Sustainable and Selective Monomethylation of Anilines by Methanol

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Sustainable and Selective Monomethylation of Anilines by Methanol with Solid Molecular NHC-Ir Catalysts Jiangbo Chen, Jiajie Wu, and Tao Tu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03246 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Sustainable Chemistry & Engineering

Sustainable and Selective Monomethylation of Anilines by Methanol with Solid Molecular NHC-Ir Catalysts Jiangbo Chen,† Jiajie Wu† and Tao Tu*,† †

Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, 220 Handan Road, Shanghai, 200433, China E-mail: [email protected].

KEYWORDS: amine synthesis, coordination assembly, iridium, methanol activation, catalytic methylation.

ABSTRACT: Using feedstock methanol as a green methylation reagent, the selective Nmonomethylation of anilines is realized under mild reaction conditions by using N-heterocyclic carbene iridium (NHC-Ir) coordination assemblies as highly efficient solid molecular catalysts. Along with a broad substrate scope and good functional group tolerance, up to quantitative yield and 2.0 × 104 turnover numbers (TONs) are obtained even at low catalyst loadings. Notably, the solid NHC-Ir molecular catalyst can be easily recovered and recycled more than 20 times without obvious loss of reactivity and selectivity. Furthermore, this selective practical protocol can be sucessfuly extended to direct methylation of highly functionalized bioactive compounds including 3-aminoestrone, cinacalcet and their analogues in excellent yields and selectivities, highlighting their potential application in pharmaceuticals.

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■ INTRODUCTION Direct N-methylation of amines is one of the most important transformations in organic chemistry and is widely applied in the syntheses of fine chemicals, pharmaceuticals and natural products.1-4 The traditional N-methylation protocols, usually involving methyl halides, dimethylsulfate and diazomethane, have several serious limitations, especially in the selectivity of the N-monomethylation of primary amines.5-7 The excess amount of strong bases, overmethylated products and toxic halogenated by-products cause tedious separation and thus hampering their practical application in industrial setting. Therefore, reductive amination of toxic formaldehyde still constitutes a chief approach in the industrial production of methyl amines.8-11 As more environmentally benign reagents, carbon dioxide,12 formic acid13-15 and dimethyl carbonate16-18 have also been demonstrated their applicability in the methylation of amines. However, high reaction temperature (>150 oC) and over amount of reducing agents are usually required to achieve satisfactory yields. An intrinsic problem is the often concurrent production of by-products tertiary amines with these substances. The formed nucleophilic mono-methylated amines prefer the tertiary amines formation. Therefore, the direct and selective Nmonomethylation of amines remains elusive. As an important biomass and cost-effective feedstock fine chemical, methanol has also been utilized for this purpose. Various Lewis acids have been demonstrated their catalytic efficiency in the N-methylation of amines with MeOH, although harsh reaction conditions are still required to achieve satisfactory results for fully-methylated products.19-22 Recently, Saito and co-workers reported a room temperature N-methylation by using Pd/TiO223 or Ag/TiO224 as nanophotocatalysts. Besides fully-methylated products are produced in the presence of primary amine substrates, however, the recyclability of the heterogeneous catalyst is not addressed in their


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studies. By using “hydrogen-borrowing” strategy,25-32 N-methylation of primary amines by MeOH in the presence of bifunctional homogenous catalysts has been disclosed. In general, the bifunctional catalysts not only favor the oxidative dehydrogenation of methanol but also accelerate the hydrogenation of the imine intermediate to produce the desired amines.29-32 Among them, Li and co-workers realized the first Ir-catalyzed N-monomethylation of aromatic primary amines,33 although ortho-substituted aromatic amines are not suitable substrates and the high reaction temperature is still required. By using air sensitive [RuCp*Cl2]2/dpePhos as a catalyst, Seayad and co-workers fulfilled N-monomethylation of less-bulky aromatic primary amines with MeOH.34 In light of their robustness and high catalytic activity in various transformations,35-49 Crabtree and co-workers firstly applied N-heterocyclic carbene (NHC) Ir complexes in this challenging reaction.50 However, high catalyst loading and limited substrate scope even under the microwave assistant conditions still hindered the protocol feasibility. Furthermore, besides these advancements achieved by the homogenous catalysts, the difficulty in the separation and recyclability of these noble catalysts is still considered as a stumbling issue in the practical industrial application. In addition, methanol is also well applied in the hydrogen transfer reactions51-53 and intermolecular dehydrogenated coupling with amines,54-56 the study on the competition and selectivity in the N-methylation of primary amines containing sensitive groups including olefins, ketones, nitriles, hydroxyl groups and etc. is still less studied. Therefore, searching efficient, selective and robust recyclable catalysts for the sustainable Nmethylation is very concerned. ■ EXPERIMENTAL SECTION General synthetic procedure of N-heterocyclic carbene iridium (NHC-Ir) coordination assemblies: Benzobisimidazolium salts (0.5 mmol) and corresponding iridium precursor (0.5


