- Modular palladium bipyrazoles for the isomerization of allylbenzenes - Mechanistic considerations and insights into catalyst design and activity, role of solvent, and additive effects
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The catalytic activity of novel bidentate N,N-chelated palladium complexes derived from electron excessive, backbone fused 3,3'-bipyrazoles in the selective isomerization of terminal arylpropenoids and 1-alkenes is described. The catalysts are easily modified by appropriate wing tip substitution, while maintaining the same bulky, rigid unreactive aliphatic backbone. Eleven novel palladium complexes with different electronic and steric properties were investigated. Their performance in the palladium(II)-catalyzed isomerization of a series of substituted allylbenzenes was evaluated in terms of electronic as well as steric effects. Besides the clear finding of a general trend towards higher catalyst activity with more electron-donating properties of the coordinated N,N-bidentate ligands, we found that the catalytic process strongly depends on the choice of solvents and additives. Extensive solvent screening revealed that reactions run best in a 2:1 toluene-methanol mixture, with the alcohol employed being a crucial factor in terms of electronic and steric factors. A reaction mechanism involving a hydride addition-elimination mechanism starting with a palladium hydride species generated in situ in alcoholic solutions, as corroborated by experiments using deuterium labeled allylbenzene, seems to be most likely. The proposed mechanism is also supported by the observed reaction rate orders of κobs[cat.]≈1 (0.94), κobs [substrate]=0.20→1.0 (t→∞) and κobs [methanol]=-0.51 for the isomerization of allylbenzene. Furthermore, the influence of acid and base, as well as the role of the halide coordinated to the catalyst, are discussed. The system catalyzes the isomerization of allylbenzenes very efficiently yielding high E:Z selectivities under very mild conditions (room temperature) and at low catalyst loadings of 1 mol% palladium even in unpurified solvents. The integrity and stability of the catalyst system were confirmed by multiple addition reaction cycles, successive filtration and isolation experiments, and the lack of palladium black formation. Copyright
- Spallek, Markus J.,Stockinger, Skrollan,Goddard, Richard,Trapp, Oliver
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- Synthesis of Homoallylic Alcohols with Acyclic Quaternary Centers through CoIII-Catalyzed Three-Component C?H Bond Addition to Internally Substituted Dienes and Carbonyls
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An efficient CoIII-catalyzed three-component strategy to prepare homoallylic alcohols containing acyclic quaternary centers is disclosed. This transformation enables the introduction of two C?C σ bonds through C?H bond activation and sequential addition to internally substituted dienes and a wide range of aldehydes and activated ketones. Isoprene and other internally monosubstituted dienes are effective inputs, with the reaction proceeding with high diastereoselectivity for those substrate combinations that result in more than one stereogenic center. Moreover, the opposite relative stereochemistry can be achieved by employing 1,2-disubstituted dienes. A mechanism for the transformation is proposed based upon the relative stereochemistry of the products and studies with isotopically labeled starting materials.
- Dongbang, Sun,Shen, Zican,Ellman, Jonathan A.
