Hot water-promoted catalyst-free reductive cycloamination of (bio-)keto acids with HCOONH4 toward cyclic amides

https://doi.org/10.1016/j.supflu.2019.104698Get rights and content

Highlights

  • A combo hot-water/HCOONH4 system enables cycloamination of bio-based levulinic acid.

  • HCOONH4 is capable of acting as both H and N source without any catalyst or additive.

  • H2O greatly improves reactivity/rate by promoting H and N source release from HCOONH4.

  • This catalyst-free/sustainable protocol is applicable to giving various cyclic amides.

  • The reaction pathways are elucidated by deuterium-labeling and kinetic studies.

Abstract

Controllable functionalization of targeted oxygen-containing species of biomass feedstocks is one of the prevalent approaches to biofuels and important chemicals, and well-designed functional catalytic materials are typically required to promote the smooth proceeding of the desired conversion processes. In this work, HCOONH4 was demonstrated to be capable of acting as both hydrogen and nitrogen source in the absence of any catalyst and additive for the reductive cycloamination of bio-based levulinic acid (LA) to 5-methyl-2-pyrrolidone (MPL) with more than 90 % in just 60 min at 180 °C. Pressurized hot water remarkably enabled the reaction efficiency and rate by promoting the hydrolysis of HCOONH4 to liberate ammonia and formic acid for the cascade reactions, and this catalyst-free protocol is also applicable to the efficient synthesis of various cyclic amides from relevant keto acids. Moreover, the reaction pathways were investigated by conducting deuterium-labeling experiments and kinetic studies of selected reactions.

Introduction

As a unique and abundant source of non-edible while renewable organic carbon, lignocellulosic biomass has been deemed as a promising substitute for the dwindling fossil fuel reserves [[1], [2], [3], [4]]. In view of the oxygen-rich feature of biomass-derived feedstocks (e.g., over 50 % oxygen by weight for pentose and hexose sugars, and 30−40 wt% oxygen content for lignin monomers), a variety of catalytic strategies such as pyrolysis and hydrodeoxygenation have been developed to selectively accomplish Csingle bondO bond scission through removal of water, which can be further integrated with either Csingle bondC coupling or Csingle bondC bond cleavage for the production of liquid fuels and chemicals [[5], [6], [7], [8], [9], [10]]. It is worth noting that bio-based platform molecules functionalized with different types of oxygen species (e.g., carboxyl, carbonyl, and hydroxy groups) formed by partial oxidation or hydrogenation open new avenues to access to further downstream chemical commodities (e.g., plastic polymers and pharmaceuticals) [[11], [12], [13], [14], [15], [16], [17]].

Nitrogen-containing compounds are versatile chemical feedstocks extensively applied in the pharmaceutical industry, and also successfully employed as catalytic materials, dyes, surfactants, solvents, and so on [[18], [19], [20]]. Among these nitrogenous molecules, N-substituted pyrrolidones have been disclosed to be efficiently synthesized by catalytic tandem reductive amination and cyclization of various primary amines with levulinic acid (LA, Fig. 1) [[21], [22], [23], [24], [25]], which is facilely accessible from pentose and hexose carbohydrates in the presence of an acidic catalyst [[26], [27], [28]]. To implement the two-step reaction process (Fig. 1), metal catalysts (e.g., Au, Ru, Ir, Pt, Ni, Cu, and Fe) in combination with HCOOH or H2 as H-donor are developed to be efficient for the synthesis of N-substituted pyrrolidones in good yields (84–99 %) at moderate reaction temperatures of 80−150 °C for appropriate control of product selectivity but requiring relatively long reaction time (6−24 h) [[29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]]. Accompanying with the great achievements obtained by metal-mediated catalytic approaches, a metal-free reaction system consisting of triethylamine (Et3N) and HCOOH developed by Wei et al. [40] was able to mimic the Leuckart-Wallach reaction to afford N-substituted pyrrolidones (72–93 % yields) from LA and primary amines in DMSO (dimethyl sulfoxide) at 100 °C after 12−15 h. However, the prerequisite of a basic additive Et3N and non-volatile organic solvent DMSO makes the overall reaction process produce unwanted waste, posing difficulty in product separation.

