Catalytic Reductions and Tandem Reactions of Nitro Compounds

Letter pubs.acs.org/OrgLett

Catalytic Reductions and Tandem Reactions of Nitro Compounds Using in Situ Prepared Nickel Boride Catalyst in Nanocellulose Solution Kaniraj Jeya Prathap,† Qiong Wu,‡ Richard T. Olsson,‡ and Peter Dinér*,† †

Department of Chemistry, Organic Chemistry, KTH Royal Institute of Technology, Teknikringen 30, 10044 Stockholm, Sweden Department of Fiber and Polymer Technology, Polymeric Materials, KTH Royal Institute of Technology, Teknikringen 58, 10044 Stockholm, Sweden

‡

S Supporting Information *

ABSTRACT: A mild and efficient method for the in situ reduction of a wide range of nitroarenes and aliphatic nitrocompounds to amines in excellent yields using nickel chloride/sodium borohydride in a solution of TEMPOoxidized nanocellulose in water (0.01 wt %) is described. The nanocellulose has a stabilizing effect on the catalyst, which increases the turnover number and enables low loading of nickel catalyst (0.1−0.25 mol % NiCl2). In addition, two tandem protocols were developed in which the in situ formed amines were either Boc-protected to carbamates or further reacted with an epoxide to yield β-amino alcohols in excellent yields.

A

Scheme 1. Strategies for Reduction of Nitroarenes by Transition Metals

romatic and heteroaromatic amines are important intermediates in the chemical and pharmaceutical industries.1 There are several ways to generate these compounds, and the most used synthetic routes are the hydrogenation of nitroarenes2 and metal-catalyzed C−N coupling chemistry.3 During recent years, nanostructured particles (NPs) have emerged as important catalysts due to their high catalytic activity and high surface areas. Several NPs have been utilized for nitroarene reductions4 using both precious metals (gold,5 platinum,6 and palladium6a,7) and cheaper, more abundant metals (e.g., cobalt,8 nickel,9 and iron10). There is a drive to develop organic reactions that take place in “green” solvents, e.g., water or solvents from renewable sources, in order to reduce the waste generated from fossil fuel based solvents. For example, recently Lipshutz and co-workers developed a mild and ligand-free procedure with Fe/Pd nanoparticles (ppm) that catalyzed the reduction of nitroarenes at room temperature in water using sodium borohydride in the presence of a designed PEG surfactant (Scheme 1).11 However, the green methodologies based on biosourced additives and “cheaper” transition metals are more challenging. In the early 1950s, Schlesinger and Paul reported that the reduction of nickel salts with sodium borohydride in aqueous or alcoholic solvents gave a finely divided black precipitate that contains both nickel and boron, i.e., nickel boride.12 Nickel boride promotes different hydrogenation reactions with similar activity as Raney nickel12i,j and has mainly been utilized in organic synthesis as a stoichiometric reagent, particularly in hydrogenation of alkenes, alkynes,13 N-heterocycles,14 nitroarenes,15 etc.16 However, only a few practical catalytic applications have been reported for synthetically useful procedures.17 The main reasons for the use of stoichiometric amounts of nickel boride are related to the inherent instability of © 2017 American Chemical Society

nickel boride in aqueous media under oxygen-containing atmosphere in which the nickel boride converts to nickel oxide and nickel metal.18 Different templates and supports, e.g., sulfonic acid−based hydrogels and polymers, silica, and MgO, have been used together with nickel boride in order to increase its catalytic performance.19 We envisioned that a biosourced support such as nanocellulose could be used to stabilize the nickel boride nanoparticles and, hence, develop a protocol for nitro compound reduction under green conditions, i.e., low catalyst loading in water containing 0.01 wt % nanocellulose. The use of nanocellulose as support for NP catalysts is appealing due to its high surface area (300 m2/gram), thermal Received: July 10, 2017 Published: August 31, 2017 4746

DOI: 10.1021/acs.orglett.7b02090 Org. Lett. 2017, 19, 4746−4749

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Organic Letters

evenly distributed within the network of fibers and consequently prevented from forming clusters/aggregates, which would restrict/limit their accessible catalytic surfaces. An increase of the concentration of nickel chloride led to larger aggregation of the formed nickel boride (see SI, Figure S4). Several parameters, such as nickel source, amount of nickel, amount of nanocellulose, and equivalent of sodium borohydride, were screened in order to find the optimal reaction conditions for the reduction of nitroarenes with in situ prepared nickel boride in the presence of TEMPO-oxidized nanocellulose in water. The optimal performing nanoparticle catalyst was made using NiCl2· 6H2O (0.25 mol %), NaBH4 (2.5 equiv), and TEMPO-NC (3.0 mL, 0.01 wt %), and these fiber-associated particles showed the ability to reduce 4-nitrophenol (0.1 mmol) to 4-aminophenol in 120 min. Reduced reaction times were achieved by an increase of nickel loading (0.5 mol %), while a decrease of the amount of nickel catalyst led to incomplete reaction, but with a higher turnover number (from 390 to 920) (see Scheme 2 and the SI).

stability,20 and functionalizability. The primary hydroxyl groups on the surface of the nanocellulose can be chemically modified to carboxylic acids in a 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-mediated oxidation, which separates cellulose fibrils from their natural bundle configuration in nature to nanocellulose nanofibers due to repulsion between the negatively charged carboxylic acids functionalities.21 Nanocellulose suspensions in water, and in a range of other polar solvents, are very stable and add enhanced stability to catalysts supported on them. Previously, nanocelluloses has been used as support for a range of metal nanoparticles (NPs) that has been used for catalysis, e.g., silver, palladium, gold, copper, and platinum nanoparticles.22 However, the use of nickel−nanocellulose in synthetically useful catalytic applications has been more scarce, e.g., the hydrogenation of alkenes.23 In the present paper, we report on the facile reduction of nitro groups using in situ prepared nickel boride (NiCl2 and NaBH4) in the presence of TEMPO-oxidized nanocellulose (TEMPO-NC) fibrils in water. Initially, we hypothesized that the negatively charged carboxyl groups of the TEMPO-NC could stabilize and aid the formation of dispersed nickel boride nanoparticles. Indeed, the addition of NiCl2·6H2O to a solution of TEMPO-NC in water led to a gellike suspension (SI, Figure S1A). UV−vis spectroscopy of the gel-like suspension confirmed the interactions between the Ni2+ ions and the carboxyl groups by the appearance of a new shoulder at 390−400 nm (see the SI, Figure S2), which correlates with the UV maximum at 395 nm for nickel acetate (SI, Figure, S2). Addition of an excess of sodium borohydride (3 equiv) to the nanocellulose−nickel complex leads to the formation of a dispersed nickel boride−nanocellulose system (SI, Figure S1B). This contrasts the black granular precipitate that forms without the presence of nanocellulose (see SI, Figure S1C). The nanocelluloseassociated nickel boride nanoparticles were investigated using transmission electron microscopy (TEM). The TEM images reveal the formation of spherical nickel boride nanoparticles of different sizes ranging from 10 to 40 nm in diameter, embedded in the network of TEMPO-NC consisting of ca. 5−7 nm thick fibrils (SI). Figure 1C shows that the nickel boride particles were

Scheme 2. Screening of Reaction Parameters for Reduction

Reaction conditions: 4-NP (0.1 mmol), NiCl2·6H2O (X mol %), NaBH4 (3.0 equiv), TEMPO-NC (1.0 mL, 0.01 wt %), rt. bReaction conditions: nitro compound (0.3 mmol), NiCl2·6H2O (X mol %), NaBH4 (2.5 equiv), TEMPO-NC (3.0 mL, 0.01 wt %), rt. cReaction conditions: only H2O as solvent. a

An increase of the amount of TEMPO-NC (0.02 wt %) led to decreased reaction times (from 120 to 70 min), and full conversion was then obtained even at 0.1 mol % nickel catalyst loading (see the SI). This suggests that the TEMPO-nanocellulose has a stabilizing and/or accelerating effect on the catalyst system. Other hydroxyl-containing solvents/stabilizers were investigated, such as polyethylene glycol, cellulose (microcrystalline), glycerol, and ethylene glycol, but they all gave lower turnover number and isolated yields compared to the TEMPO-NC (see the SI, Table S4). In order to verify enhanced efficacy by TEMPO-nanocellulose on the reaction, three different nitro compounds were reduced in the presence and absence of the TEMPO-NC. The results clearly demonstrate that higher isolated yields (84−95%) are obtained for the reactions performed in the presence of nanocellulose than the reactions without (43−28%) (Scheme 2). Decreasing the

Figure 1. Schematic illustration of using nanocellulose as stabilizing support for nickel boride: (A) electrostatic interactions between the carboxyl groups in TEMPO-NC and Ni2+ ions; (B) formation of nanocellulose-associated nickelboride; (C) TEM image of nickel boride nanoparticles supported in TEMPO-NC solution; (D) enlarged picture of TEM analysis of nickel boride in TEMPO-NC solution. 4747

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tandem reactions were developed on the basis of the in situ functionalization of the formed amines. The formed amino functionality can easily be protected as a Boc-carbamate in a tandem procedure, where all substrates are mixed in one pot. Both aromatic and aliphatic nitro groups were smoothly reduced to Boc-protected amines in excellent yields (22, 23) (see Scheme 4). The tandem process was also able to convert the more difficult trans-β-nitrostyrene to the reduced carbamate in excellent yield (24, 96%) (Scheme 4).

amount of nickel chloride (from 0.25 to 0.1 mol %) led to a larger difference in isolated yields (about 8 times) in the reactions performed with and without the nanocellulose. The catalytic system could be recycled up to four times for 4-nitrobenzoic acid with full conversion (SI, Table S5). With the optimized reaction conditions at hand, several mono-, di-, and trisubstituted nitroarene compounds were reduced in order to assess the generality and the scope of the reaction. Nitrobenzene was fully converted in 1.5 h, and the product was isolated in excellent yield (1, 93%) (Scheme 3).

Scheme 4. Nitro Group Reduction in Combination with in Situ Boc Protection and in Situ Epoxide Ring-Opening

Scheme 3. Nitro Group Reduction in Combination with in Situ Boc Protection

Further synthetic applicability of the reduction was demonstrated in another tandem reaction involving an epoxide ringopening yielding biological important β-amino alcohols. Nitrobenzene 1 was easily reduced, and the epoxide ring-opening gave the corresponding β-amino alcohol in excellent isolated yield (25, 96%) (Scheme 4). With this methodology, we also prepared racemic propranolol, which is a nonselective beta blocker of the β-adrenergic receptors. In this tandem reaction, 2-nitropropane was reduced to the corresponding amine in the presence of the epoxide, derived from epichlorohydrin and 1-naphthol, to yield propranolol in excellent yield (26, 96%) (Scheme 4) after the in situ epoxide ring-opening. In conclusion, we have developed a mild and efficient method for the in situ reduction of a wide range of nitroarenes to amines using nickel chloride/sodium borohydride in a solution of TEMPO-oxidized nanocellulose in water (0.01 wt %). The protocol uses low catalyst loading (0.1−0.25 mol % of NiCl2) due to the stabilizing effect of the nanocellulose. In addition, two tandem protocols were developed in which the in situ formed amines were either Boc-protected to carbamates or further reacted with an in situ epoxide to yield β-amino alcohols.

Nitro compounds 2−4 with halogen substituents (F, Cl, I) were all reduced, but lower yields were obtained for the chloro and the iodo substituents (3, 84% and 4, 76%) (Scheme 3), and unreacted starting material was recovered. Substrates containing hydroxyl, amino, and methoxy groups in the ortho, meta, and para position (5−9, 16) were all reduced smoothly and were isolated in excellent yields. Remarkably, the 4-nitrobenzoic acid containing the highly challenging free carboxylic acid was successfully reduced in only 10 min and excellent yield (10, 97%) (Scheme 3). This is probably due to its increased solubility in aqueous solution. The protocol also tolerates nitrogen-containing heteroaromatic compounds such as substituted pyridines (11, 12). An ester group (13), an amide group (14), and a cyano group (17) are compatible with the applied conditions and yield the reduced amines in 95−97% yield. In addition, aliphatic nitroalkanes 18a−c are completely converted to the corresponding amines, while in nitro compounds containing aldehydes (19) and ketones (20, 21) both groups were reduced to the corresponding amino alcohols (19−21) as expected. The reduction of trans-β-nitrostyrene showed no selectivity, and both the double bond and the nitro group were reduced, making it difficult to isolate the aliphatic amine. In order to increase the synthetic utility of the nanocellulosesupported nickel-catalyzed reduction of nitro compounds, two

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ASSOCIATED CONTENT

S Supporting Information *

TThe Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02090. Experimental procedures, TEM analysis, and compound characterization data (PDF) 4748

DOI: 10.1021/acs.orglett.7b02090 Org. Lett. 2017, 19, 4746−4749

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peter Dinér: 0000-0001-6782-6622 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.J.P. and P.D. thank the Wenner-Gren Foundation (Grant No. UPD2016-0084) for financial support. REFERENCES

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DOI: 10.1021/acs.orglett.7b02090 Org. Lett. 2017, 19, 4746−4749