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- Mechanistic insights into catalytic linear cross-dimerization between conjugated dienes and styrenes by a ruthenium(0) complex
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The mechanistic studies for linear cross-dimerization between 2,3-dimethylbuta-1,3-diene and styrene by a Ru(0) complex, Ru(η6-naphthalene)(η4-1,5-COD) (1), are performed both by kinetic and computational studies. This reaction is ba
- Hirano, Masafumi,Ueda, Takao,Komine, Nobuyuki,Komiya, Sanshiro,Nakamura, Saki,Deguchi, Hikaru,Kawauchi, Susumu
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- Rh-Catalyzed Coupling of Acrylic/Benzoic Acids with α-Diazocarbonyl Compounds: An Alternative Route for α-Pyrones and Isocoumarins
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A coupling of acrylic acids/benzoic acids with α-diazocarbonyl compounds has been realized by a combined catalytic system of rhodium catalyst and Zn(OAc)2 additive. The presence of Zn(OAc)2 obviously accelerates the C(sp2)
- Hong, Chao,Liu, Zhanxiang,Yu, Shuling,Zhang, Yuhong
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p. 815 - 820
(2022/02/07)
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- Iron-Catalyzed Reductive Cyclization by Hydromagnesiation: A Modular Strategy Towards N-Heterocycles
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A reductive cyclization to prepare a variety of N-heterocycles, through the use of ortho-vinylanilides, is reported. The reaction is catalyzed by an inexpensive and bench-stable iron complex and generally occurs at ambient temperature. The transformation likely proceeds through hydromagnesiation of the vinyl group, and trapping of the in situ generated benzylic anion by an intramolecular electrophile to form the heterocycle. This iron-catalyzed strategy was shown to be broadly applicable and was utilized in the synthesis of substituted indoles, oxindoles and tetrahydrobenzoazepinoindolone derivatives. Mechanistic studies indicated that the reversibility of the hydride transfer step depends on the reactivity of the tethered electrophile. The synthetic utility of our approach was further demonstrated by the formal synthesis of a reported bioactive compound and a family of natural products.
- Loup, Joachim,Larin, Egor M.,Lautens, Mark
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p. 22345 - 22351
(2021/09/09)
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- Iron Catalyzed Double Bond Isomerization: Evidence for an FeI/FeIII Catalytic Cycle
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Iron-catalyzed isomerization of alkenes is reported using an iron(II) β-diketiminate pre-catalyst. The reaction proceeds with a catalytic amount of a hydride source, such as pinacol borane (HBpin) or ammonia borane (H3N?BH3). Reactivity with both allyl arenes and aliphatic alkenes has been studied. The catalytic mechanism was investigated by a variety of means, including deuteration studies, Density Functional Theory (DFT) and Electron Paramagnetic Resonance (EPR) spectroscopy. The data obtained support a pre-catalyst activation step that gives access to an η2-coordinated alkene FeI complex, followed by oxidative addition of the alkene to give an FeIII intermediate, which then undergoes reductive elimination to allow release of the isomerization product.
- Woof, Callum R.,Durand, Derek J.,Fey, Natalie,Richards, Emma,Webster, Ruth L.
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p. 5972 - 5977
(2021/03/17)
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- Nickel-Catalyzed Multicomponent Coupling: Synthesis of α-Chiral Ketones by Reductive Hydrocarbonylation of Alkenes
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A nickel-catalyzed, multicomponent regio- and enantioselective coupling via sequential hydroformylation and carbonylation from readily available starting materials has been developed. This modular multicomponent hydrofunctionalization strategy enables the straightforward reductive hydrocarbonylation of a broad range of unactivated alkenes to produce a wide variety of unsymmetrical dialkyl ketones bearing a functionalized α-stereocenter, including enantioenriched chiral α-aryl ketones and α-amino ketones. It uses chiral bisoxazoline as a ligand, silane as a reductant, chloroformate as a safe CO source, and a racemic secondary benzyl chloride or an N-hydroxyphthalimide (NHP) ester of a protected α-amino acid as the alkylation reagent. The benign nature of this process renders this method suitable for late-stage functionalization of complex molecules.
- Chen, Jian,Zhu, Shaolin
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supporting information
p. 14089 - 14096
(2021/09/13)
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- Palladium-Catalyzed Asymmetric Markovnikov Hydroxycarbonylation and Hydroalkoxycarbonylation of Vinyl Arenes: Synthesis of 2-Arylpropanoic Acids
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Asymmetric hydroxycarbonylation is one of the most fundamental yet challenging methods for the synthesis of carboxylic acids. Herein, we reported the development of a palladium-catalyzed highly enantioselective Markovnikov hydroxycarbonylation of vinyl arenes with CO and water. A monodentate phosphoramidite ligand L6 plays vital role in the reaction. The reaction tolerates a range of functional groups, and provides a facile and atom-economical approach to an array of 2-arylpropanoic acids including several commonly used non-steroidal anti-inflammatory drugs. The catalytic system has also enabled an asymmetric Markovnikov hydroalkoxycarbonylation of vinyl arenes with alcohols to afford 2-arylpropanates. Mechanistic investigations suggested that the hydropalladation is irreversible and is the regio- and enantiodetermining step, while hydrolysis/alcoholysis is probably the rate-limiting step.
