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(a) Trends in carbonyl reactivity. (b) Resonance stabilization. (c) Manganese-catalyzed transfer hydrogenation of amides, carbamates, urea derivatives, and polyurethanes reported in this work.
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The reduction of polar bonds, in particular carbonyl groups, is of fundamental importance in organic chemistry and biology. Herein, we report a manganese pincer complex as a versatile catalyst for the transfer hydrogenation of amides, carbamates, urea derivatives, and even polyurethanes leading to the corresponding alcohols, amines, and methanol as...
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... 28-30 urea derivatives [31][32][33] as well as polyurethanes 34,35 can be produced from CO 2 , and their hydrogenation offers a facile approach to the indirect reduction of CO 2 to methanol. 36 However, compared to other carbonyl derivatives these compounds are less reactive towards hydrogenation and nucleophilic attack to the carbonyl group (Fig. 1a). 2 This can be ascribed to resonance stabilization (Fig. 1b). Amide resonance is leading to the delocalization of the nitrogen electronic lone pair which lowers the reactivity of the carbonyl group (Fig. 1b, I). 37 Also, intermolecular hydrogen bonding between amide groups can increase their stability additionally (Fig. 1b, II). 38 In ...
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... 34,35 can be produced from CO 2 , and their hydrogenation offers a facile approach to the indirect reduction of CO 2 to methanol. 36 However, compared to other carbonyl derivatives these compounds are less reactive towards hydrogenation and nucleophilic attack to the carbonyl group (Fig. 1a). 2 This can be ascribed to resonance stabilization (Fig. 1b). Amide resonance is leading to the delocalization of the nitrogen electronic lone pair which lowers the reactivity of the carbonyl group (Fig. 1b, I). 37 Also, intermolecular hydrogen bonding between amide groups can increase their stability additionally (Fig. 1b, II). 38 In comparison with amides, carbamates and urea derivatives are ...
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... compared to other carbonyl derivatives these compounds are less reactive towards hydrogenation and nucleophilic attack to the carbonyl group (Fig. 1a). 2 This can be ascribed to resonance stabilization (Fig. 1b). Amide resonance is leading to the delocalization of the nitrogen electronic lone pair which lowers the reactivity of the carbonyl group (Fig. 1b, I). 37 Also, intermolecular hydrogen bonding between amide groups can increase their stability additionally (Fig. 1b, II). 38 In comparison with amides, carbamates and urea derivatives are even less reactive, mainly due to the additional resonance stabilization by the second oxygen or nitrogen atom, respectively (Fig. 1b, III and IV). 39 ...
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... to the carbonyl group (Fig. 1a). 2 This can be ascribed to resonance stabilization (Fig. 1b). Amide resonance is leading to the delocalization of the nitrogen electronic lone pair which lowers the reactivity of the carbonyl group (Fig. 1b, I). 37 Also, intermolecular hydrogen bonding between amide groups can increase their stability additionally (Fig. 1b, II). 38 In comparison with amides, carbamates and urea derivatives are even less reactive, mainly due to the additional resonance stabilization by the second oxygen or nitrogen atom, respectively (Fig. 1b, III and IV). 39 Numerous procedures have been developed for the reduction of amide-related substrates, e.g. hydrogenation, 40 ...
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... of the carbonyl group (Fig. 1b, I). 37 Also, intermolecular hydrogen bonding between amide groups can increase their stability additionally (Fig. 1b, II). 38 In comparison with amides, carbamates and urea derivatives are even less reactive, mainly due to the additional resonance stabilization by the second oxygen or nitrogen atom, respectively (Fig. 1b, III and IV). 39 Numerous procedures have been developed for the reduction of amide-related substrates, e.g. hydrogenation, 40 hydrosilylation 41-43 and hydroborylation. 44,45 Among them, hydrogenation methods stand out as a green approach. During the past decade, hydrogenation of amides through either C-N or C-O bond cleavage were widely studied. ...