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mmol) were dissolved in DMF under N2 at room temperature, LiHMDS (1 mmol) solution in THF was added in dropwise. The resulting mixture was stirred at 80 oC for 12 h. The dark brown solids were isolated after filtration and washed with deionized water for three times. The coordination polymer solids 1a-1f were then dried over under vacuum. General procedure of N-monomethylation of aniline by methanol and the solid molecular catalyst recovery: A 15 mL sealed tube equipped with a magnetic stirring bar was charged with aniline (279 mg, 3 mmol), solid molecular catalyst 1a (8.1 mg, 0.5 mol%), KOtBu (336 mg, 3 mmol) and methanol (6 mL). After heating to the desired temperature for 12 h, the mixture was cooled to room temperature; the solid catalysts were readily recovered after centrifugation and decantation. The recovered solid was washed with methanol (3ⅹ5 mL), and reused directly in the next run without additional activation steps, just by simply recharging aniline, KOtBu and MeOH in the sealed tube. General procedure for the N-methylation of primary amines by methanol: Amine (1.0 mmol), solid catalyst 1a (2.6 mg, 0.5 mol%), KOtBu (112 mg, 1.0 equiv.) were charged in a 15 mL pressure tube with magnetic bar. Then, 2 mL of methanol was added into the mixture. The reaction tube was closed with the Teflon stopper and heated to 130 oC. After reaction completion, the reaction mixture was allowed to cool to room temperature. The solvent was removed under vacuum and the resulting mixture was purified by flash column chromatography. The isolated products were fully characterized. Synthetic procedure of secondary amines 25a-d: The aliphatic primary amine and the same equivalent ketone or aldehyde was condensed with Ti(O-iPr)4 at room temperature. After amine consumption, methanol was added. The resulting mixture was cooled to 0 oC. Then NaBH4 was added portion to give the desired secondary amines 25a-d in good to excellent yields.


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■ RESULTS AND DISCUSSION

Scheme 1. NHC-Ir coordination assemblies 1a-f and corresponding molecular complexes 2a-b.

Recently, using a coordination assembly strategy,57-60 we have prepared a series of coordination assemblies based on bis-NHC-M complexes (M = Ir and Ru),61-63 which function as solid bifunctional molecular catalysts not only in oxidative dehydrogenation of polyols,61 but also in the reductive amination of levulinic acid and its analogues.62 Notably, these solid assemblies could be easily recovered and reused dozens of times without obvious observation of activity or selectivity loss, highlighting their potential in industrial application. Among these transformations, we found that these solid assemblies exhibited better catalytic activity than their corresponding molecular NHC-M complexes. Therefore, in our continuing interests in developing solid molecular catalysts and their catalytic application in the selective biomass transformation, herein, we wish to report the results of the studies by uncovering NHC-Ir coordination assemblies (Scheme 1) functioning as efficient and recyclable solid bifunctional molecular catalysts in the selective N-monomethylation of diverse anilines by MeOH via reductive amination process.


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Table 1 Reaction optimization of N-monomethylation of aniline by methanol.a

Entry

[Cat]

t (h)

Yield (%)b

1

1a

12

80

2

1b

12

trace

3

1c

12

60

4

1d

12

trace

5

1e

12

6

6

1f

12

8

7

2a

12

85

8

2b

12

55

9

1a

18

>99

10c

1a

12

>99

a

Reaction was carried out with 4 mmol aniline, 0.1 mol% [Cat], 1.0 equiv KOtBu in 8 mL methanol; b Yield was determined by GC-MS using p-xylene as the internal standard; c with 1 mmol aniline, 0.5 mol% [Cat], 1.0 equiv KOtBu and 2 mL methanol.

