Quasi-Catalytic Approach to N-Unprotected Lactams via Transfer

Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Quasi-Catalytic Approach to N‑Unprotected Lactams via Transfer Hydro-amination/Cyclization of Biobased Keto Acids Hongguo Wu,† Wenshuai Dai,‡ Shunmugavel Saravanamurugan,§ Hu Li,*,† and Song Yang*,†

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†

State Key Laboratory Breeding Base of Green Pesticide & Agricultural Bioengineering, Key Laboratory of Green Pesticide & Agricultural Bioengineering, Ministry of Education, State-Local Joint Engineering Lab for Comprehensive Utilization of Biomass, Center for R&D of Fine Chemicals, Guizhou University, 2708 Huaxi Road, Huaxi District, Guiyang, Guizhou 550025, China ‡ Beijing National Laboratory of Molecular Science, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, Beijing 100049, China § Laboratory of Bioproduct Chemistry, Center of Innovative and Applied Bioprocessing (CIAB), Sector 81 (Knowledge City), Mohali 140306, Punjab, India S Supporting Information *

ABSTRACT: Levulinic acid (LA) and formic acid (FA) are concurrently derivable from biomass sugars, and recognized as sustainable feedstock for producing biofuels and chemicals. Herein, a benign and eco-friendly approach using low-cost formamide (FAM) and FA as nitrogen and hydrogen source, respectively, was developed to be highly efficient for the synthesis of 5-methyl-2pyrrolidone (MPLD, up to 93% yield) from LA under quasi-catalytic and solvent-free conditions in a relatively short reaction time of 90 min at 160 °C. Deuterium-labeled and control experiments with 2D NMR illustrated the occurrence of transfer hydroamination process, where FA acted as an acid as well as H-donor. The smooth proceeding of the initial C−N bond formation process mainly contributed to the rapid substrate conversion, while the concurrent amidation process was favorable for the subsequent cyclization to give the desired lactam, as proved by computational calculations and kinetic studies. In addition, this quasi-catalytic system was applicable to the synthesis of various N-unprotected lactams in 76−95% yields from keto acids under benign conditions, and the target product could be simply isolated by extraction. KEYWORDS: Transfer hydrogenation, Biomass conversion, Sustainable catalysis, Amination and amidation, Solvent-free, Levulinic acid

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INTRODUCTION Biomass is sustainable material from plants or waste from animals, which has been developed as promising feedstock for producing various value-added chemicals, functional materials, and biofuels.1−5 The value-added products derived from

biomass are oxygen-rich, featuring functional groups such as hydroxy, ether, carbonyl, carboxyl, and ester groups, which enrich the product variety.6 Among the developed biorefinery products, levulinic acid (LA) is listed in “top 10” chemical opportunities from carbohydrates, as addressed by the U.S. Department of Energy (DOE) in 2004.7 As one of the most promising and sustainable platform chemicals, LA can be further upgraded to commodity chemicals and biofuels, such as angelica lactone, 1,4-pentadiol, alkyl levulinate, 2-methyltetrahydrofuran, and γ-valerolactone.8−10 In addition to the coupling and cleavage of C−O and C−C bonds for catalytic biomass valorization, recent work has also focused on the sustainable production of valuable nitrogen-containing chemicals via C−N bond formation.11,12 N-Substituted lactams, especially N-protected pyrrolidones, are important core scaffolds and intermediates in a wide range of pharmaceutical products, fiber dyes and printing ink, and

Scheme 1. Syntheses of Lactams from Keto Acids

Received: January 21, 2019 Revised: April 16, 2019

© XXXX American Chemical Society

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DOI: 10.1021/acssuschemeng.9b00412 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

Figure 1. Computed model of formamide (FAM) activated by formic acid (FA) via a double proton-transfer transition state with calculated free energies (IS, initial state; TS, transition state; FS, final state).

Figure 2. Distribution of products in conversion of LA to MPLD in the time-course of 0−480 min at varying reaction temperature: (A) 120 °C, (B) 140 °C, (C) 160 °C, and (D) 180 °C. Conditions: 2 mmol LA, 6 mmol FA, 20 mmol FAM.