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... have not been reported. Based on our interest in the transfer hydrogenation 64,65 and borrowing hydrogen reactions, 66 we envisioned to overcome this limitation by using earth-abundant metal catalysts. Herein, we report the rst examples of the transfer hydrogenation of carbamates, urea derivatives and amides under C-N bonds cleavage (Fig. 1c). Even polyurethanes can be reduced to the corresponding diols, amines and methanol, realizing a transfer hydrogenative degradation of commercial polyurethane materials into valuable ...
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The selective transformation of a less reactive carbonyl moiety in the presence of more reactive ones can realize straightforward and environmentally benign chemical processes. However, such a transformation is highly challenging because the reactivity of carbonyl compounds, one of the most important functionalities in organic chemistry, depends on...
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... Double phenylation of 7g afforded diphenylsulfone 4,4′bis(diarylsulfonium) salt 20 (Fig. 8c). Subsequent uorination of 20 gave bis(4-uorophenyl)sulfone (21) in 87% yield, which is a monomer of diphenylsulfone-based polymers such as PSU, [72][73][74][75][76][77][78][79][80] PESU, 81,82 and PEES. 83 In addition, 20 reacted with p-methoxyphenol in the presence of Cs 2 CO 3 to give 4,4'-bis(p-anisyloxy)diphenylsulfone (22) in 79% yield. ...
As the effective use of carbon resources has become a pressing societal issue, the importance of chemical recycling of plastics has increased. The catalytic depolymerization method for plastics is a promising approach for creating valuable products under efficient and mild conditions. Although depolymerization methods for various commodity plastics and several engineering plastics have been developed, the degradation of robust super engineering plastics that have very high heat resistance, chemical resistance, and low solubility is nearly unexplored. Herein, we report the catalytic depolymerization of oxyphenylene-based super engineering plastics such as polyetheretherketone, polysulfone, and polyetherimide using thiols via selective carbon–oxygen main chain cleavage to form monomer-type molecules, electron-deficient arenes with sulfur functional groups and bisphenols. The catalyst combination of a bulky phosphazene base P 4 - t Bu with inorganic bases such as tripotassium phosphate or cesium carbonate enabled smooth depolymerization by activating the thiols to form reactive thiolates. This depolymerization method could be utilized with carbon- or glass fiber-enforced polyetheretherketone materials and a consumer resin. The sulfur functional groups in one product could be transformed to amino and sulfonium groups and fluorine by using suitable catalysts. Notably these fluorinated products are the monomers of the parent super engineering plastics.
... In fact, Milstein reported that a competitive reaction of urea and formamide using a Ru catalyst resulted in the selective hydrogenolysis of formamide 29 . Since Milstein reported the Ru-catalyzed hydrogenolysis of urea derivatives into two molecules of amine and methanol in 2011 29 , several catalytic systems, including Ru 30-35 , Mn [35][36][37] , and Ir 38 , have been used for the same conversion (Fig. 1d). As formamide intermediates are more reactive than urea, urea derivatives are fully reduced to amines and methanol under these catalytic systems, with the exception using a Ru-triphos catalyst to give a mixture of formamides and amines from different diaryl-and dialkylureas 31,39 . ...
... In this catalytic system, however, not only urea but also ester, amide, and other carbonyl functionalities were hydrogenated under similar conditions (vide infra) 31 . Indeed, urea-selective hydrogenolysis in the presence of more reactive carbonyl functionalities, such as esters, has never been achieved using these catalysts and is believed to be unfeasible [29][30][31][32][33][34][35][36][37][38][39] . ...
... Next, we focused on the hydrogenolysis of unsymmetric urea derivatives (Fig. 3d). Because two C-N bonds are cleaved in the hydrogenolysis of urea derivatives in previous reports [29][30][31][32][33][34][35][36][37][38] , the regioselectivity of the first C-N bond cleavage in unsymmetric ureas has not yet been addressed. When a methyl group is introduced onto the one nitrogen atom of 1,3-diphenylurea (1ao), the hydrogenolysis regioselectively occurred at the C-N bond of the secondary amine to give formanilide (2a) and N-methylaniline (3o) in 84 and 88% yields, respectively, along with small amounts of 3a (3%) under Condition A. Although exchange reaction between an amide and an amine occasionally takes place under harsh conditions 7 , the reaction between products 2a and 3o was negligible for urea 1ao under Condition A ( Supplementary Fig. 19). ...
