Green and Efficient: Oxidation of Aldehydes to Carboxylic Acids and

Mar 7, 2018 - (36−46) With our continuing interest in oxidation reaction(47−50) and green organic synthesis,(51−55) herein we disclose an extern...
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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Green and Efficient: Oxidation of Aldehydes to Carboxylic Acids and Acid Anhydrides with Air Kai-Jian Liu,†,§ Yu-Ling Fu,† Long-Yong Xie,† Chao Wu,† Wei-Bao He,† Sha Peng,† Zheng Wang,† Wen-Hu Bao,† Zhong Cao,§ Xinhua Xu,‡ and Wei-Min He*,†,‡,§ †

Hunan Provincial Engineering Research Center for Ginkgo biloba, Hunan University of Science and Engineering, Yongzhou 425100, China ‡ State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China § Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation, Changsha University of Science and Technology, Changsha 410114, China S Supporting Information *

ABSTRACT: An economic, safe, practical, and environmentally benign protocol for the oxidation of aldehydes to carboxylic acids and acid anhydrides with ambient air as the sole oxidant was developed. This oxidation is operationally simple and external catalyst-, initiator-, and base-free, with outstanding functional group tolerance (moisture-, acid-, base-, and oxidant-sensitive groups). It also provides a practical protocol for large scale synthesis (>100 g), late-stage modification of polyfunctional compounds, and one-pot sequential transformation starting from aldehydes. KEYWORDS: Oxidation, Aldehyde, Carboxylic acid, Acid anhydride, Air

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INTRODUCTION Oxidation is a fundamental research field in both laboratories and industries.1−10 Among the oxidation products, carboxylic acids and acid anhydrides are important structural motifs and versatile synthetic intermediates for the manufacture of fine chemicals, pharmaceuticals, and functional materials, some of which are produced in millions of tons each year. For example, 1,4-phthalicacid, the monomer for the polyester (PET), has a worldwide annual consumption of 12.6 million tons. Numerous powerful methods have been developed for oxidizing aldehydes to carboxylic acids11−18 and acid anhydrides.19−21 However, most current oxidations employ a (super)stoichiometric amount of hazardous inorganic or organic oxidants (Scheme 1a). To overcome such drawbacks, the development of an environmentally benign and practical protocol for the oxidation of aldehydes is highly desirable. With its environment-friendly and natural abundance advantages, oxygen has been a more ideal oxidant22,23 than other oxidants for oxidation of aldehydes.24−30 Recently, the base-promoted metal-complex (Rh, Ag, Cu, and Fe) catalyzed oxidation of aldehydes to carboxylic acids with pure O2 as the oxidant in water was reported by Wang,31 Li,32,33 and Han’s © XXXX American Chemical Society

Scheme 1. Oxidation of Aldehydes

groups34 (Scheme 1b). Although these strategies represent a considerable advance, all approaches require two-step operation procedures, metal catalysts, and bases to facilitate the oxidation, which not only leads to environmental issues and high cost but also prevents subsequent transformations (without isolation Received: November 23, 2017 Revised: January 22, 2018

A

DOI: 10.1021/acssuschemeng.7b04400 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

amount of 2a was observed when employing nonether solvents (entries 8−13). Reducing the loading of DPDME to 3 equiv leads to a slight decrease in the yield of 2a (entry 14). Increasing the reaction temperature to 90 °C had little influence on the reaction outcome (entry 15), whereas a trace amount of 2a was observed when the reaction temperature was decreased to 70 °C (entry 16). Conducting the oxidation in the dark did not influence the reaction outcome (entry 17).57,58 These results demonstrated that the oxidation might be a thermodynamically controlled process. The employment of pure oxygen, which is frequently used as an oxidant in aerobic oxidative processes, provided the same yield of 2a in 7 h (entry 18). With the optimal system in hand, we next investigated the substrate scope of this oxidation. As shown in Table 2,