(boiling point: 189 °C) was used as a solvent with triethylamine as an additive (E-factor: 14.6, Table S2), which pose challenges for waste reduction as well as product isolation and purification.16,17 Different types of primary amines have been employed as nitrogen sources in previous reports (Table S1), where the formation of byproducts (i.e., formamides) from the condensation of the amine and FA is most likely to be an unavoidable disadvantage.18,19 Although several synthetic strategies have been recently reported for the efficient preparation of N-substituted lactams, to the best of our knowledge, direct synthesis of N-unprotected lactams from keto acids is not previously reported, particularly

can also be directly used as solvents and surfactants.13,14 Starting from biomass-derived LA, a number of homogeneous and heterogeneous catalysts have been developed to be efficient for the synthesis of N-protected pyrrolidones (PLD) in good yields using H2, hydrosilane, or bioderivable formic acid (FA) as a hydrogen source (Scheme 1, Table S1).15 Alongside of the remarkable achievements in metal-catalyzed conversion of LA to PLD, a metal-free Leuchart−Wallach reaction system was also developed to be capable of mimicking the tandem reductive amination and amidation process, which shows great potential in practical application with reduced production cost.16 However, the relatively nonvolatile DMSO B


Letter

ACS Sustainable Chemistry & Engineering Scheme 2. Control Experiments*

*

(A) Comparison of the reaction rate of transfer hydro-amination and amidation with or without FA. Conditions: 2 mmol pentan-2-one or pentanoic acid, 6 mmol FA, 20 mmol FAM, 160 °C, 10−300 min. (B) Relative reactivity in amidation. Conditions: 2 mmol 2-pentanone or pentanoic acid, 6 mmol FA, 20 mmol FAM, 160 °C, 90 min for each step. (C) Product distribution in conversion of EL to MPLD. Conditions: 2 mmol EL, 6 mmol FA, 20 mmol FAM, 160 °C, 0−300 min.

Figure 3. 1H−13C NMR HSQC (DEPT 90) spectra of LA-to-MPLD conversion using (A) normal FA and (B) deuterium-labeled FA. Conditions: 2 mmol LA, 6 mmol FA, 20 mmol FAM, 160 °C, 90 min.

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in the absence of a metal catalyst. Herein, a low-cost and bulk commodity formamide (H2NCHO, FAM) is used in place of primary amines as a nitrogen source, which significantly restricts the undesired condensation reaction with FA due to the strong electron-withdrawing effect of −CHO in FAM to passivate −NH2.20 Notably, FA can serve a catalytic role, activating FAM via a double proton-transfer transition state apart from donating hydride,21 as illustrated by density functional theory (DFT) calculation (Figure 1), thus facilitating the desired amination/amidation process that affords N-unprotected lactams (Scheme 1). Our developed approach was also applicable to a variety of keto acids, and the reaction mechanism was explicitly elucidated.

RESULTS AND DISCUSSION

Initially, the quasi-catalytic and solvent-free conversion of LA to MPLD was investigated with a constant LA/FA/FAM molar ratio of 1:3:10 at varying reaction temperatures of 120−180 °C. The reaction time courses at varying temperatures are shown in Figure 2. It can be clearly seen that the reaction temperature remarkably affects the reaction efficiency. At a relatively low temperature of 120 °C, LA was gradually transformed and reached ca. 90% conversion after 480 min, while the highest MPLD yield was only 15% (Figure 2A). Instead, 4-formamidopentanoic acid (BP-1) and 4-formamidopentanamide (BP-2) were formed in the early stage (0−120 C


Letter


Figure 4. Computed free energy profiles for hydro-amination of LA to BP-1. TS, transition state; IM, intermediate. Conditions and basis set: [T = 160 °C, M062X/6-311g(d,p)]. Values in parentheses are free energies (kJ mol−1) with respect to the starting energy of LA, FA, and FAM.

Figure 5. Schematic illustration of synthesizing purified MPLD from aqueous equivalent LA and FA solution (with 50 equiv. water) derived from hexose sugars.

