Sustainable catalysis with fluxional acridine-based PNP pincer complexes

Kar, Sayan; Milstein, David

doi:10.1039/d2cc00247g

Cited by 27 publications

(16 citation statements)

References 98 publications

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“…The fac isomer was calculated to be 9.1 kcal/mol higher in energy than the mer isomer. 67 Interestingly, the DFT studies indicate that two distinct mechanisms are possible for the initial amine dehydrogenation. The first is N–H activation and H 2 liberation, followed by β-hydride elimination, which can proceed without water ( Figure 6 e).…”

Section: Oxidation Of Amines By Watermentioning

confidence: 99%

“…The reaction requires a catalytic amount of base to convert the employed precatalyst 5c to the actual catalyst. Mechanistic investigations revealed that in the presence of base and cyclic amine, the aromatic complex 5c is first converted to a dearomatized complex 6 , which then catalyzes the reaction (Figure c). , Consequently, the dearomatized complex 6 catalyzes the reaction even under neutral conditions in the absence of base. Important insights regarding the catalytic cycle were obtained by DFT studies.…”

Section: Oxidation Of Amines By Watermentioning

confidence: 99%

“…It was found that while the most thermodynamically stable form of complex 6 features the ligand bound in a meridional fashion, the complex is nonetheless fluxional, with the fac isomer facilitating its catalytic activity (Figure d). The fac isomer was calculated to be 9.1 kcal/mol higher in energy than the mer isomer . Interestingly, the DFT studies indicate that two distinct mechanisms are possible for the initial amine dehydrogenation.…”

Section: Oxidation Of Amines By Watermentioning

confidence: 99%

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Oxidation of Organic Compounds Using Water as the Oxidant with H₂ Liberation Catalyzed by Molecular Metal Complexes

Kar

Milstein

2022

Acc. Chem. Res.

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Conspectus Oxidation reactions of organic compounds play a central role in both industrial chemical and material synthesis as well as in fine chemical and pharmaceutical synthesis. While traditional laboratory-scale oxidative syntheses have relied on the use of strong oxidizers, modern large-scale oxidation processes preferentially utilize air or pure O 2 as an oxidant, with other oxidants such as hydrogen peroxide, nitric acid, and aqueous chlorine solution also being used in some processes. The use of molecular oxygen or air as an oxidant has been very attractive in recent decades because of the abundance of air and the lack of wasteful byproduct generation. Nevertheless, the use of high-pressure air or, in particular, pure oxygen can lead to serious safety concerns with improper handling and also necessitates the use of sophisticated high-pressure reactors for the processes. Several research groups, including ours, have investigated in recent times the possibility of carrying out catalytic oxidation reactions using water as the formal oxidant, with no added conventional oxidants. Along with the abundant availability of water, these processes also generate dihydrogen gas as the reaction coproduct, which is a highly valuable fuel. Several well-defined molecular metal complexes have been reported in recent years to catalyze these unusual oxidative reactions with water. A ruthenium bipyridine-based PNN pincer complex was reported by us to catalyze the oxidation of primary alcohols to carboxylate salts with alkaline water along with H 2 liberation, followed by reports by other groups using other complexes as catalysts. At the same time, ruthenium-, iridium-, and rhodium-based complexes have been reported to catalyze aldehyde oxidation to carboxylic acids using water. Our group has combined the catalytic aqueous alcohol and aldehyde oxidation activity of a ruthenium complex to achieve the oxidation of biomass-derived renewable aldehydes such as furfural and 5-hydroxymethylfurfural (HMF) to furoic acid and furandicarboxylic acid (FDCA), respectively, using alkaline water as the oxidant, liberating H 2 . Ruthenium complexes with an acridine-based PNP ligand have also been employed by our group for the catalytic oxidation of amines to the corresponding lactams, or to carboxylic acids via a deaminative route, using water. Similarly, we also reported molecular complexes for the catalytic Markovnikov oxidation of alkenes to ketones using water, similar to Wacker-type oxidation, which, however, does not require any terminal oxidant and produces H 2 as the coproduct. At the same time, the oxidation of enol ethers to the corresponding esters with water has also been reported. This account will highlight these recent advances where water was used as an oxidant to carry out selective oxidation reactions of organic compounds, catalyzed by well-defined molecular complexes, with H 2 lib...

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Section: Oxidation Of Amines By Watermentioning

confidence: 99%

Section: Oxidation Of Amines By Watermentioning

confidence: 99%

Section: Oxidation Of Amines By Watermentioning

confidence: 99%

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Oxidation of Organic Compounds Using Water as the Oxidant with H₂ Liberation Catalyzed by Molecular Metal Complexes

Kar

Milstein

2022

Acc. Chem. Res.

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“…Its distinct photophysical properties such as strong emission and long lifetime in the excited states are utilized not only for biological and material sciences but also for organic photocatalysis such as acridinium salts . While acridine-based PNP-pincer ligands and complexes have recently been developed for some catalytic reactions, transition metal–acridine complexes have not been applied to photochemical reactions . We suppose that visible-light excitation of the metal–acridine complex would induce metal-to-ligand charge transfer (MLCT) efficiently, generating a highly reactive electron-deficient metal center.…”

Section: Introductionmentioning

confidence: 99%

Shining Visible Light on Reductive Elimination: Acridine–Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids

2022

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Despite the recent tremendous progress on transition-metal/photoredox dual catalysis in organic synthesis, single transition-metal catalysis under visible-light irradiation, which can utilize light energy more efficiently, is still underdeveloped. Herein, we report the design of photosensitizing phosphinoacridine bidentate ligands for visible-light-induced transition-metal catalysis, expecting that the electron-accepting acridine moiety would create a highly reactive electron-deficient metal center toward reductive elimination via metal-to-ligand charge transfer (MLCT). Using these ligands, we have achieved a palladium-catalyzed cross-coupling reaction of aryl halides with carboxylic acids under visible-light irradiation. Electronic tuning of the phosphinoacridine ligands not only enabled the use of a variety of aryl halides as the coupling partner, including less reactive aryl chlorides, under blue light irradiation, but also realized the employment of lower-energy green and red light for the cross-coupling. Experimental mechanistic studies have proved that the reductive elimination of aryl esters is induced by photoirradiation of phosphinoacridine-ligated arylpalladium(II) carboxylate complexes. The theoretical calculation suggests that the reductive elimination in the excited state is promoted by decreasing the electron density of the Pd center through photoinduced intramolecular electron transfer, i.e., MLCT, in the transition state owing to the electron-deficient acridine scaffold. This is a very rare example of photoinduced reductive elimination on palladium(II) complexes.

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“…1), which have previously shown good catalytic activities for the transformation of different carbonyl and alcohol functionalities. 21,22,[26][27][28][29][30][31][32][33][34][35][36][37][38] We also studied the addition of a cosolvent to maximize the productivity of the catalyst under ADC conditions for the transformation of EtOH.…”

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confidence: 99%

Base-free, acceptorless dehydrogenative coupling of ethanol to ethyl acetate with PNP complexes

Paz

Pramanick

et al. 2023

Dalton Trans.

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Sustainable catalysis with fluxional acridine-based PNP pincer complexes

Abstract: Due to the widespread use of fossil fuels and the resulting global warming, development of sustainable catalytic transformations is now more important than ever to obtain our desired fuels and...

Cited by 27 publications

References 98 publications

Oxidation of Organic Compounds Using Water as the Oxidant with H₂ Liberation Catalyzed by Molecular Metal Complexes

Oxidation of Organic Compounds Using Water as the Oxidant with H₂ Liberation Catalyzed by Molecular Metal Complexes

Shining Visible Light on Reductive Elimination: Acridine–Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids

Base-free, acceptorless dehydrogenative coupling of ethanol to ethyl acetate with PNP complexes

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Sustainable catalysis with fluxional acridine-based PNP pincer complexes

Abstract: Due to the widespread use of fossil fuels and the resulting global warming, development of sustainable catalytic transformations is now more important than ever to obtain our desired fuels and...

Cited by 27 publications

References 98 publications

Oxidation of Organic Compounds Using Water as the Oxidant with H2 Liberation Catalyzed by Molecular Metal Complexes

Oxidation of Organic Compounds Using Water as the Oxidant with H2 Liberation Catalyzed by Molecular Metal Complexes

Shining Visible Light on Reductive Elimination: Acridine–Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids

Base-free, acceptorless dehydrogenative coupling of ethanol to ethyl acetate with PNP complexes

Contact Info

Product

Resources

About

Oxidation of Organic Compounds Using Water as the Oxidant with H₂ Liberation Catalyzed by Molecular Metal Complexes

Oxidation of Organic Compounds Using Water as the Oxidant with H₂ Liberation Catalyzed by Molecular Metal Complexes