Initially, the N-monomethylation of aniline in pure MeOH was selected as a model reaction to investigate the catalyst efficiency. After intensively screening of various bases (see the Supporting Information, Table S1), delightedly, in the presence of 0.1 mol% solid catalyst 1a (Ir content), KOtBu was found as a suitable base for this heterogeneous catalytic reaction and to deliver the best GC-yield at 130 oC (80%, entry 1, Table 1). The reaction temperature affected the methylation outcomes dramatically: the yield of product 3 was descended along with the


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reaction temperature decreasing. For example, a 98% GC-yield of the desired product 3 was found at 130 oC, while only a 34% yield was detected at 100 oC with the same reaction duration (Table S1, entry 15 vs 3). Subsequently, other solid catalysts (1b-f) were probed. In contrast with anion effects, ancillary ligands show stronger impact on the methylation process. When iodide containing solid catalyst 1c was applied, product 3 was still formed in a 60% yield (entry 3, Table 1). However, when COD (cyclooctadiene) ligand containing catalysts 1b and 1d were utilized, almost no methylated products were detected (entries 2 and 4, Table 1). Slightly increased yields were observed with the solid catalysts containing longer alkyl groups (6% and 8% for 1e and 1f, respectively, entries 5 and 6, Table 1). When corresponding homogenous NHC-Ir catalyst 2a was applied instead of solid 1a, a slightly better yield was obtained (85%, entry 7, Table 1). However, benzimidazolium analogue 2b only resulted in a 55% yield (entry 8, Table 1). To our delight, slightly extending the reaction time to 18 hours or increasing the catalyst loading to 0.5 mol%, quantitative yields were finally achieved (entries 8 and 9, Table 1). 100 80 60 40 20 0 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th 14th 15th 16th 17th 18th 19th 20th 21th 22th 23th

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Recycling and reuse of the NHC-Ir coordination assemblies 1a in the Nmonomethylation of aniline: 3 mmol scale for each run (reaction time: blue 12 h and brown 24h).


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Due to its insolubility in water, substrates and the most organic solvents, the solid catalyst 1a is readily recovered quantitatively by convenient centrifugation. To our delight, after simple filtration and washing with additional methanol, the recovered solids could be directly applied in the next run by simply adding base, aniline and methanol. Almost quantitative yields (>99%) were obtained in the first 13 runs (blue column, Figure 1) under the optimized reaction conditions. Slightly decreased yields were observed in the 14th (96%) and 15th (95%) runs. After extending the reaction time to 24 hours (brown column, Figure 1), the yield could keep quantitatively until the 20th run. By using scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDS) mapping, a comparison between the freshly prepared and recovered solid catalyst 1a was carried out. Delightedly, the recovered solid 1a after 23 runs display similar images as the fresh prepared one (Figure 2a-c vs d-f) which indicate that assembly 1a preserves its molecular structural features during recycling. In order to exclude the possibility of the formation of Ir nanoparticles during the reaction, mercury poisoning experiments were performed. After heating, one drop of mercury was added into the reaction mixture after 0 and 4 hours. To our delight, the reactions are not prohibited and excellent yields are still observed (98% and 97%, respectively). These outcomes exclude the possibility of nanoparticle formation, which is also consistent with our previous study that NHC-Ir coordination assemblies 1a functioned as a solid molecular catalyst in these transformations.62,63 Furthermore, the filtrates after each run were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) for potential iridium leaching (see the Supporting Information, Table S3). Approximately 0.2 µg/mL iridium leaching was observed for each in the initial three runs, which might be the trace amount of the inactive iridium species or precursors entrapped in


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the assemblies’ matrix cavities during the metal coordination process. In later, the real leaching amount of every run was negligible, which further indicated the robustness of NHC-Ir assemblies even in the polar solvent like methanol. The yield decreasing after 20 runs might be caused by the catalyst loss during the dozens of centrifugation, decantation and washing processes. Encouraged by the performance of solid molecular catalyst 1a observed so far, a scale-up reaction with 100 mmol aniline in 75 mL of methanol was then conducted (Scheme S5). In the presence of 5×10-3 mmol 1a, aniline was completely converted to N-methyl aniline 3 at 150 oC with an extended reaction time, and the highest TONs, 2×104, was achieved for this challenging transformation.

Figure 2. (a-c)TEM, SEM, EDS images of freshly prepared NHC-Ir coordination assemblies 1a; (d-f) images of recovered solid 1a after the 23th run.

With this practical and efficient protocol in hand, the scope on other amines was then investigated (Table 2). The substituent position on the aromatic shows limited impact on the


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transformation: meta- and para-methyl substituted anilines give excellent isolated yields (96% and 91% for N-monomethylated products 4b and 4c, respectively); and sterically hindered orthosubstituted aniline still results in a moderate yield (60%, 4a). Gratifyingly, the yield of 4a could readily be improved to 92% by simply prolonging the reaction time. Remarkably, amines containing both electron-rich (4-7) and electron-deficient (8-11) substitutes were fully compatible; the corresponding N-monomethylated amines were produced in excellent yields (89−99%). It is worth noting that nitrile group, easily reduced in the hydrogen transfer reactions,64 was not affected under the reaction conditions. The halide groups were also fully tolerated and the corresponding N-monomethylated products (11-13) were isolated in 85−99% yields, the iodo-, and chloro groups are kept and readily for further transformations. To our delight, bulky 1-naphthalamine and heterocyclic aryl amines were all considered as suitable substrates, leading good to excellent yields (82%-99%) for the desired products (14-19). Moreover, sulfonamide was also found as a suitable partner, and selectively Nmonomethylated to produce compound 20 in an excellent isolated yield, although extended reaction time was required. When the substrate containing both amino and amide groups, the Nmonomethylation selectively took place at the amine side resulting in the desired product 21 in an 85% isolated yield. No amide-methylated product was observed. When aniline with unprotected hydroxymethyl group was employed, a 91% yield of product 22 was obtained. No intermolecular dehydrogenated coupling products was detected,49-51 further highlighting the reaction selectivity and compatibility. Pleasingly, the protocol was successfully extended to diamine and triamine substrates and the corresponding dimethyl and trimethyl products (23 and 24) were produced in 99% and 72% isolated yields, respectively. Notably, this protocol is readily extended to the methylation of crucial secondary alkyl-amines. For instance, the methylated


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cinacalcet 25a, which is used to treat secondary hyperparathyroidism,65 was readily prepared in an 88% isolated yield. Its analogues (25b-d) were efficiently synthesized in good to excellent yields (84%-94%). Table 2. N-Methylation of aromatic primary amine.a

a

1 mmol scale in 2 mL methanol with 0.5 mol % solid catalyst 1a; isolated yield; b after 48 h; c

with 10 mol % solid catalyst 1a for 3.5d.


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It is well known that olefins can be readily reduced via hydrogen transfer process in alcohol.5153

When 4-aminostyrene was selected to test the selectivity of two competitive reactions. As we

expected, poor selectivity was observed under the optimized conditions, and the fully reductive product 26 and N-monomethylated product 27 with olefin group were produced almost in equal amounts (Scheme 2). The optimization studies of reaction conditions (see the Supporting Information, Table S1) revealed that bases dramatically influenced the methylation process. Therefore, we conceived the alternation of different bases may tune the selectivity between the hydrogenation of olefin and methylation of amine. It was found that (see the Supporting Information, Table S2), when NaOtBu was employed instead of KOtBu, delightedly, up to 81% isolated yield of N-monomethylated product 27 was obtained (Scheme 2). Alternatively, it was possible to selectively produce full reductive product 26 in a 95 % isolated yield by using KOtBu as the base with extended reaction time (Scheme 2).

Scheme 2. The competition between olefin hydrogen transfer and N-monomethylation of 4aminostyrene.

To examine the potential feasibility of this selective, efficient and environmental benign methodology, we would like to demonstrate its application in the N-monomethylation of bioactive complex aromatic amine, which could be exploited for drug discovery. 3-


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Aminoestrone is known as a versatile intermediate for the synthesis of a number of biologically active compounds for the treatment of hormono-sensitive diseases including prostate and breast cancers.66,67 Therefore, direct N-monomethylation of 3-aminoestrone was then carried out with the solid molecular catalyst 1a (Scheme 3). Due to the competitive C-methylation at the ortho site of the carbonyl group,68,69 product 28 was still obtained in a 31% isolated yield. In order to avoid this influence, the protected 3-aminoestrone 29 was then synthesized and applied under the identical reaction conditions, a good yield (80%) was observed with N-monomethylated product 30. After simple hydrolysis, secondary amine 30 was readily converted to 28 almost in a quantitative yield.