min−1) in the presence of FA and FAM, based on the reaction kinetics for conversion of 2-pentanone and pentanoic acid at 160 °C, respectively (Scheme 2A, Figures S2 and S3). In this regard, BP-1 seemed to be preferably formed in the early reaction period, while BP-2 would be produced by either prolonging reaction time or elevating temperature, which is consistent with the results provided in Figure 2. However, in the absence of FA, the reaction does not afford N-(pentan-2yl)formamide from 2-pentanone (k < 0.001 min−1), and gave relatively lower conversion of pentanoic acid (51%) with a reaction rate of ca. 0.008 min−1, compared to that with FA (70%) after 180 min. These results demonstrate that FA not only acts as H-donor in the hydro-amination process but also behaves as an acid to promote the C−N bond formation. The relative reactivity in the cyclization of BP-1 and BP-2 to MPLD was evaluated by using in situ generated N-(pentan-2yl)formamide from 2-pentanone and/or pentanamide from pentanoic acid as starting material, followed by amidation (Scheme 2B). Among the tested three experiments, the amidation reaction between N-(pentan-2-yl)formamide and pentanamide was faster than those between pentanoic acid and N-(pentan-2-yl)formamide as well as pentanamide and 2pentanone (Scheme 2B), suggesting that BP-2 rather than BP1 cyclizes to yield MPLD. Furthermore, considering that carboxylate group in LA may influence the reaction progress, ethyl levulinate (EL) instead of LA was also used as substrate for the synthesis of MPLD (Scheme 2C). At the identical temperature of 160 °C, the conversion of EL was found to be comparable to that of LA in the time-course of 0−300 min (Figures S4 and 2C), indicating that the first reaction step should be the hydro-amination of 2-keto in EL and LA to give ethyl 4-formamidopentanoate (EFPT) and BP-1, respectively.

min) and then gradually consumed, with a maximum yield of 6% and 17%, respectively. These results indicate the conversion of BP-1 to BP-2 and to 2-methyl-5-oxo-1pyrrolidinecarboxaldehyde (BP-3), which seemed stable at 120 °C and continuously increased to 55% throughout the reaction (Figure 2A). The increase of reaction temperature results in faster conversion of BP-3 to MPLD, and LA was completely consumed in roughly 90, 30, and 20 min at 140, 160, and 180 °C, respectively (Figure 2B−D). The yields of BP-1 and BP-2 do not exceed 4%, while the BP-3 yield first reaches the plateau and then drops down with prolonged time. These results explicitly show that BP-1, BP-2, and BP-3 are all the key intermediates leading to the target product MPLD. Notably, a high MPLD yield of 82% was obtained at 160 °C in 90 min, and no significant increase in MPLD yield was observed when the reaction duration was expanded to 180 min (Figure 2C). In contrast, longer reaction time was required at lower temperatures (140 °C), and side reactions took place at a temperature of 180 °C (Figure 2B,D), thus resulting in decreased MPLD yields. In addition to the reaction temperature and time, the dosage of both FAM and FA was also found to affect closely the formation of MPLD from LA under above-optimized conditions (160 °C, 90 min), and a moderate MPLD yield (82%) was obtained with the LA/FAM/FA molar ratio of 1:10:3 (Figure S1). This result implies it is crucial to have sufficient nitrogen and hydrogen sources for amination and transfer hydrogenation, respectively. To illustrate the respective function of the reactants, several control experiments were conducted (Scheme 2). It was found that the hydro-amination (k = 0.14 min−1) was much faster than the amidation (k = 0.01 D