and purification). From a practical and safety point of view, the risk of using pure O2 (routinely stocked in gas cylinders) restricts application of these protocols for most organic chemistry laboratories, especially for large scale industrial manufactures. In addition, the extraction workup to isolate the carboxylic acids from the aqueous mixture will consume more organic solvent than isolation of products from organic reaction medium.35 On the other hand, there are no examples of preparation of acid anhydrides using oxygen as the oxidant. Thus, it is important to develop an economic, safe, practical, and more environmentally benign protocol for the oxidation of aldehydes to carboxylic acids and particularly for direct synthesis of phthalandiones. In contrast to pure oxygen, atmospheric air is a more attractive ideal oxidant and could be employed to eliminate the security issue of oxidation with pure O2.36−46 With our continuing interest in oxidation reaction47−50 and green organic synthesis,51−55 herein we disclose an external catalyst-, initiator-, and base-free oxidation of aldehyde to carboxylic acids and acid anhydrides with ambient air as a sole oxidant and 4 equiv of eco-friendly dipropylene glycol dimethyl ether (DPDME)56 as the solvent.

Table 2. Scope of Aldehydesa

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RESULTS AND DISCUSSION Our investigation started with the oxidation of benzaldehyde 1a in DPDME (4 equiv) with air as the sole oxidant at 80 °C for 10 h, affording equivalent amounts of benzoic acid 2a (Table 1, Table 1. Optimization of Reaction Conditions for the Oxidation of Benzaldehyde to Benzoic Acida

entry

solvent

temp. (°C)

2ab

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17c 18d

DPDME (4 equiv) dimethoxyethane (4 equiv) diethyl ether (4 equiv) diglyme (4 equiv) diethyldigol (4 equiv) 1,4-dioxane (4 equiv) THF (4 equiv) MeCN (4 equiv) EtOH (4 equiv) DCE (4 equiv) toluene (4 equiv) DMSO (4 equiv) DMF (4 equiv) DPDME (3 equiv) DPDME (4 equiv) DPDME (4 equiv) DPDME (4 equiv) DPDME (4 equiv)

80 80 80 80 80 80 80 80 80 80 80 80 80 80 90 70 80 80

>99% 42% 51% 72% 78% 80% trace N.R N.R N.R. N.R. N.R. N.R. 95% >99% trace >99% >99% a Condition: 1 (0.6 mmol), DPDME (2.4 mmol), air, 80 °C; isolated yields were reported. bOxygen balloon was used.

a

Conditions: 1a (0.6 mmol), solvent, temperature, air balloon. bYield was determined by GC-MS. cReaction was performed in the dark room. dPure oxygen was used as the sole oxidant.

monosubstituted benzaldehyde with a set of distinct valuable functional groups can be transformed smoothly into the desired oxidation products (2a−2w). Neither the electronic effect nor the steric factor of benzaldehydes had much influence on the oxidation. Oxidation of 4-nitrobenzaldehyde 1p with pure oxygen as the sole oxidant resulted in 55% conversion of 1p and the desired 4-nitrobenzoic acid 2p was isolated in 51% yield, probably because the strongly electron-withdrawing

entry 1). As anticipated, the nature of the solvent exerted a crucial role on the oxidation outcome, while other related ether solvents provided results in lower yields (entries 2−6). When THF was used instead of DPDME, only a trace amount of 1a was oxidized (entry 7). This can possibly be attributed to the loss of contact between the substrate and air (atmospheric boiling point of THF is 66 °C). No reaction or only a trace B

DOI: 10.1021/acssuschemeng.7b04400 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 3. Large-Scale Synthesis, Scopes of Complex Molecules, and One-Pot Sequential Transformationsa

a

Condition: 1 (0.6 mmol), DPDME (2.4 mmol), air, 80 °C; isolated yields were reported.

Table 4. Scope of Acid Anhydridesa

a

Condition: 1 (0.6 mmol), DPDME (2.4 mmol), air, 80 °C; isolated yields were reported.