Using sub- or supercritical water as conversion medium that is widely available, non-toxic, eco-friendly and inexpensive, hydrothermal processes have being developed as green chemistry approaches for upgrading of biomass to biofuels and bio-based chemicals, especially applicable to the direct conversion of wet biomass (70−90 wt% natural water content) without drying [41,42]. Hydrothermal gasification is an important tool for a one-step biorefinery to produce hydrogen, and methane in some cases [43], while typically suffering from high-energy consumption, corrosion caused by water above its critical point, and largely unexplored reaction kinetics and routes [44,45]. Unlike industrial biomass gasification processes, scale-up of hydrothermal carbonization and liquefaction is quite limited because of high organic loadings in the aqueous phase [44], and heating up and cooling down a large amount of water are the dominant challenge toward all hydrothermal processes [46]. At relatively low processing temperatures (≤180 °C), employing pressurized hot water without any additive is likely able to eliminate the complex processes of desalination and neutralization and avoid the recovery of organic solvents, thus possibly showing great potential in minimizing the waste generation [47].

In the present study, ammonium formate (HCOONH4) assisted by pressurized hot water is capable of in situ releasing HCOOH and NH3·H2O to be respectively used as hydrogen and nitrogen source, which remarkably enables the reductive cycloamination of LA to yield 5-methyl-2-pyrrolidone (MPL), an unsubstituted cyclic amide rather than N-substituted pyrrolidones that have been reported everywhere. This catalyst-free, single-step protocol is also illustrated to be suitable for synthesizing a wide range of cyclic amides from various keto acids. Moreover, the reaction kinetics and mechanism are also investigated to understand the predominant pathways.

Section snippets

Materials

Levulinic acid (LA, 99 %), 3-benzoylpropionic acid (99 %), 5-methyl-2-pyrrolidone (MPL, 98 %), ethyl levulinate (≥98 %), 4-acetylbutyric acid (97 %), 3-(4-fluorobenzoyl)propionic acid (97 %), 7-oxooctanoic acid (98 %), 4-oxo-4-(2-thienyl)butyric acid (97 %), deuterium oxide (D2O, 99.994 atom % D), and DMSO-d6 (99.9 atom % D) were bought from Sigma-Aldrich (Shanghai). Ammonium formate (HCOONH4, 99 %), 4-(4-fluorobenzoyl)butyric acid (98 %), 6-oxoheptanoic acid (98 %), 4-benzoylbutyric acid (97

Effect of water content and HCOONH4 dosage on reductive cycloamination of LA

Typically, it is necessary to include both hydrogen (H2 or HCOOH) and nitrogen sources (primary amines) for implementing the reductive amination process, followed by cyclization capable of leading to the formation of N-heterocycles [[48], [49], [50]]. In this study, HCOONH4 was proposed to simultaneously act as H- and N-donor in the reductive cycloamination of LA toward MPL. Preliminary experiments were conducted using 2 mmol LA and 6 equiv. HCOONH4 in the absence of any catalyst or additive at

Conclusions

Without using any catalyst or additive, the developed reaction system consisting of HCOONH4 and water was illustrated to be highly efficient for the reductive cycloamination of biomass-derived LA to MPL with >90 % yields in a reaction time of as short as 60 min under thermal conditions (180 °C), which is comparable and even superior to the previously reported results of metal-mediated catalytic systems. The pressurized hot water played a promotional effect in the in situ release of H and N

Declaration of Competing Interest

The authors declare no conflicts of interest.

Acknowledgments

The authors thank the National Natural Science Foundation of China (21576059, 21666008, 21908033), Fok Ying-Tong Education Foundation (161030), Key Technologies R&D Program of China (2014BAD23B01), and Guizhou Science & Technology Foundation ([2018]1037) for financial support.

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