- Guan, Zheng-Hui,Ren, Zhi-Hui,Wang, Yuan,Yang, Hui-Yi,Yao, Ya-Hong,Zou, Xian-Jin
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supporting information
p. 23117 - 23122
(2021/09/18)
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- Asymmetric Markovnikov Hydroaminocarbonylation of Alkenes Enabled by Palladium-Monodentate Phosphoramidite Catalysis
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A palladium-catalyzed asymmetric Markovnikov hydroaminocarbonylation of alkenes with anilines has been developed for the atom-economical synthesis of 2-substituted propanamides bearing an α-stereocenter. A novel phosphoramidite ligand L16 was discovered which exhibited very high reactivity and selectivity in the reaction. This asymmetric Markovnikov hydroaminocarbonylation employs readily available starting materials and tolerates a wide range of functional groups, thus providing a facile and straightforward method for the regio- and enantioselective synthesis of 2-substituted propanamides under ambient conditions. Mechanistic studies revealed that the reaction proceeds through a palladium hydride pathway.
- Yao, Ya-Hong,Yang, Hui-Yi,Chen, Ming,Wu, Fei,Xu, Xing-Xing,Guan, Zheng-Hui
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supporting information
p. 85 - 91
(2021/01/12)
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- RhIII-Catalyzed C-H (Het)arylation/Vinylation of N-2,6-Difluoroaryl Acrylamides
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RhIII-catalyzed sp2 C-H cross-coupling of acrylamides with organoboron reactants has been accomplished using a commercially available N-2,6-difluoroaryl acrylamide auxiliary. A broad range of aryl and vinyl boronates as well as a variety of heterocyclic boronates with strong coordinating ability can serve as the coupling partners. This transformation proceeds under moderate reaction conditions with excellent functional group tolerance and high regioselectivity.
- Wang, Huai-Wei,Qiao, Yu-Han,Wu, Jia-Xue,Wang, Qiu-Ping,Tian, Meng-Xin,Li, Yong-Fei,Yao, Qing-Xia,Li, Da-Cheng,Dou, Jian-Min,Lu, Yi
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supporting information
p. 656 - 662
(2021/02/01)
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- Formal Allylation and Enantioselective Cyclopropanation of Donor/Acceptor Rhodium(II) Azavinyl Carbenes
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A highly efficient formal allylation of dihydronaphthotriazoles with alkenes under rhodium(II) catalysis is reported. Various allyl dihydronaphthalene derivatives were furnished via rhodium(II) azavinyl carbenes with moderate to good yields and excellent chemoselectivity. When monosubstituted alkenes are used, cyclopropanation occurs and good to excellent enantioselectivities have been achieved. Particularly noteworthy is the allylic C(sp2)-H activation instead of traditional C(sp3)-H activation in the formal allylation process.
- Liu, Zhili,Chen, Lianfen,Zhu, Dong,Zhu, Shifa
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supporting information
p. 1275 - 1279
(2021/02/20)
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- Organocatalytic Enantioselective γ-Elimination: Applications in the Preparation of Chiral Peroxides and Epoxides
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An organocatalyzed enantioselective γ-elimination process has been achieved and applied in the kinetic resolution of peroxides to access chiral peroxides and epoxides. The reaction provided a pathway for the preparation of two useful synthetic and biologically important structural motifs through a single-step reaction. A range of substrates has been resolved with a selectivity factor up to 63. The obtained enantioenriched peroxides and epoxides allowed a series of transformations with retained optical purities.
- Chen, Zhili,Gong, Xiangnan,Hu, Fangli,Huang, Shengli,Jia, Shiqi,Qin, Wenling,Tan, Yu,Xu, Da,Yan, Hailong
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supporting information
p. 1934 - 1940
(2020/03/24)
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- Platinum ω-Alkenyl Compounds as Chemical Vapor Deposition Precursors. Mechanistic Studies of the Thermolysis of Pt[CH2CMe2CH2CH= CH2]2in Solution and the Origin of Rapid Nucleation
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The compound cis-bis(η1,η2-2,2-dimethylpent-4-en-1-yl)platinum, Pt[CH2CMe2CH2CH= CH2]2 (3), is a recently discovered chemical vapor deposition (CVD) precursor for the deposition of highly smooth platinum thin films without nucleation delays on a variety of substrates. This paper describes detailed mechanistic studies of the pathway by which 3 reacts upon being heated in solution. In various solvents between 90 and 130 °C, 3 decomposes to generate ~1 equiv of 4,4-dimethylpentenes by addition of a hydrogen atom to the pentenyl ligands in 3. The "extra"hydrogen atoms arise by dehydrogenation of other pentenyl ligands; some of these dehydrogenated ligands are released as methyl-substituted methylenecyclobutanes and cyclobutenes. A combination of isotope labeling and kinetic studies suggests that 3 decomposes by C-H activation of both allylic and olefinic C-H bonds to give transient platinum hydride intermediates, followed by reductive elimination steps to form the pentene products, but that the exact mechanism is solvent-dependent. In C6F6, solvent association occurs before C-H bond activation, and the rate-determining step for thermolysis is most likely the formation of a Pt σ complex. In hydrocarbon solvents, the solvent is little involved before C-H bond activation, and the rate-determining step is most likely the formation of a Pt σ complex only for γ-C-H and ?-C-H bond activation, but cleavage or formation of a C-H bond for δ-C-H bond activation. A comparison of the thermolysis reactions under CVD conditions and in solution suggests that the high smoothness of the CVD-grown films is due in part to rapid nucleation (which is a consequence of the availability of low-barrier C= C bond dissociation pathways) and in part to the formation of carbon-containing species that passivate the Pt surface.
- Liu, Sumeng,Zhang, Zhejun,Abelson, John R.,Girolami, Gregory S.
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supporting information
p. 3817 - 3829
(2020/11/13)
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- Experimental and Computational Studies on Rh(I)-Catalyzed Reaction of Siloxyvinylcyclopropanes and Diazoesters
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The Rh(I)-catalyzed reaction of siloxyvinylcyclopropanes and diazoesters leads to the formation of siloxyvinylcyclobutane and 1,4-diene derivatives. With [Rh(cod)Cl]2 as the catalyst, the formation of 1,4-diene was favored over the formation of siloxyvinylcyclobutane. By changing the catalyst to [Rh(cod)2OTf], siloxyvinylcyclobutane derivatives are formed with excellent chemoselectivities and in moderate to good yields. The alkene products are also obtained as single E configured isomers. A detailed mechanism for this transformation is proposed on the basis of mechanistic experiments and DFT calculations. The effect of catalysts on the chemoselectivity of these reactions is also examined computationally.
- Feng, Sheng,Wang, Kang,Ping, Yifan,Wang, Jianbo
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supporting information
p. 21032 - 21039
(2020/12/21)
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- An α-Cyclopropanation of Carbonyl Derivatives by Oxidative Umpolung
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The reactivity of iodine(III) reagents towards nucleophiles is often associated with umpolung and cationic mechanisms. Herein, we report a general process converting a range of ketone derivatives into α-cyclopropanated ketones by oxidative umpolung. Mechanistic investigation and careful characterization of side products revealed that the reaction follows an unexpected pathway and suggests the intermediacy of non-classical carbocations.
- Bauer, Adriano,Di Mauro, Giovanni,Li, Jing,Maulide, Nuno
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supporting information
p. 18208 - 18212
(2020/08/21)
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- Rhodium(i)-catalyzed mono-selective C-H alkylation of benzenesulfonamides with terminal alkenes
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The Rh(i)-catalyzed ortho-alkylation of benzenesulfonamides with alkenes with the aid of an 8-aminoquinoline directing group is reported. The reaction is applicable to a variety of benzenesulfonamide derivatives and various alkenes. Curiously, unactivated
- Rej, Supriya,Chatani, Naoto
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supporting information
p. 10503 - 10506
(2019/09/12)
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- Palladium-Catalyzed Enantioselective Thiocarbonylation of Styrenes
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A highly enantioselective thiocarbonylation of styrenes with CO and thiols has been achieved by Pd catalysis, providing highly enantioenriched thioesters in good to excellent yields. Key to the successful execution of this reaction is the use of a chiral sulfoxide-(P-dialkyl)-phosphine (SOP) ligands. This thiocarbonylation proceeds smoothly under mild reaction conditions (1 atm CO and 0 °C) and displays broad substrate scope. Also demonstrated is that this transformation can be conducted using surrogates of CO, greatly increasing the safety aspects of running the reaction. The generality and utility of the method is manifested by its application to the synthetic transformations of thioester products and the direct acylation of cysteine-containing dipeptides. A primary mechanism was investigated and a plausible catalytic cycle was proposed.
- Wang, Xihong,Wang, Bing,Yin, Xuemei,Yu, Wangzhi,Liao, Yang,Ye, Jialin,Wang, Min,Hu, Lianrui,Liao, Jian
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supporting information
p. 12264 - 12270
(2019/08/01)
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- Direct Synthesis of Alkenylboronates from Alkenes and Pinacol Diboron via Copper Catalysis
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We report an efficient approach for the direct synthesis of alkenylboronates using copper catalysis. The Cu/TEMPO catalyst system (where TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxyl) exhibits both excellent reactivity and selectivity for the synthesis of alkenylboronates, starting from inexpensive and abundant alkenes and pinacol diboron. This approach allows for the direct functionalization of both aromatic and aliphatic terminal alkenes. Mechanistic experiments suggest that the alkenylboronates arise from oxyboration intermediates.
- Lu, Wenkui,Shen, Zengming
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supporting information
p. 142 - 146
(2019/01/11)
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- Electrochemistry-Enabled Ir-Catalyzed Vinylic C-H Functionalization
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Synergistic use of electrochemistry and organometallic catalysis has emerged as a powerful tool for site-selective C-H functionalization, yet this type of transformation has thus far mainly been limited to arene C-H functionalization. Herein, we report the development of electrochemical vinylic C-H functionalization of acrylic acids with alkynes. In this reaction an iridium catalyst enables C-H/O-H functionalization for alkyne annulation, affording α-pyrones with good to excellent yields in an undivided cell. Preliminary mechanistic studies show that anodic oxidation is crucial for releasing the product and regeneration of an Ir(III) intermediate from a diene-Ir(I) complex, which is a coordinatively saturated, 18-electron complex. Importantly, common chemical oxidants such as Ag(I) or Cu(II) did not give significant amounts of the desired product in the absence of electrical current under otherwise identical conditions.
- Yang, Qi-Liang,Xing, Yi-Kang,Wang, Xiang-Yang,Ma, Hong-Xing,Weng, Xin-Jun,Yang, Xiang,Guo, Hai-Ming,Mei, Tian-Sheng
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supporting information
p. 18970 - 18976
(2019/12/04)
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- Cobaloxime Catalysis: selective synthesis of alkenylphosphine oxides under visible light
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Direct activation of H-phosphine oxide to react with an unsaturated carbon-carbon bond is a straightforward approach for accessing alkenylphosphine oxides, which shows significant applications in both synthetic and material fields. However, expensive metals and strong oxidants are typically required to realize the transformation. Here, we demonstrate the utility of earth-abundant cobaloxime to convert H-phosphine oxide into its reactive radical species under visible light irradiation. The radical species thus generated can be utilized to functionalize alkenes and alkynes without any external photosensitizer and oxidant. The coupling with terminal alkene generates E-alkenylphosphine oxide with excellent chemo- A nd stereoselectivity. The reaction with terminal alkyne yields linear E-alkenylphosphine oxide via neutral radical addition, while addition with internal ones generates cyclic benzophosphine oxides and hydrogen. Mechanistic studies on radical trapping experiments, electron spin resonance studies, and spectroscopic measurements confirm the formation of phosphinoyl radical and cobalt intermediates that are from capturing the electron and proton eliminated from H-phosphine oxide. The highlight of our mechanistic investigation is the dual role played by cobaloxime, viz., both as the visible light absorber to activate the P(O)-H bond as well as a hydrogen transfer agent to influence the reaction pathway. This synergetic feature of the cobaloxime catalyst preforming multiple functions under ambient condition provides a convergent synthetic approach to vinylphosphine oxides directly from H-phosphine oxides and alkenes (or alkynes).
- Liu, Wen-Qiang,Lei, Tao,Zhou, Shuai,Yang, Xiu-Long,Li, Jian,Chen, Bin,Sivaguru, Jayaraman,Tung, Chen-Ho,Wu, Li-Zhu
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supporting information
p. 13941 - 13947
(2019/09/30)
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- Cobaloxime Catalysis: Selective Synthesis of Alkenylphosphine Oxides under Visible Light
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Direct activation of H-phosphine oxide to react with an unsaturated carbon-carbon bond is a straightforward approach for accessing alkenylphosphine oxides, which shows significant applications in both synthetic and material fields. However, expensive metals and strong oxidants are typically required to realize the transformation. Here, we demonstrate the utility of earth-abundant cobaloxime to convert H-phosphine oxide into its reactive radical species under visible light irradiation. The radical species thus generated can be utilized to functionalize alkenes and alkynes without any external photosensitizer and oxidant. The coupling with terminal alkene generates E-alkenylphosphine oxide with excellent chemo- and stereoselectivity. The reaction with terminal alkyne yields linear E-alkenylphosphine oxide via neutral radical addition, while addition with internal ones generates cyclic benzophosphine oxides and hydrogen. Mechanistic studies on radical trapping experiments, electron spin resonance studies, and spectroscopic measurements confirm the formation of phosphinoyl radical and cobalt intermediates that are from capturing the electron and proton eliminated from H-phosphine oxide. The highlight of our mechanistic investigation is the dual role played by cobaloxime, viz., both as the visible light absorber to activate the P(O)-H bond as well as a hydrogen transfer agent to influence the reaction pathway. This synergetic feature of the cobaloxime catalyst preforming multiple functions under ambient condition provides a convergent synthetic approach to vinylphosphine oxides directly from H-phosphine oxides and alkenes (or alkynes).
- Liu, Wen-Qiang,Lei, Tao,Zhou, Shuai,Yang, Xiu-Long,Li, Jian,Chen, Bin,Sivaguru, Jayaraman,Tung, Chen-Ho,Wu, Li-Zhu
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supporting information
p. 13941 - 13947
(2019/09/30)
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- Modular Access to Substituted Azocanes via a Rhodium-Catalyzed Cycloaddition-Fragmentation Strategy
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A short entry to substituted azocanes by a Rh-catalyzed cycloaddition-fragmentation process is described. Specifically, exposure of diverse N-cyclopropylacrylamides to phosphine-ligated cationic Rh(I) catalyst systems under a CO atmosphere enables the directed generation of rhodacyclopentanone intermediates. Subsequent insertion of the alkene component is followed by fragmentation to give the heterocyclic target. Stereochemical studies show, for the first time, that alkene insertion into rhodacyclopentanones can be reversible.