DPDME was used as the solvent, and DPDME loading lower than that would decrease the yield of oxidized product. Pleasingly, performing the oxidation of benzaldehyde 1a with an air bag (1 mol, 106 g) in DPDME (3 equiv) at 80 °C afforded the desired 2a in 93% yield (Table 3, 2a). In addition, the benzoic acid 2a was easily isolated and purified by recrystallization. Second, a series of drug intermediates and natural product derivatives underwent the present oxidation well, producing the target products in good to excellent yields (2at−2ay). In order to prove such an operational superiority, several classical chemical processes were conducted. The crude reaction mixture could smoothly perform esterification (1a → 3a),59 Friedel-crafts acylation (1a → 3b),60 and cyanation (1a → 3c)61 in good to excellent yield. Delightfully, the oxidation could be suited for synthesis of phthalic anhydrides (Table 4). A set of substituted phthalandiones could smoothly undergo the oxidation and dehydration, providing the corresponding phthalic anhydrides in good yields (4a−4d). Not surprisingly, the expected double oxidized and dehydrated product 5,5′-oxybis(isobenzofuran1,3-dione) (4e) was obtained smoothly in good yields. No reaction was observed when glutaraldehyde or adipaldehyde was used as the substrates. From the viewpoint of resource reutilization and environmental benignancy, the employment of the reaction system would be favorable with simple separation and reusing. Therefore, the recyclability and reusability of the present system was evaluated under the standard reaction conditions. After the completion of the reaction, the DPDME was recycled

group reduced the activity of CO bond of the benzaldehyde. Preliminary investigations showed that 4-aminobenzaldehyde and 4-((tetrahydro-2H-pyran-2-yl)oxy)benzaldehyde substrates failed to give the desired oxidation products. It is noteworthy that the oxidizable (−SH, −SMe, and −SCF3, 2j−2l), acidsensitive (−OTBDMS, 2h), and base-labile groups (−CN and −CO2Me, 2r and 2s) did not have any influence on yields and remained intact during the oxidation. Pleasingly, the poly substituted benzaldehydes and heterocyclic aromatic as well as polycyclic aldehydes afforded the corresponding benzoic acids with good to excellent yields (2x− 2aa and 2ac−2ag). 1,4-Phthalaldehyde proceeded smoothly to provide the double oxidation product 2ab in 97% yield. Additionally, the oxidation of the ferrocenecarboxaldehyde led to the expected ferrocenecarboxylic acid (2ah) in good yield, which highlighted the feasibility of this mild reaction system for the synthesis of metal-containing carboxylic acids. Various chain lengths and diverse functional groups in a series of aliphatic aldehydes did not affect the oxidation outcome (2ai−2ao). This reaction also permitted the use of formaldehyde substituted with a bulky cyclopentyl group (2ap), cyclohexyl group (2aq), and an adamantyl group (2ar), and good yields were obtained. Moreover, an acid-sensitive N-Boc-L-prolinal substrate is also effective to afford the desired product N-Boc-Lproline (2as) in 91% yield. Once the scope of the oxidation was established, the synthetic practicability of this green chemical process was investigated. First, a large-scale synthesis experiment was conducted. For the small-scale experiment, 4 equiv of C

DOI: 10.1021/acssuschemeng.7b04400 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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On the basis of the above-mentioned results and related work,26,62,63 a possible mechanistic pathway was outlined in Scheme 4. The DPDME first reacts with O2 under heatedcondition to produce the hydroperoxide radical intermediate, which serves as a radical initiator to oxidize benzaldehyde 1a to perbenzoic acid intermediate A. Then the Criegee intermediate B is generated by a nucleophilic addition of A with benzaldehyde 1a, and undergoes intramolecular rearrangement to afford the benzoic acid 2a.

through vacuum distillation and was reused for the next oxidation. As shown in Scheme 2, comparable yields (on average 95.6%) were obtained throughout all of the oxidation cycle, which strongly demonstrated that the DPDME is a reusable solvent and initiator. Scheme 2. Reusability of DPDMEa

