solvents. The detailed examination of Δlog(k/s-1)/Δ[LiClO4] for 1-adamantyl bromide vs. the contents of organic solvents (CH3CN, DMA, and DMSO) suggested that the observed different salt effects were caused by the different solvation abilities of the solvents toward the leaving-group anion as well as the metal cation. As a new highlight in the present paper, we were able to demonstrate a proportionality or correlation between the LiClO4 effects in the solvolysis rates and the carbocation stabilities expressed by the Gibbs free energy values (ΔGo) of RX in the gas phase. Based on the Raman spectra of DMA-H2O and DMF-D2O as well as DMF-D2O mixed solvents, we discuss the distortion of the bulk water structure.
- Hojo, Masashi,Ueda, Tadaharu,Ueno, Eriko,Hamasaki, Tensei,Nakano, Toshiyuki
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supporting information; experimental part
p. 401 - 414
(2010/07/08)
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- Synthesis of enantiomers of exo-2-norbornyl-N-n-butylcarbamate and endo-2-norbornyl-N-n-butylcarbamate for stereoselective inhibition of acetylcholinesterase
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The acetylcholinesterase inhibition by enantiomers of exo- and endo-2-norbornyl-N-n-butylcarbamates shows high stereoselelectivity. For the acetylcholinesterase inhibitions by (R)-(+)- and (S)-(-)-exo-2-norbornyl-N-n- butylcarbamates, the R-enantiomer is more potent than the S-enantiomer. But, for the acetylcholinesterase inhibitions by (R)-(+)- and (S)-(-)-endo-2-norbornyl- N-n-butylcarbamates, the S-enantiomer is more potent than the R-enantiomer. Optically pure (R)-(+)-exo-, (S)-(-)-exo-, (R)-(+)-endo-, and (S)-(-)-endo-2-norbornyl-N-n-butylcarbamates are synthesized from condensations of optically pure (R)-(+)-exo-, (S)-(-)-exo-, (R)-(+)-endo-, and (S)-(-)-endo-2-norborneols with n-butyl isocyanate, respectively. Optically pure norborneols are obtained from kinetic resolutions of their racemic esters by lipase catalysis in organic solvent.
- Chiou, Shyh-Ying,Huang, Chuan-Fu,Yeh, Shyh-Jei,Chen, I-Ru,Lin, Gialih
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experimental part
p. 267 - 274
(2010/12/24)
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- Asymmetric hydrogenation of bicyclic ketones catalyzed by BINAP/IPHAN-Ru(II) complex
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(Equation Presented). Hydrogenation of 3-quinuclidinone and bicyclo[2.2.2]octan-2-one with a combined catalyst system of RuCl 2[(S)-binap][(R)-iphan] and t-C4H9OK in 2-propanol afforded the chiral alcohols in 97-98% ee. 2-Diphenylmethyl-3- quinuclidinone was hydrogenated with the same catalyst to the cis alcohol with perfect diastereo-and enantioselectivity. The reaction of unsymmetrical ketones with a bicyclo[2.2.1] or-[2.2.2] skeleton gave the corresponding alcohols with high stereoselectivity.
- Arai, Noriyoshi,Akashi, Masaya,Sugizaki, Satoshi,Ooka, Hirohito,Inoue, Tsutomu,Ohkuma, Takeshi
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supporting information; experimental part
p. 3380 - 3383
(2010/11/04)
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- Hydrogen-transfer reduction of carbonyl compounds catalysed by nickel nanoparticles
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We report for the first time the hydrogen-transfer reduction of carbonyl compounds catalysed by well-defined nickel(0) nanoparticles. The nickel nanoparticles could be reutilised several times in a very simple reaction medium composed of the nickel nanoparticles, isopropanol and the substrate, without any added base.
- Alonso, Francisco,Riente, Paola,Yus, Miguel
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p. 1939 - 1942
(2008/09/19)
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- Water: The ideal hydrogen-atom source in free-radical chemistry mediated by TiIII and other single-electron-transfer metals?
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(Chemical Equation Presented) Hydrogen-atom transfer from water to free radicals can be mediated by aqua complexes of titanium(III) (see scheme). Asymmetric epoxidation in combination with [Cp2TiCl]/H 2O-mediated reductive epoxide opening can be viewed as an alternative with complementary stereoselectivity to the hydroboration-epoxidation method for the enantioselective synthesis of anti-Markovnikov alcohols from alkenes.
- Cuerva, Juan M.,Campa?a, Araceli G.,Justicia, José,Rosales, Antonio,Oller-López, Juan L.,Robles, Rafael,Cárdenas, Diego J.,Bu?uel, Elena,Oltra, J. Enrique
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p. 5522 - 5526
(2007/10/03)
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- Efficient tetrahydropyranylation of alcohols and detetrahydropyranylation reactions in the presence of catalytic amount of trichloroisocyanuric acid (TCCA) as a safe, cheap industrial chemical
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Preparation and cleavage of THP ethers of different hydroxy functional groups are easily and efficiently performed in the presence of trichloroisocyanuric acid (TCCA) in the absence of solvent with high yields.
- Firouzabadi, Habib,Iranpoor, Nasser,Hazarkhani, Hassan
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p. 3623 - 3630
(2007/10/03)
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- Enantioselective rhodium catalyzed hydroboration of olefins using chiral bis(aminophosphine) ligands
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The synthesis of new chiral bis(aminophosphine) ligands 1-5 was achieved and assessed in the enantioselective rhodium catalyzed hydroboration of various olefins with catecholborane. Enantioselectivities up to 77% were obtained.
- Brunel, Jean-Michel,Buono, Gerard
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p. 3561 - 3564
(2007/10/03)
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- Tertiary phosphine compound and transition metal complex comprising the same as ligand
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A tertiary phosphine compound of the formula (1): STR1 wherein R1 and R2 represent independently from each other a hydrogen atom or a methyl group, or together form --CH=CH--CH=CH--; R3 is a hydrogen atom or a cycloalkyl group having 5 to 7 carbon atoms or a lower alkyl group which may be substituted with a halogen atom, a lower alkoxy group, a lower alkoxyalkoxy group or a phenyl group; X1 is a halogen atom when both R1 and R2 are hydrogen atoms, or a hydrogen atom, a halogen atom, a lower alkyl group or a lower alkoxy group when at least one of R1 and R2 is not a hydrogen atoms; and m is an integer of 1 to 5, which is useful as a ligand of a transition metal complex that can catalyze various reactions.
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- ENANTIOSELECTIVE HYDROBORATIONS CATALYZED BY RHODIUM(+1) COMPLEXES
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Enantioselective hydroborations of alkenes 1 - 4 with catecholborane were performed via catalysis with complexes containing the chiral ligands DIOP, BINAP, CHIRAPHOS, DIPAMP, BDPP, 2-MeODIOP, and 3-MeODIOP.Alkenes 1 - 3 were also hydroborated with catecholborane derivatives 5 - 8 in the presence of Rh(+1)/DIOP catalysts.Trends in the observed optical purities are discussed.
- Burgess, Kevin,Donk, Wilfred A. van der,Ohlmeyer, Michael J.
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p. 613 - 621
(2007/10/02)
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- ENZYMATIC RECOGNITION OF DIASTEREOMERIC ESTERS
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Aiming to improve the enantioselectivity of enzymatic resolution of esters, lipase catalyzed hydrolysis of (D)- and (L)-2-chloropropanoates of four racemic alcohols and transesterification of ethyl (DL)-2-chloropropanoate with optically pure alcohols was investigated.Thus, (rac)-endo-2-norbornyl (L)-2-chloropropanoate was hydrolized by lipase P about 5 times more selectively than its corresponding (D)-counterpart and optically pure (1R,2S,5R)-menthol was obtained by transesterification of its racemate with ethyl (D)-2-chloropropanoate using Candida cylindracea (CC)lipase.From the results obtained it seems obvious that lipases CC and P mainly can recognize the chirality of an alcohol moiety rather than that of an acid
- Rabiller, C. G.,Koenigsberger, .,Faber, K.,Griengl, H.
