- Photoenzymatic Hydrogenation of Heteroaromatic Olefins Using ‘Ene’-Reductases with Photoredox Catalysts
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Flavin-dependent ‘ene’-reductases (EREDs) are highly selective catalysts for the asymmetric reduction of activated alkenes. This function is, however, limited to enones, enoates, and nitroalkenes using the native hydride transfer mechanism. Here we demonstrate that EREDs can reduce vinyl pyridines when irradiated with visible light in the presence of a photoredox catalyst. Experimental evidence suggests the reaction proceeds via a radical mechanism where the vinyl pyridine is reduced to the corresponding neutral benzylic radical in solution. DFT calculations reveal this radical to be “dynamically stable”, suggesting it is sufficiently long-lived to diffuse into the enzyme active site for stereoselective hydrogen atom transfer. This reduction mechanism is distinct from the native one, highlighting the opportunity to expand the synthetic capabilities of existing enzyme platforms by exploiting new mechanistic models.
- Biegasiewicz, Kyle F.,Black, Michael J.,Chung, Megan M.,Hyster, Todd K.,Meichan, Andrew J.,Nakano, Yuji,Sandoval, Braddock A.,Zhu, Tianyu
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supporting information
p. 10484 - 10488
(2020/04/29)
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- Atroposelective Synthesis of Axially Chiral Styrenes via an Asymmetric C–H Functionalization Strategy
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Axially chiral styrenes, which exhibit a chiral axis between a substituted alkene and an aromatic ring, have been largely overlooked. The hurdle is the lower barriers to rotation compared with that of their biaryl counterparts, rendering their asymmetric synthesis more difficult. We report herein the highly atroposelective synthesis via a C?H functionalization strategy of axially chiral styrenes with an open-chained alkene. Various axially chiral styrenes were produced by Pd(II)-catalyzed C?H alkenylation and alkynylation in good yields (up to 99%) and enantioselectivities (up to 99% ee) by using L-pyroglutamic acid as an inexpensive chiral ligand. The potent application of the styrene atropisomers is demonstrated by a Co(III)-catalyzed enantioselective C?H amidation of ferrocene with axially chiral styrene-type acid as chiral ligand. Experimental and computational studies were conducted to elucidate the reaction mechanism. The chiral induction model of the enantioselectivity-determining C?H bond activation step was also provided based on DFT calculations. Atropisomerism, which stems from the hindered rotation around a chiral axis, is widely present in natural products, pharmaceuticals, and chiral catalysts or ligands. In contrast to the well-investigated biaryl atropisomers, the asymmetric synthesis of axially chiral styrenes bearing a chiral axis between an alkene and an aromatic ring remains a significant challenge. Here, we report a highly atroposelective synthesis of styrene atropisomers with open-chained alkene by asymmetric C?H functionalization by using available L-pyroglutamic acid as a chiral ligand. This strategy enables rapid access to a broad range of enantio-enriched axially chiral styrenes under mild conditions in an atom- and step-economical manner. The resulting axially chiral styrenes are important precursors for further elaborations, including the transformation into axially chiral styrene-type acids, which were demonstrated to be efficient chiral ligands in Co(III)-catalyzed enantioselective C?H amidation reactions. An asymmetric C–H functionalization strategy with L-pGlu-OH as chiral ligand has been developed for the atroposelective synthesis of styrene atropisomers with open-chained alkene. The strategy allows quick access to a wide range of enantio-enriched axially chiral styrenes in high yields and enantioselectivities. The axially chiral styrene-derived chiral acids have been demonstrated to be an efficient type of chiral ligands in Co(III)-catalyzed enantioselective C?H amidation reactions.
- Jin, Liang,Yao, Qi-Jun,Xie, Pei-Pei,Li, Ya,Zhan, Bei-Bei,Han, Ye-Qiang,Hong, Xin,Shi, Bing-Feng
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supporting information
p. 497 - 511
(2020/02/20)
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- A convenient and practical heterogeneous palladium-catalyzed carbonylative Suzuki coupling of aryl iodides with formic acid as carbon monoxide source
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A practical heterogeneous palladium-catalyzed carbonylative Suzuki coupling of aryl iodides with arylboronic acids under carbon monoxide gas-free conditions has been developed using a bidentate phosphino-functionalized magnetic nanoparticle-immobilized palladium(II) complex as catalyst. Formic acid was utilized as the carbon monoxide source with dicyclohexylcarbodiimide as the activator, and a wide variety of biaryl ketones were generated in moderate to high yields. The new heterogeneous palladium catalyst can be prepared via a simple procedure and can easily be separated from a reaction mixture by simply applying an external magnet and recycled up to 10 times without any loss of activity.
- You, Shengyong,Yan, Chenyu,Zhang, Rongli,Cai, Mingzhong
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- Isolation and characterization of a trinuclear cobalt complex containing trigonal-prismatic cobalt in secondary alcohol aerobic oxidation
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An unusual trinuclear cobalt complex was successfully isolated and characterized in the Co(OAc)2·4H2O-catalyzed aerobic oxidation of pyridine-based secondary alcohols. In this complex, a cobalt ion (the one in the middle, labeled Co2) has a novel trigonal-prismatic structure coordinated with six oxygen atoms from the substrate. The molecular oxygen present in air plays a major role in enabling this transformation to be catalytic. This aerobic catalytic reaction is very selective to pyridine-based secondary alcohols over primary alcohols.
- Karthikeyan,Alamsetti,Sekar
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p. 1665 - 1671
(2014/05/06)
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- Palladium-N-heterocyclic carbene an efficient catalytic system for the carbonylative cross-coupling of pyridine halides with boronic acids
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Carbonylative cross-coupling of different pyridyl halides with various boronic acids was studied using catalytic systems constituted of N-heterocyclic carbene type ligands and palladium. These systems easily obtained in situ from the corresponding imidazolium salt and palladium acetate appear more efficient toward bromopyridines than catalysts based on hindered and basic alkylphosphines such as tricyclohexylphosphine. Their higher efficiency was also evidenced by coupling using chloro- or dichloropyridines and chloroquinolines, which practically do not react with catalytic systems based on phosphines.
- Maerten, Eddy,Sauthier, Mathieu,Mortreux, André,Castanet, Yves
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p. 682 - 689
(2007/10/03)
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- Kinetic Energy Release and Position of Transition State During the Intramolecular Substitution of Ionized 2-Benzoyl Pyridines
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The loss of substituents X from molecular ions of ortho substituted 2-benzoyl pyridines has been investigated as a function of the dissociation energy of the C-X bond.Comparison of unimolecular and collisional induced decompositions of the resulting + ions and reference ions arising from 3-hydroxypyridoindole shows that cyclic fragment ions are formed in every case by an intramolecular substitution reaction with the exception of the parent compound (X=H), which gives rise to a mixture of + ions with different structures.The heat of formation of the cyclic ion has been estimated experimentally and by calculation using thermochemical data, and from this value and the appearance energies, the activation energies of the reverse reactions have been evaluated for the different reaction systems.Measurement of the kinetic energy release during the substitution reactions shows that only part of the reverse activation energy is released as kinetic energy.The energy partitioning quotient varies from 0.37 to 0.08 depending on the dissociation energy of the C-X bond or the reaction enthalpy.A sudden change in the energy partitioning quotient is observed with increasing exothermicity of the reaction, paralleling the behaviour of similar reaction systems.These results are interpreted as a demonstration of the influence of the variation of transition state position on the energy partitioning quotient.
- Schubert, Ralf,Gruetzmacher, Hans-Friedrich
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p. 122 - 130
(2007/10/02)
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