Letter


times (Figure S5). The gradual consumption of LA with the formation of MPLD could be clearly observed with the extension of reaction time, by comparing with the NMR spectra of pure MPLD, LA, FA, and FAM (Figure S6). However, no evident peaks belonging to BP-1 and BP-2 intermediates were observed even in a short time (15 min), possibly due to their low concentrations. Although BP-3 was in a relatively high concentration, its characteristic peaks overlapped with FAM (CHO) and MPLD (CH3, CH2, and CH). Gratifyingly, 1H−13C NMR HSQC coupled with DEPT 90 spectra using normal and deuterium-labeled FA could discern the incorporation of D species from FA (Figure 3). This result was confirmed by GC−MS, where MPLD was generated with +1 mass shift besides 99 (m/z, amu) utilizing DCOOD instead of HCOOH (Figure S7). In addition, a primary kinetic isotope effect (kH/kD = 2.6) was detected, consistent with the fact that the hydride is derived from FA via the hydrogen transfer process.22 Either reductive amination or amidation have previously been reported to be key reaction steps in the conversion of LA to MPLD over metal catalysts.23−25 In the present study, the reaction pathways proceeding via both transfer hydroamination and amidation were calculated to be thermodynamically favorable (Figures S8 and S9). However, the activation energies to overcome the transition state (TS) for the initial amination of LA (TS1: ΔG = 149 kJ mol−1) followed by transfer hydrogenation (TS3: ΔG = 174 kJ mol−1; Figure 4) were relatively lower than that for first amidation (TS1*: ΔG = 192 kJ mol−1; Figure S10) and then transfer hydrogenation (TS3*: ΔG = 183 kJ mol−1; Figure S11). It is worth noting that the experimental activation energy was found to be 140 kJ mol−1 (Figure S12), which is close to that of the initial LA amination (149 kJ mol−1). These results explicitly demonstrate that the pronounced conversion of LA is ascribed to the rapid hydro-amination with FAM with the assistance of FA, while the reductive amidation process cannot be eliminated due to its favorable thermodynamics despite of inferior reaction kinetics, which is in good agreement with the formation of BP1 and BP-2 as the key intermediates toward MPLD (Figure 2). In view of the relatively higher stability of BP-3 (free energy: −75 kJ mol−1, relative to LA and FAM; Figure S8) with moderate yields during the reaction (Figure 2), a certain amount of water (1−50 equiv) was added and proposed to undergo hydrolysis to release MPLD. Interestingly, the coaddition of 30 equiv. water (relative to LA) could significantly raise the yield of MPLD from 82% to 93% at 160 °C for 90 min, or even increase to 95% after 120 min under otherwise identical conditions (Figure S13). Typically, equivalent LA and FA was obtained from around 15 wt % aqueous sugar solutions with an acid.26 Thanks to the excellent compatibility of our developed reaction system with water, good MPLA yield (81%) could be achieved from aqueous equivalent LA and FA solution (with 50 equiv. water) at 160 °C after 120 min. A moderate yield of MPLD (ca. 43%) could be obtained directly from glucose via a two-step reaction process including (i) aqueous HCl-catalyzed glucose conversion to LA and FA, and then (ii) transfer hydro-amination/ cyclization with FAM to give the product. It is interesting to note that MPLD was able to be isolated in ca. 95% purity by extraction with methyl isobutyl ketone (MIBK) or CH2Cl2 followed by evaporation under reduced pressure (Figure 5). Encouraged by the predominant performance of the developed benign and eco-friendly protocol (Table S1), the

Table 1. Transfer Hydroamination/Cyclization of Various Keto Acids with FAM to N-Unprotected Lactams*

*

Conditions: 2 mmol keto acid, 6 mmol FA, 20 mmol FAM, 30 equiv. water, 160 °C. aAmmonia solution (28% NH3 in H2O) instead of FAM and water was used.

However, EFPT was found be more stable than BP-1 (Figures S4 and 2C), and a much longer reaction time (180 min) was required to achieve comparable MPLD yield of 83% from EL, in comparison with that from LA (90 min, 82% yield). According to these results, it can be concluded that BP-1 rapidly derived from LA, as compared with EFPT, was more prone to be further converted to BP-2 that is easier to undergo cyclization under identical conditions. To track the product distribution in the conversion of LA to MPLD, ex situ 1H NMR studies were performed at various E


Letter

ACS Sustainable Chemistry & Engineering Notes

substrate scope was further expanded to a variety of keto acids to produce corresponding N-unprotected lactams (Table 1). Satisfactorily, this reaction system is applicable to the synthesis of both γ- and δ-lactams substituted with either alkyl or aromatic groups, in good yields of up to 95%. Although the electron-withdrawing substituents on aromatic rings restrict the reactivity to a certain extent, moderate to good yields of lactams (78−86%) could be achieved by slightly prolonging the reaction time to 120−180 min. It was noteworthy that ammonia solution could be used instead of FAM as nitrogen source, affording MPLD in 76% yield (Table 1, entry 10). Despite of obtaining relatively lower MPLD yield, the reaction system using the nitrogen source NH3 generates much less waste (E-factor: 2.0), and has a relatively higher reaction mass efficiency (RME) and atom economy than that using FAM (Table S2). These results indicate that the cascade transfer hydro-amination and cyclization of NH3 and keto acids would be promising for the synthesis of N-unprotected lactams from an environmental perspective, while further optimizing the reaction system with respect to improving the product yield is highly desirable. On the other hand, the decline of FAM and FA dosage by designing a suitable reactor can be another promising approach to promote the reaction progress while fulfilling the green chemistry metrics.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21666008, 21576059), Fok Ying-Tong Education Foundation (161030), Key Technologies R&D Program of China (2014BAD23B01), and Guizhou Science & Technology Foundation ([2018]1037, [2017] 5788). S.S. thanks Department of Biotechnology (Government of India), New Delhi, India. We also acknowledge the Computer Network Information Center of the Chinese Academy of Sciences for performing theoretical calculations.

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CONCLUSION In summary, a quasi-catalytic solventless protocol was developed to be capable of efficiently producing a wide range of N-unprotected lactams (76−95% yields) from keto acids in the presence of FA and FAM or ammonia solution at 160 °C in 90−180 min. Deuterium-labeled experiments illustrated the occurrence of transfer hydro-amination, as evidenced by 2D NMR. DFT calculations and experimental kinetic studies verified that the fast initial transfer hydroamination contributed to the superior reaction rate, while the concurrent amidation process was favorable for the subsequent cyclization to give the desired lactams. This benign and ecofriendly reaction system shows great potential in large-scale synthesis, in which the target product (e.g., MPLD) could be facilely isolated by extraction and evaporation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00412.

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REFERENCES

(1) Hu, L.; Xu, J.; Zhou, S.; He, A.; Tang, X.; Lin, L.; Xu, J.; Zhao, Y. Catalytic advances in the production and application of biomassderived 2, 5-dihydroxymethylfuran. ACS Catal. 2018, 8, 2959−2980. (2) Tang, X.; Zuo, M.; Li, Z.; Liu, H.; Xiong, C.; Zeng, X.; Sun, Y.; Hu, L.; Liu, S.; Lei, T.; Lin, L. Green processing of lignocellulosic biomass and its derivatives in deep eutectic solvents. ChemSusChem 2017, 10, 2696−2706. (3) Filiciotto, L.; Luque, R. Biomass promises: A bumpy road to a renewable economy. Curr. Green Chem. 2018, 5, 47−59. (4) Rodríguez-Padrón, D.; Algarra, M.; Tarelho, L. A.; Frade, J.; Franco, A.; de Miguel, G.; Jiménez, J.; Rodríguez-Castellón, E.; Luque, R. Catalyzed microwave-assisted preparation of carbon quantum dots from lignocellulosic residues. ACS Sustainable Chem. Eng. 2018, 6, 7200−7205. (5) Li, H.; Riisager, A.; Saravanamurugan, S.; Pandey, A.; Sangwan, R. S.; Yang, S.; Luque, R. Carbon-increasing catalytic strategies for upgrading biomass into energy-intensive fuels and chemicals. ACS Catal. 2018, 8, 148−187. (6) Li, H.; Fang, Z.; Smith, R. L., Jr; Yang, S. Efficient valorization of biomass to biofuels with bifunctional solid catalytic materials. Prog. Energy Combust. Sci. 2016, 55, 98−194. (7) Werpy, T.; Petersen, G. Top value added chemicals from biomass. Vol. 1 - Results of screening for potential candidates from sugars and synthesis gas; Department of Energy: Washington DC, 2004. (8) Xue, Z.; Liu, Q.; Wang, J.; Mu, T. Valorization of levulinic acid over non-noble metal catalysts: Challenges and opportunities. Green Chem. 2018, 20, 4391−4408. (9) Pileidis, F. D.; Titirici, M. M. Levulinic acid biorefineries: New challenges for efficient utilization of biomass. ChemSusChem 2016, 9, 562−582. (10) Li, H.; Fang, Z.; Luo, J.; Yang, S. Direct conversion of biomass components to the biofuel methyl levulinate catalyzed by acid-base bifunctional zirconia-zeolites. Appl. Catal., B 2017, 200, 182−191. (11) Sun, Z.; Barta, K. Cleave and couple: Toward fully sustainable catalytic conversion of lignocellulose to value added building blocks and fuels. Chem. Commun. 2018, 54, 7725−7745. (12) Hülsey, M. J.; Yang, H.; Yan, N. Sustainable routes for the synthesis of renewable heteroatom-containing chemicals. ACS Sustainable Chem. Eng. 2018, 6, 5694−5707. (13) Ogiwara, Y.; Uchiyama, T.; Sakai, N. Reductive amination/ cyclization of keto acids using a hydrosilane for selective production of lactams versus cyclic amines by switching of the indium catalyst. Angew. Chem., Int. Ed. 2016, 55, 1864−1867. (14) Sun, Z.; Chen, J.; Tu, T. NHC-based coordination polymers as solid molecular catalysts for reductive amination of biomass levulinic acid. Green Chem. 2017, 19, 789−794. (15) Yan, L.; Yao, Q.; Fu, Y. Conversion of levulinic acid and alkyl levulinates into biofuels and high-value chemicals. Green Chem. 2017, 19, 5527−5547. (16) Wei, Y.; Wang, C.; Jiang, X.; Xue, D.; Liu, Z. T.; Xiao, J. Catalyst-free transformation of levulinic acid into pyrrolidinones with formic acid. Green Chem. 2014, 16, 1093−1096.