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CONCLUSIONS In summary, we have developed an economic, practical, and eco-friendly approach for the solvent-dependent oxidation of aldehydes to carboxylic acids and acid anhydrides. Significant advantages of the present transformation are as follows: (1) the oxidation can be performed without any exrernal catalyst, initiator, base, or additive; (2) the oxidation proceeds under very mild conditions with outstanding functional group compatibility, as proved by well tolerating moisture, acid- and base-sensitive groups as well as easily oxidizable groups, and oxidation of complex poly functionalized compounds; (3) both aromatic and aliphatic aldehydes, as well as heteroaromatic aldehydes, are suitable for oxidation; (4) a series of one-pot sequential transformations starting from unactivated aldehydes were achieved; (5) the desired carboxylic acids crystallized directly from DPDME and were isolated via simple filtration to afford highly pure products without further isolation and purification; and (6) the approach only uses 4 equiv of reusable green solvent and ambient air as the oxidant and can be easily scalled up (>100 g, 3 equiv of DPDME was used). Based on these principal advantages, the present process can be considered as a beneficial complement in the field of aldehyde oxidations.

Condition: 1 (3 mmol), DPDME (12 mmol), air balloon, 80 °C; GC yields were reported. a

To gain insight into the reaction mechanism, some control experiments were conducted (Scheme 3). When the oxidation Scheme 3. Mechanism Research

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EXPERIMENTAL SECTION

General Information. Unless otherwise specified, all reagents and solvents were obtained from commercial suppliers and used without further purification. All reagents were weighed and handled in air at room temperature. 1H NMR spectra were recorded at 400 MHz, and 13 C NMR spectra were recorded at 100 MHz by using a Bruker Avance 400 spectrometer. Chemical shifts were calibrated using residual undeuterated solvent as an internal reference (1H NMR: CDCl3 7.26 ppm, DMSO 2.50 ppm, 13C NMR: CDCl3 77.0 ppm, DMSO 40.0 ppm). The following abbreviations were used to describe peak splitting patterns when appropriate: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, and brs = broad singlet. IR spectra were recorded on a Nicolet iS10 FT-IR spectrometer, and only major peaks were reported in cm−1. Mass spectra were performed on a spectrometer operating on ESI-TOF. General Procedure for the Synthesis of Carboxylic Acids and Acid Anhydrides. A mixture of aldehyde 1 (0.6 mmol) and DPDME (0.43 mL, 2.4 mmol) was added to a 5 mL round flask with an air balloon at room temperature. The reaction typically took 10 h. The progress of the reaction was monitored by TLC or GC-MS. Upon completion, the reaction was cooled to room temperature and concentrated under reduced pressure. The resultant residue was

was conducted in the presence of di-tert-butylhydroxytoluene (BHT) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a radical trapping reagent, the oxidation was seriously suppressed and only a trace amount of 2a was observed (Scheme 3a). These results indicated that a free radical intermediate was involved in this oxidation. However, when H2O2 or anhydrous TBHP was applied as the sole oxidant under nitrogen atmosphere, only a trace amount of 2a could be detected via GC-MS (Scheme 3b). With a catalytic amount of DPDME at 80 °C for 12 h, the desired product 2a was formed in 75% GC yield, leaving 25% of 1a unconsumed. In the absence of DPDME, only an 11% GC yield of 2a was observed after a reaction time of 15 h (Scheme 3c). These results suggested that the DPDME plays a vital catalytic role in the oxidation. Scheme 4. Plausible Mechanism

D

DOI: 10.1021/acssuschemeng.7b04400 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering purified by silica gel column chromatography to afford the desired 2 and 4.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b04400. 1 H and 13C NMR spectra of compounds 2−4. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wei-Min He: 0000-0002-9481-6697 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (Nos. 21302048, 21273068, and 21545010), Scientific Research Hunan Provincial Education Department (No. 15C0464), Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province (2012-318), and the Construct Program of the Key Discipline in Hunan Province.

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DOI: 10.1021/acssuschemeng.7b04400 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX