Combined experimental/theoretical study of D-glucosamine promoted Ullmann-type C–N coupling catalyzed by copper(I): does amino really count?

Abstract

Ullmann type C–N coupling reaction catalyzed by copper(I) with D-glucosamine derivatives as promoters was studied by means of combined experimental/theoretical investigation. The catalytic role of D-glucosamine was addressed. In contrast with previous speculations, the amino group may not count in the catalytic cycle in which the oxidative addition/reductive elimination mechanism works. Experimental results are in good agreement with theoretical findings. Extensive work indicates the wide applicability of the C–N coupling strategy exploited in this work.

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Introduction

The copper-catalyzed Ullmann coupling reaction is an important strategy for C–N coupling, which plays a fundamental role in pharmaceutical and agrochemical industry.¹ Application of the traditional copper-catalyzed Ullmann coupling reaction was limited by the high temperature (about 200 °C), stoichiometric usage of catalyst and fastidiousness about the substrate. These imperfections have inspired development of the prevalent palladium-catalyzed cross-coupling reaction.² In 2001, great progress reported by Buchwald indicated that ligand diamines could efficiently improve the copper-catalyzed cross-coupling of N–H heterocycles with aryl halides;³ the afterward work done by Cristau and Taillefer also provided such C–N coupling strategies with high efficiencies alternatively by using Schiff bases and oximes as ligand.⁴ The prospect of Ullmann coupling reaction catalyzed by copper catalysts is to perform such processes in more economic way. Up till now, while various ligands such as diamines,^3,5 glycol,⁶ Schiff base,^4,7 amino acids,⁸ diketones,⁹ phenanthroline,¹⁰ 2,2′-bipyridine,¹¹ carbohydrates,¹² oxalamic acid,¹³ diamide,¹⁴ have already been proved efficient for promoting the targeted coupling reactions, it is always challenging and interesting to develop eco-friendly and more efficient ligand,¹⁵ as well as to unravel the associated mechanistic aspects.

Due to the high density of chiral information on the backbone and environmental benignity, carbohydrates have been widely used in organic synthesis, especially as efficient ligands. Several reports has proved the effectiveness of carbohydrates in promoting copper-catalyzed Ullmann coupling reaction. For example, Cheng et al. found that D-glucosamine could efficiently promote CuI catalyzed C–N coupling reaction;^12a per-6-amino-β-cyclodextrin was prepared as a supramolecular ligand for CuI catalyzed N-arylation of imidazole with aryl bromide;^12c Zhang et al. also reported Ullmann type reaction by using D-glucosamine as ligand.^12b In spite of the abundant findings described above, the associated catalytic mechanisms deduced at a molecular level remain unclear. So far as reported, most mechanistic investigations focused on the reactions catalyzed by β-diketone-, 1,10-phenanthroline- and diamine-ligated copper complexes.¹⁶ Compared with the single-electron-transfer (SET) and atom-transfer mechanisms, the oxidative addition/reductive elimination mechanism was more widely accepted,^16b,17 the corresponding intermediates are shown in Scheme 1. As indicated by the oxidative addition/reductive mechanism, the intermediate of Cu^III was formed by the oxidative addition of a ligand (L_n)–Cu^I–nucleophile (Nu) complex with the aryl halide.^16e,18 Moreover, it was also concluded that the rate-limiting step was oxidative addition of ligand (L_n)–Cu^I–nucleophile (Nu) complex with the aryl halide.^16b,17,19

Scheme 1 The intermediates reported in previous studies.

Our recently interest focuses on the ligand- and catalytic effect of carbohydrates,²⁰ we herein present mechanistic study on D-glucosamine promoted C–N coupling reactions catalyzed by copper catalyst. The experimental study on fine-tuning and design of D-glucosamine-derived ligand was complemented by theoretical investigation using DFT calculation.