Experimental section; collection of previous results; kinetic study; reaction parameter effects; computational free energy diagrams; 1H and 13C NMR data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel: (+86)851 88292171. Fax: (+86)851 88292170. E-mail: [email protected] (S. Yang). *Tel: (+86)851 88292171. Fax: (+86)851 88292170. E-mail: [email protected] (H. Li). ORCID

Shunmugavel Saravanamurugan: 0000-0002-3980-5020 Hu Li: 0000-0003-3604-9271 Song Yang: 0000-0003-1301-3030 F


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ACS Sustainable Chemistry & Engineering (17) Vidal, J. D.; Climent, M. J.; Concepcion, P.; Corma, A.; Iborra, S.; Sabater, M. J. Chemicals from biomass: Chemoselective reductive amination of ethyl levulinate with amines. ACS Catal. 2015, 5, 5812− 5821. (18) Martínez, J. J.; Silva, L.; Rojas, H. A.; Romanelli, G. P.; Santos, L. A.; Ramalho, T. C.; Brijaldo, M. H.; Passos, F. B. Reductive amination of levulinic acid to different pyrrolidones on Ir/SiO2SO3H: Elucidation of reaction mechanism. Catal. Today 2017, 296, 118−126. (19) Ledoux, A.; Sandjong Kuigwa, L.; Framery, E.; Andrioletti, B. A highly sustainable route to pyrrolidone derivatives−direct access to biosourced solvents. Green Chem. 2015, 17, 3251−3254. (20) Li, H.; Guo, H.; Su, Y.; Hiraga, Y.; Fang, Z.; Hensen, E. J.; Watanabe, M.; Smith, R. L. N-formyl-stabilizing quasi-catalytic species afford rapid and selective solvent-free amination of biomass-derived feedstocks. Nat. Commun. 2019, 10, 699. (21) Cybulski, H.; Sadlej, J. A computational study of the nuclear magnetic resonance parameters for double proton exchange pathways in the formamide-formic acid and formamide-formamidine complexes. Phys. Chem. Chem. Phys. 2009, 11, 11232−11242. (22) Boddien, A.; Mellmann, D.; Gärtner, F.; Jackstell, R.; Junge, H.; Dyson, P. J.; Laurenczy, G.; Ludwig, R.; Beller, M. Efficient dehydrogenation of formic acid using an iron catalyst. Science 2011, 333, 1733−1736. (23) Gao, G.; Sun, P.; Li, Y.; Wang, F.; Zhao, Z.; Qin, Y.; Li, F. Highly stable porous-carbon-coated Ni catalysts for the reductive amination of levulinic acid via an unconventional pathway. ACS Catal. 2017, 7, 4927−4935. (24) Touchy, A. S.; Hakim Siddiki, S. M. A.; Kon, K.; Shimizu, K. I. Heterogeneous Pt catalysts for reductive amination of levulinic acid to pyrrolidones. ACS Catal. 2014, 4, 3045−3050. (25) Rodríguez-Padrón, D.; Puente-Santiago, A. R.; Balu, A. M.; Romero, A. A.; Munoz-Batista, M. J.; Luque, R. Benign-by-design orange peel-templated nanocatalysts for continuous flow conversion of levulinic acid to N-heterocycles. ACS Sustainable Chem. Eng. 2018, 6, 16637−16644. (26) Du, X. L.; He, L.; Zhao, S.; Liu, Y. M.; Cao, Y.; He, H. Y.; Fan, K. N. Hydrogen-independent reductive transformation of carbohydrate biomass into γ-valerolactone and pyrrolidone derivatives with supported gold catalysts. Angew. Chem., Int. Ed. 2011, 50, 7815−7819.

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Quasi-Catalytic Approach to N-Unprotected Lactams via Transfer

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