The selective transformation of a less reactive carbonyl moiety in the presence of more reactive ones can realize straightforward and environmentally benign chemical processes. However, such a transformation is highly challenging because the reactivity of carbonyl compounds, one of the most important functionalities in organic chemistry, depends on the substituents on the carbon atom. Herein, we report an Ir catalyst for the selective hydrogenolysis of urea derivatives, which are the least reactive carbonyl compounds, affording formamides and amines. Although formamide, as well as ester, amide, and carbamate substituents, are considered to be more reactive than urea, the proposed Ir catalyst tolerated these carbonyl groups and reacted with urea in a highly chemoselective manner. The proposed chemo- and regioselective hydrogenolysis allows the development of a strategy for the chemical recycling of polyurea resins.
... However, it is worth noting that the common tendency in the activity of a carbonyl group towards hydrogenation reaction depends on its polarizability, so urea derivatives are the most difficult to be hydrogenated within carbonyl compounds including aldehyde, ester, acid, amide, carbonate, and urea. [8] Moreover, the hydrogenation of formamides is more easily than the urea derivatives, mainly due to the additional resonance stabilization of the second nitrogen atom of the urea starting material. [4b,c,9] As previously first reported by Milstein and coworkers, [4b] hydrogenation of urea derivatives to the corresponding amines and methanol, formamides existed as intermediates without excessive accumulation. ...
The utilization of CO2 as a non‐toxic and cheap feedstock for C1 is a desirable route to achieve high value‐added chemicals. In this context, we report a highly efficient ruthenium‐catalyzed semi‐hydrogenation reaction of CO2‐derived ureas. Various alkyl and aryl urea derivatives were successfully hydrogenated to obtain the corresponding recyclable amines and formamides (up to 97 % yield), highlighting the good substrate applicability of this method, which makes this method a sustainable alternative for the hydrogenation of CO2 to formamides in the presence of amines. In the meantime, we have discovered a new pathway that enables rapid hydrogenation of urea derivatives even at lower H2 pressure (<5 bar). This methodology might provide a new insight into the reduction functionalization of CO2 under mild pressure to form new C−N bond. Based on the control experiments and the observed intermediate products, we clarify the mechanism for selective semi‐hydrogenation of ureas.
... In the process of establishing the circular economy for PU, Werner and co-workers developed a manganese PNP pincer complex (49) that catalyzed depolymerization of the PU using the transfer dehydrogenation method. 207,208 This report successfully demonstrated the reduction of carbamates, amides, urea derivatives, and PU using isopropanol as a hydride source. The authors reported the first example of transfer dehydrogenation on PUs. ...
Polymers have become an indispensable part of our daily lives, and today we produce around 370 MT of plastic per year. Only about 20% of it is being recycled, and the rest, 80%, is unleashed into the environment without appropriate treatment. This calls forth the evaluation of strategies available for mitigating the menace of “after-use” plastic waste. Various approaches have evolved over a decade and are at different levels of development. Plastic depolymerization and upcycling are considered some of the most prominent and long-term solutions. The metal-catalyzed depolymerization of plastic waste to chemical feedstocks has emerged as one of the promising ways to address global plastic pollution. Therefore, this review aims to examine the available metal-catalyzed depolymerization methods, notify recent progress, pinpoint current gaps, and gauge the potential of this strategy. Both homogeneous and heterogeneous catalysts have been reported to depolymerize various polymers over the last decade. Considerable advances have been reported in the metal-mediated depolymerization of polyolefins, polyesters, polycarbonates, polyurethanes, polyamides, and polyethers. The depolymerization of the above polymers produces the monomers or intermediates, which can be used again for polymerization and thus brings the waste polymers back into circularity. The overview debates the usage of high temperatures, sophisticated ligands, expensive metals, stoichiometric reagents, etc., in metal-catalyzed depolymerization. Thus, this review summarizes the current understanding of the fundamental science of metal-catalyzed plastic depolymerization, the remaining scientific challenges, and the potential opportunities.
