- Benzimidazoles as Metal-Free and Recyclable Hydrides for CO2 Reduction to Formate
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We report a novel metal-free chemical reduction of CO2 by a recyclable benzimidazole-based organo-hydride, whose choice was guided by quantum chemical calculations. Notably, benzimidazole-based hydride donors rival the hydride-donating abilities of noble-metal-based hydrides such as [Ru(tpy)(bpy)H]+ and [Pt(depe)2H]+. Chemical CO2 reduction to the formate anion (HCOO-) was carried out in the absence of biological enzymes, a sacrificial Lewis acid, or a base to activate the substrate or reductant. 13CO2 experiments confirmed the formation of H13COO- by CO2 reduction with the formate product characterized by 1H NMR and 13C NMR spectroscopy and ESI-MS. The highest formate yield of 66% was obtained in the presence of potassium tetrafluoroborate under mild conditions. The likely role of exogenous salt additives in this reaction is to stabilize and shift the equilibrium toward the ionic products. After CO2 reduction, the benzimidazole-based hydride donor was quantitatively oxidized to its aromatic benzimidazolium cation, establishing its recyclability. In addition, we electrochemically reduced the benzimidazolium cation to its organo-hydride form in quantitative yield, demonstrating its potential for electrocatalytic CO2 reduction. These results serve as a proof of concept for the electrocatalytic reduction of CO2 by sustainable, recyclable, and metal-free organo-hydrides.
- Lim, Chern-Hooi,Ilic, Stefan,Alherz, Abdulaziz,Worrell, Brady T.,Bacon, Samuel S.,Hynes, James T.,Glusac, Ksenija D.,Musgrave, Charles B.
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- Diazaphosphinanes as hydride, hydrogen atom, proton or electron donors under transition-metal-free conditions: Thermodynamics, kinetics, and synthetic applications
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Exploration of new hydrogen donors is in large demand in hydrogenation chemistry. Herein, we developed a new 1,3,2-diazaphosphinane 1a, which can serve as a hydride, hydrogen atom or proton donor without transition-metal mediation. The thermodynamics and kinetics of these three pathways of 1a, together with those of its analog 1b, were investigated in acetonitrile. It is noteworthy that, the reduction potentials (Ered) of the phosphenium cations 1a-[P]+ and 1b-[P]+ are extremely low, being-1.94 and-2.39 V (vs. Fc+/0), respectively, enabling corresponding phosphinyl radicals to function as neutral super-electron-donors. Kinetic studies revealed an extraordinarily large kinetic isotope effect KIE(1a) of 31.3 for the hydrogen atom transfer from 1a to the 2,4,6-tri-(tert-butyl)-phenoxyl radical, implying a tunneling effect. Furthermore, successful applications of these diverse P-H bond energetic parameters in organic syntheses were exemplified, shedding light on more exploitations of these versatile and powerful diazaphosphinane reagents in organic chemistry.
- Cheng, Jin-Pei,Yang, Jin-Dong,Zhang, Jingjing
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p. 3672 - 3679
(2020/04/21)
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- Neutral Organic Super Electron Donors Made Catalytic
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Neutral organic super electron donors (SEDs) display impressive reducing power but, until now, it has not been possible to use them catalytically in radical chain reactions. This is because, following electron transfer, these donors form persistent radical cations that trap substrate-derived radicals. This paper unlocks a conceptually new approach to super electron donors that overcomes this issue, leading to the first catalytic neutral organic super electron donor.
- Rohrbach, Simon,Shah, Rushabh S.,Tuttle, Tell,Murphy, John A.
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supporting information
p. 11454 - 11458
(2019/07/18)
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- Bridging amines with CO2: Organocatalyzed reduction of CO2 to aminals
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The four-electron reduction of CO2 in the presence of secondary aromatic amines is described for the first time to access aminals. Under metal-free hydrosilylation conditions, the four C-O bonds of CO2 are cleaved, and the organocatalysts are able to balance the reactivity of CO2 to promote the selective formation of two C-N and two C-H bonds. The methodology enables the formation of various symmetrical and unsymmetrical aminals.
- Frogneux, Xavier,Blondiaux, Enguerrand,Thuéry, Pierre,Cantat, Thibault
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p. 3983 - 3987
(2015/11/11)
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- Iron-catalyzed reduction of CO2 into methylene: Formation of C-N, C-O, and C-C bonds
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We report herein the use of the (dihydrido)iron catalyst, Fe(H)2(dmpe)2, for the selective reduction of CO2 into either bis(boryl)acetal or methoxyborane depending on the hydroborane used as a reductant. In a one-pot two-step procedure, the in situ generated bis(boryl)acetal was shown to be a reactive and versatile source of methylene to create new C-N but also C-O and C-C bonds.
- Jin, Guanghua,Werncke, C. Gunnar,Escudié, Yannick,Sabo-Etienne, Sylviane,Bontemps, Sébastien
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supporting information
p. 9563 - 9566
(2015/08/18)
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- New reactions of terminal hydrides on a diiron dithiolate
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Mechanisms for biological and bioinspired dihydrogen activation and production often invoke the intermediacy of diiron dithiolato dihydrides. The first example of such a Fe2(SR)2H2 species is provided by the complex [(term-H)(μ-H)Fe2(pdt)(CO)(dppv) 2] ([H1H]0). Spectroscopic and computational studies indicate that [H1H]0 contains both a bridging hydride and a terminal hydride, which, notably, occupies a basal site. The synthesis begins with [(μ-H)Fe2(pdt)(CO)2(dppv)2]+ ([H1(CO)]+), which undergoes substitution to afford [(μ-H)Fe 2(pdt)(CO)(NCMe)(dppv)2]+ ([H1(NCMe)] +). Upon treatment of [H1(NCMe)]+ with borohydride salts, the MeCN ligand is displaced to afford [H1H]0. DNMR (EXSY, SST) experiments on this complex show that the terminal and bridging hydride ligands interchange intramolecularly at a rate of 1 s-1 at -40 °C. The compound reacts with D2 to afford [D1D]0, but not mixed isotopomers such as [H1D]0. The dihydride undergoes oxidation with Fc+ under CO to give [1(CO)]+ and H2. Protonation in MeCN solution gives [H1(NCMe)]+ and H2. Carbonylation converts [H1H]0 into [1(CO)]0.
- Wang, Wenguang,Rauchfuss, Thomas B.,Zhu, Lingyang,Zampella, Giuseppe
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p. 5773 - 5782
(2014/05/06)
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- A general catalytic methylation of amines using carbon dioxide
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Putting CO2 to work: Carbon dioxide is shown to be a general and selective methylating reagent for secondary and primary, aromatic and aliphatic amines under reductive conditions. A variety of tertiary amines are obtained from CO2 and commercially available silanes in high yields with good tolerance to nitrile, olefin, ether, ester, and hydroxy groups. Copyright
- Li, Yuehui,Fang, Xianjie,Junge, Kathrin,Beller, Matthias
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p. 9568 - 9571
(2013/09/23)
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- Carbon–carbon bond formation via benzoyl umpolung attained by photoinduced electron-transfer with benzimidazolines
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A photoreaction between benzoyl compounds, such as benzoylformate derivatives, and 2-(p-anisyl)-1,3-dimethylbenzimidazoline in the presence of allyl bromide was found to give various allylated alcohols. In the reaction of benzoylformates, α-hydroxy ester enolates, for which the negative charge occurs on the carbonyl carbon of benzoyl (umpolung reactivity), are proposed to be generated as intermediates by electron-transfer from benzimidazolines to the photoexcited benzoylformates; these species react with allyl bromide to produce α-allyl-α-hydroxy esters.
- Igarashi, Tomohito,Tayama, Eiji,Iwamoto, Hajime,Hasegawa, Eietsu
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supporting information
p. 6874 - 6877
(2019/04/10)
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- Thermodynamic and kinetic hydricity of ruthenium(II) hydride complexes
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Despite the fundamental importance of the hydricity of a transition metal hydride (δG°H-(MH) for the reaction M-H → M+ + H-) in a range of reactions important in catalysis and solar energy storage, ours (J. Am. Chem. Soc.2009, 131, 2794) are the only values reported for water solvent, and there has been no basis for comparison of these with the wider range already determined for acetonitrile solvent, in particular. Accordingly, we have used a variety of approaches to determine hydricity values in acetonitrile of Ru(II) hydride complexes previously studied in water. For [Ru(ν6-C6Me6)(bpy)H] + (bpy = 2,2′-bipyridine), we used a thermodynamic cycle based on evaluation of the acidity of [Ru(ν6-C6Me 6)(bpy)H]+ pKa = 22.5 ± 0.1 and the [Ru(ν6-C6Me6)(bpy)(NCCH3) 1/0]2+/0 electrochemical potential (-1.22 V vs Fc +/Fc). For [Ru(tpy)(bpy)H]+ (tpy = 2,2′:6′, 2″-terpyridine) we utilized organic hydride ion acceptors (A+) of characterized hydricity derived from imidazolium cations and pyridinium cations, and determined K for the hydride transfer reaction, S + MH+ + A+ → M(S)2+ + AH (S = CD3CN, MH + = [Ru(tpy)(bpy)H]+), by 1H NMR measurements. Equilibration of initially 7 mM solutions was slow-on the time scale of a day or more. When E°(H+/H-) is taken as 79.6 kcal/mol vs Fc+/Fc as a reference, the hydricities of [Ru(ν6-C 6Me6)(bpy)H]+ and [Ru(tpy)(bpy)H]+ were estimated as 54 ± 2 and 39 ± 3 kcal/mol, respectively, in acetonitrile to be compared with the values 31 and 22 kcal/mol, respectively, found for aqueous media. The pKa estimated for [Ru(tpy)(bpy)H] + in acetonitrile is 32 ± 3. UV-vis spectroscopic studies of [Ru(ν6-C6Me6)(bpy)]0 and [Ru(tpy)(bpy)]0 indicate that they contain reduced bpy and tpy ligands, respectively. These conclusions are supported by DFT electronic structure results. Comparison of the hydricity values for acetonitrile and water reveals a flattening or compression of the hydricity range upon transferring the hydride complexes to water.
- Matsubara, Yasuo,Fujita, Etsuko,Doherty, Mark D.,Muckerman, James T.,Creutz, Carol
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supporting information
p. 15743 - 15757,15
(2020/08/24)
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- Formal Transfers of Hydride from Carbon-Hydrogen Bonds. Thermolysis of 2,3-dihydro-2-(2-hydroxyphenyl)-1,3-dimethyl-1H-benzimidazole
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In 2,3-dihydro-2-(2-hydroxyphenyl)-1,3-dimethyl-1H-benzimidazole (3), the acidic hydrogen of a phenolic oxygen-hydrogen bond is near a carbon-hydrogen bond activated as a donor of hydride by two adjacent lone pairs in a dihydro aromatic ring.This juxtapos
- Montgrain, France,Ramos, Socorro M.,Wuest, James D.
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p. 1489 - 1492
(2007/10/02)
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