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p. 4231 - 4240
(2007/10/02)
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- Enantioselective Hydroboration Mediated by Homochiral Rhodium Catalysts
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Homochiral rhodium-phosphine complexes facilitate enantioselective hydroboration of alkenes by catecholborane; oxidation of the products affords optically active alcohols.
- Burgess, Kevin,Ohlmeyer, Michael J.
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p. 5178 - 5179
(2007/10/02)
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- Hydroboration. 82. Asymmetric Hydroboration of Representative Cis Disubstituted and Heterocyclic Olefins with Dicaranylboranes of High Enantiomeric Purity
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Dicaranylboranes (2-dIcr2BH and 4-dIcr2BH) are prepared by the reaction of borane-methyl sulfide with (+)-2- and (+)-3-carene, respectively.The reagents readily hydroborate prochiral cis disubstituted olefins to yield the corresponding trialkylboranes, which, upon oxidation, give chiral alcohols of 77-93percent ee (2-dIcr2BH) and 36-75percent ee (4-dIcr2BH). 2-dIcr2BH and 4-dIcr2BH convert cis-alkenes into alcohols of the opposite absolute configurations.The trialkylborane hydroboration products, upon treatment with benzaldehyde, eliminate (+)-2- or (+)-3-carene, providing the corresponding benzyl boronates.Thus, the chiral auxiliary is recovered, and the alcohols can be obtained by oxidation of the benzyl boronates or alternatively by their hydrolysis and oxidation of the boronic acids.Trans disubstituted and trisubstituted olefins are hydroborated by both reagents with partial elimination of (+)-2- or (+)-3-carene and low asymmetric induction.On the other hand, the hydroboration-oxidation of five-membered ring heterocyclic olefins proceeds rapidly to give the corresponding chiral alcohols of 11-85percent ee.Convenient procedures for the preparation of high purity (+)-2- and (+)-3-carene are described.
- Brown, Herbert C.,Prasad, J.V.N. Vara,Zaidlewicz, Marek
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p. 2911 - 2916
(2007/10/02)
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- UNSOLVATED MAGNESIUM DIISOPROPYLAMIDE (MDA) IN ORGANIC SYNTHESIS. THE REDUCTION OF ALDEHYDES AND KETONES TO ALCOHOLS.
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A solution of unsolvated magnesium diisopropylamide in cyclohexane has been found to reduce aldehydes and ketones to the corresponding alcohols in good yield.
- Sanchez, Ramiro,Scott, William
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p. 139 - 142
(2007/10/02)
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- A SIMPLE CONVENIENT METHOD FOR THE GENERATION OF DIBORANE FROM NaBH4 AND I2
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Treatment of NaBH4 with I2 in diglyme yields diborane which can be utilized conveniently for the preparation of a borane-N,N-diethylaniline complex and other borane-Lewis base complexes.
- Narayana, C.,Periasamy, M.
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p. 145 - 148
(2007/10/02)
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- Solvent Effects on Solvolytic Reactivity. A Diagnostic Test for Neighboring-Group Assistance by ? Conjugation in Solvolysis Reactions
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The solvolysis rates of 13 arenesulfonates have been determined in mixtures of ethanol and water (the E-series solvents) and acetic acid and formic acid (the A-series solvents).The substrates studied include the following: neophyl (1), 2-adamantyl (2), neopentyl (3), and cyclooctyl (4) tosylates; cyclopropylcarbinyl pemsylate (5a) and tosylate (5b); exo-2-norbornyl tosylate (6); cyclobutylcarbinyl (7), cyclobutyl (8), and pinacolyl (9) brosylates; cyclopentyl (10), cyclohexyl (11), and 2-propyl (12) tosylates; and cyclopentylcarbinyl brosylate (13).Correlations of the rate data by eq 1 t(Y) = a + b log kt(neophyl-OTs)> showed that the various substrates responded differently to the examined solvent effects on the solvolytic reactivity.For compounds 2-4, linear correlations were obtained.For compounds 5-9, E-line, A-line dispersions were noted which diverged with increasing ionizing power; for compounds 10-12, parallel E-line, A-line dispersions were obtained; and for compound 13, an E-line, A-line dispersion was observed which converged with increasing ionizing power.Stastistical analyses of the rate data correlations confirmed the significance of the E or A classification of data.The results are interpreted in terms of differences in the involvement of solvent in the transition state for those solvolyses assisted by bridging from those assisted by C-C ? bond hyperconjugation.
- Roberts, Donald D.
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p. 2521 - 2526
(2007/10/02)
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- MECHANISM OF THE γ-RADIOLYSIS OF 2-PROPANOL SOLUTIONS OF CYCLOHEXANONES
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The γ-radiolysis of 2-propanol solutions of cyclohexanone gives mainly hydrogen, acetone, pinacol, methane derived from 2-propanol, and cyclohexanol, 2-(2-cyclohexanonyl)-cyclohexanone, and 3-(2-hydroxy-2-propyl)cyclohexanone derived from cyclohexanone.The radiolytic yields of all these products were highly dependent on the initial cyclohexanone concentration.The formation of cyclic alcohols by radioreduction has been extended to various substituted cyclohexanones.Radiolytically generated solvated electrons are scavenged by cyclohexanone, leading to the corresponding radical anions.The protonation of these radical anions gives rise to cyclohexanol via the dismutation of the hydroxycyclohexyl radicals.Steady state radiolysis measurements were complemented by pulse radiolysis in dilute solution.It was established that radical-anions and hydroxylated radicals decayed according to a second order rate law.When ketone concentration was lower than 0.1M, radiolytic yields were in agreement with the mechanism mentioned above.However, in concentrated media the large increase in G(cyclohexanol) cannot be only accounted for by the involvement of radiolytically generated solvated electrons; probably it is due to an electron transfer from the cyclohexanone enolate to cyclohexanone itself, thus generating extra amounts of cyclohexanone radical anions.
- Alipour, E.,Vidril, D.,Micheau, J. C.,Paillous, N.,Lattes, A.,et al.
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p. 2807 - 2814
(2007/10/02)
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- Hydroboration. 61. Diisopinocampheylborane of High Optical Purity. Improved Preparation and Asymmetric Hydroboration of Representative Cis-Disubstituted Alkenes
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The convenient preparation of diisopinocampheylborane of high enentiomeric purity (99.1percent) utilizing the commercially available relatively stable borane-methyl sulfide and α-pinene of 92percent enantiomeric purity is described.Methyl sulfide liberated in the hydroboration step interferes with the equilibration needed to improve the optical purity of the reagent.However, this difficulty is readily overcome by removal of methyl sulfide under vacuum following hydroboration of the α-pinene.The raegent is then equilibrated in THF with 15percent excess α-pinene at 0 deg C for3 days.During this equilibration period, the major isomer becomes incorporated selectively into the reagent.This high optical purity diisopinocampheylborane has been utilized for asymmetric hydroboration of representative cis-disubstituted alkenes such as cis-2-butene, cis-3-hexene, cis-2-pentene, norbornene, norbornadiene, cis-4,4-dimethyl-2-pentene, and cis-propenylbenzene.Oxidation of the intermediate organoboranes provides the coprresponding alcohols in enantiomeric purities of 60-98percent.
- Brown, Herbert C.,Desai, Manoj C.,Jadhav, Prabhakar K.
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p. 5065 - 5069
(2007/10/02)
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- THE STEREOCHEMISTRY OF SODIUM DITHIONITE REDUCTIONS OF CYCLIC KETONES
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The stereochemistry of reduction of several substituted cyclohexanones, norcamphor, and camphor by sodium dithionite (sodium hydrosulfite, Na2S2O4) in aqueous DMF solution has been studied.The cyclohexanones yield mainly equatorial alcohols while the bicyclic ketones give mainly endo alcohols.
- Krapcho, A. Paul,Seidman, David A.
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p. 179 - 180
(2007/10/02)
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