Scheme 3. N-monomethylation of 3-aminoestrone.

In light of hydrogen-borrowing process may be involved in this methylation reactions,29-32, 70-72 methanol may be first oxidized to formaldehyde, which was subsequently condensed with an amine to produce the aromatic imine intermediate with the liberation of 1 equiv. water. This resulting imine was then hydrogenated to give the corresponding N-monomethylation product (Scheme 4). To gain mechanistic insights, the reaction trajectory of the N-monomethylation of aniline by the mixture of methanol and deuterated methanol (1:1 ratio) was monitored by 1H


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NMR. In order to exclude the possible oxidation under the ambient condition, the reaction was carried out in the sealed NMR tube. The expected imine –N=CH2 signal was found at 8.57 ppm (see he Supporting Information, Figure S25), supporting the proposed reaction mechanism. O

oxidative dehydrogenation

R CH2 OH

R

H

base R' NH2 [Ir]

Bifunctional [Ir]-H catalyst

H2O

R'

N H

R

R' hydrogenation

N

R

Scheme 4. Proposed mechanism for Ir-Catalyzed N-alkylation of amines using alcohol. With this plausible mechanism in hand, the protocol should be extended to other long-chain alcohols. When ethanol and 1-propanol were applied as the solvent and alkylating reagents, the desired N-mono-alkylated products were still detected in 59% and 29% yields, respectively (eq. 1 and eq.2), which further prove the reaction mechanism and protocol efficiency. The low yields of N-monoalkylated products may be caused by the self-aldol condensation reaction of the aldehydes under the basic reaction conditions.73-75 Moreover, the unsaturated aldehyde product from the self-aldol condensation would react with amine to give various alkylated byproducts, which also contributed to the diminished yield of the desired N-monoalkylated products (Figure S120).


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■ CONCLUSIONS In conclusion, by using NHC-Ir solid molecular catalyst, we have developed the selective Nmonomethylation of aromatic anilines with methanol under mild conditions. Besides good functional group tolerance and broad substrates scope even at low catalyst loading, the solid molecular catalyst was readily recovered by simple centrifugation and reused more than 20 times without obvious loss of activity. Up to 2×104 TONs was achieved and clearly demonstrated the efficiency of our developed method. In addition, due to the high selectivity observed in the Nmonomethylation primary amines containing competitive groups including olefins, ketones, nitriles and free hydroxyls, we successfully applied this method to the methylation of bioactive molecule with excellent performance, highlighting its great practical potential in the sustainable amine synthesis and pharmaceuticals. ■ ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details and characterization data (PDF) ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.


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■ ACKNOWLEDGMENT Financial support from the National Key R&D Program of China (2016YFA0202902), National Natural Science Foundation of China (No. 21572036 and 91127041), the Shanghai International Cooperation Program (14230710600), the External Cooperation Program of Jiangxi Province (20151BDH80045), and Department of Chemistry, Fudan University is gratefully acknowledged. ■ REFERENCES (1) Modern Amination Methods; Ricci, A., Ed.; Wiley-VCH: Weinheim, Germany, 2000. (2) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, 4th ed.; Kluwer Academic: New York 2001; Part B, Chapter 3.2.5. (3) Amines: Synthesis, Properties, and Applications; Lawrence, S. A., Ed.; Cambridge University: Cambridge, 2006. (4) Amino Group Chemistry: From Synthesis to the Life Sciences; Ricci, A., Ed.; Wiley-VCH: Weinheim. 2008. (5) Selva, M.; Bomben, A.; Tundo, P. Selective mono-N-methylation of primary aromatic amines by dimethyl carbonate over faujasite X- and Y-type zeolites. J. Chem. Soc., Perkin Trans. 1997, 1, 1041-1046. (6) Selva, M.; Perosa, A.; Fabris, M. Sequential coupling of the transesterification of cyclic carbonates with the selective N-methylation of anilines catalysed by faujasites. Green Chem. 2008, 10, 1068-1077.


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For Table of Contents Use Only

Using recoverable solid molecular NHC-Ir catalyst, a sustainable and selective N-methylation of aniline by methanol with broad substrate scope was successfully developed.


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Sustainable and Selective Monomethylation of Anilines by Methanol

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