... The recycling of real-life polyurethanes has not been addressed in the report. [11] However, it is highly desirable to develop an efficient catalytic system based on an earth-abundant metal, as a cheaper and greener alternative to precious transition metal catalysts. ...
Chemical recycling, in particular hydrogenative depolymerization, offers a promising way to utilize plastic waste. This report covers the manganese‐catalyzed hydrogenation of polyurethane materials to the corresponding monomeric units. The key to success is a Mn pincer complex as a potent hydrogenation catalyst in combination with elevated temperatures (up to 200 °C) and appropriate solvents to ensure sufficient solubility of the polymers. A wide range of polyurethane samples of varying polyol and isocyanate compositions, some of which feature significant amounts of urea functionalities, are depolymerized, releasing polyetherols and diaminotoluene (TDA) in yields of up to 89 % and 76 %, respectively.
As the effective use of carbon resources has become a pressing societal issue, the importance of chemical recycling of plastics has increased. The catalytic chemical decomposition for plastics is a promising approach for creating valuable products under efficient and mild conditions. Although several commodity and engineering plastics have been applied, the decompositions of stable resins composed of strong main chains such as polyamides, thermoset resins, and super engineering plastics are underdeveloped. Especially, super engineering plastics that have high heat resistance, chemical resistance, and low solubility are nearly unexplored. In addition, many super engineering plastics are composed of robust aromatic ethers, which are difficult to cleave. Herein, we report the catalytic depolymerization-like chemical decomposition of oxyphenylene-based super engineering plastics such as polyetheretherketone and polysulfone using thiols via selective carbon–oxygen main chain cleavage to form electron-deficient arenes with sulfur functional groups and bisphenols. The catalyst combination of a bulky phosphazene base P4-tBu with inorganic bases such as tripotassium phosphate enabled smooth decomposition. This method could be utilized with carbon- or glass fiber-enforced polyetheretherketone materials and a consumer resin. The sulfur functional groups in one product could be transformed to amino and sulfonium groups and fluorine by using suitable catalysts.
The industrial production of methanol through CO hydrogenation using the Cu/ZnO/Al2O3 catalyst requires harsh conditions, and the development of new catalysts with low operating temperatures is highly desirable. In this study, organic biomimetic FLP catalysts with good tolerance to CO poison are theoretically designed. The base-free catalytic reaction contains the 1,1-addition of CO into a formic acid intermediate and the hydrogenation of the formic acid intermediate into methanol. Low-energy spans (25.6, 22.1, and 20.6 kcal/mol) are achieved, indicating that CO can be hydrogenated into methanol at low temperatures. The new extended aromatization–dearomatization effect involving multiple rings is proposed to effectively facilitate the rate-determining CO 1,1-addition step, and a new CO activation model is proposed for organic catalysts.
The hydrogenation of amides and other less electrophilic carbonyl derivatives with an N–C=O functionality requires significant improvements in scope and catalytic activity to be a genuinely useful reaction in industry. Here, we report the results of a study that examined whether such reactions are further disadvantaged by nitrogen-containing compounds such as aliphatic amines acting as inhibitors on the catalysts. In this case, an enantiomerically pure manganese catalyst previously established to be efficient in the hydrogenation of ketones, N-aryl-imines, and esters was used as a prototype of a manganese catalyst. This was accomplished by doping a model ester hydrogenation with various nitrogen-containing compounds and monitoring progress. Following from this, a protocol for the catalytic hydrogenation of amides and polyurethanes is described, including the catalytic hydrogenation of an axially chiral amide that resulted in low levels of kinetic resolution. The hypothesis of nitrogen-containing compounds acting as an inhibitor in the catalytic hydrogenation process has also been rationalized by using spectroscopy (high-pressure infrared (IR), nuclear magnetic resonance (NMR)) and mass spectrometry studies.