- Shaw, Megan H.,Croft, Rosemary A.,Whittingham, William G.,Bower, John F.
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supporting information
p. 8054 - 8057
(2015/07/15)
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- Branch-selective, Iridium-catalyzed hydroarylation of monosubstituted alkenes via a cooperative destabilization strategy
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Highly branch-selective, carbonyl-directed hydroarylations of monosubstituted alkenes are described. The chemistry relies upon a cationic Ir(I) catalyst modified with an electron deficient, wide bite angle bisphosphine ligand. This work provides a regioisomeric alternative to the Murai hydroarylation protocol.
- Crisenza, Giacomo E. M.,McCreanor, Niall G.,Bower, John F.
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supporting information
p. 10258 - 10261
(2014/08/05)
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- Preparation of deuterium labelled organophosphonium salts (Wittig Salts) under hydrothermal condition catalyzed by molecular sieves
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Organophosphonium salt was converted into deuterium labelled ones under hydrothermal condition in the presence of molecular sieves. The hydrothermal condition can be obtained by either heating D2O in an autoclave or irradiating with microwave to D2O in a sealed vial.
- Yamamoto, Mitsuru,Oshima, Koichiro,Matsubara, Seijiro
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p. 846 - 847
(2007/10/03)
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- Transition-State Structure Variation in the Diels-Alder Reaction from Secondary Deuterium Kinetic Isotope Effects: The Reaction of Nearly Symmetrical Dienes and Dienophiles Is Nearly Synchronous
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Secondary deuterium kinetic isotope effects (KIEs) in the reaction of acrylonitrile, fumaronitrile, vinylidene cyanide, dimethyl fumarate and maleate, and methyl trans-β-cyanoacrylate with isoprene were determined by competition with isoprene-d0 (d0), 4,4-dideuterioisoprene (4,4-d2), and 1,1,4,4-tetradeuterioisoprene (d4).The d4 experiment gives the KIE for 1,1-dideuterioisoprene from the KIEs for 4,4-d2.The KIEs for bond making to the β site of acrylonitrile and vinylidene cyanide are much more inverse than those at the α bond making site but are less than the maximum value expected, which indicates an early, unsymmetrical, concerted transition state.The effects wit fumaronitrile are the same for both 1,1- and 4,4-dideuterioisoprene, indicating equivalent effects at both sites and not bonding to the preferred C-1 site of isoprene most of the time.The KIEs with methyl trans-β-cyanoacrylate are inverse at both bond-making sites to the dienophile and both nearly one-third of the maximum value expected.The results with nearly symmetrical addends are not consistent with a two-step reaction, particularly one with the second step being rate determining, or with a concerted reaction with a highly unsymmetrical transition state, but rather with a concerted pathway that is nearly synchronous.The KIEs with methyl fumarate and maleate suggest an unsymmetrical transition state, but they are within experimental error of a symmetrical transition state.
- Gajewski, Joseph J.,Peterson, Karen B.,Kagel, John R.,Huang, Y. C. Jason
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p. 9078 - 9081
(2007/10/02)
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- Synthesis and EPR and ENDOR Investigations of Coppinger's Radical with Perdeuteriated tert-Butyl Groups
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The synthesis of Coppinger's radical (galvinoxyl) with perdeuteriated tert-butyl groups is described.Its EPR spectrum exhibits markedly decreased line widths (0.015 mT) compared with the unlabelled radical (0.039 mT).The correspondingly higher resolution
- Gersdorff, J. von,Kirste, B.,Kurreck, H.
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p. 1077 - 1079
(2007/10/02)
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