Result and discussion

To rationally obtain some mechanistic information experimentally, the D-glucosamine-derived ligand were finely tuned and designed. The coupling of 4-iodoanisole (1.0 equiv.) and imidazole (1.2 equiv.) was adopted as the model reaction, and detailed results are shown in Table 1. First, when D-glucose (L1) and D-glucosaminium chloride (L2) were used as ligands, the coupling product 3a was obtained with yields of 73% and 85%, respectively, which is consistent with previous reports.^12a,b Pitchumani speculated the catalytic mechanism of CuI/per-6-amino-β-cyclodextrin that the ligand–Cu^I complex was obtained by interaction between the amino groups of per-6-amino-β-cyclodextrin and CuI.^12c Zhang et al. elaborated the similar viewpoint in D-glucosamine/CuI catalyzed the Ullmann type C–S coupling reaction.²¹ However, considering the very activity of L1, possibly, rather than the amino it is the hydroxyls that work in the present C–N coupling processes. To verify this speculation, we protected the hydroxyl and exposed the amino groups of L2, respectively. Thus, L6, L7, L8 and L9 were obtained by protecting the hydroxyl groups in the position C-4 and C-6. However, copper complexes prepared with these ligands are completely inactive in promoting the coupling reaction, no matter if the amino group is acylated (L6, L7) or not (L8, L9). Alternatively, by just protecting the amino group and exposing some hydroxyl groups, L3, L4, and L5 were obtained, which turned out to be very active for the model coupling reaction. Moreover, only protecting the hydroxyl group in the position C-1 of L3 (i.e. L4 and L5) does not affect the activities of ligands obviously. Compared with L1–L5, while L6–L9 are much more conformationally rigid, and sterically hindered, the sharp contrast in Table 1 clearly indicates that the amino or imino group maybe not count in the catalytic process. Thus, as the most effective promoter, the prototype of L2 and L3, i.e. D-glucosamine, was selected for theoretical investigation presented later.

Table 1 Ligand effect on CuI-catalyzed coupling of 4-iodoanisole and imidazole^a

a Reaction conditions: 4-iodoanisole 1a (1.0 mmol), imidazole 2a (1.2 mmol), K2CO3 (2.0 mmol), CuI (0.1 mmol) and ligand (0.1 mmol) in the DMSO–water (5 mL, 1 : 1/v/v) at 110 °C for 24 h.b Isolated yield.
Ligand	L1	L2	L3	L4	L5	L6	L7	L8	L9
Yield^b/%	73	85	84	76	73	0	0	0	0

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The reaction mechanisms were interrogated with DFT calculations. As found in the experiments, the amino maybe not count in the catalytic process. We attempted to clarify the active ligand–Cu^I complex structure by DFT first. As concluded above, other than the amine, hydroxyls on the C3, C4 and C6 positions are crucial for the CuI catalyzed Ullmann reaction promoted by glucosamine derivatives. Thus, two oxo-ligated Cu complexes, i.e. Cat. A and B and two N–O ligated Cu complexes, i.e. Cat. C and D (as shown in Scheme 2), are considered as the possible active species that directly catalyze the reaction. However, the structure of Cat. C and D can't converge by DFT. It tallies with the experiment results. In addition, C–N couplings involving either N atom of 1H-imidazole are taken account. The potential energy surfaces (PES) and relevant structural sketches of the most favorable reaction pathways are shown in Fig. 1–3; more detailed structural information is given in the ESI.†

Scheme 2 Possible active complexes that are directly involved in the C–N coupling processes.

Fig. 1 PES and relevant structural sketches of the most favorable reaction pathways for D-glucosamine promoted C–N(1) coupling reaction; charges are omitted for the sake of clarity.

Fig. 2 PES and relevant structural sketches of the most favorable reaction pathways for D-glucosamine promoted C–N(3) coupling reaction; charges are omitted for the sake of clarity.

Fig. 3 PES and relevant structural sketches of the most favorable reaction pathways for bare CuI catalyzed reaction; charges are omitted for the sake of clarity.

For C–N(1) coupling, as shown in Fig. 1, the reactions promoted by Cat. A and B proceed via the same mechanisms, and three most probable pathways were located. Being found as the thermodynamically most favorable one, pathway c is selected for more detailed elucidation. From an encounter complex IM1c, in which the N–H bond has already been activated barrierlessly and the N(1) atom coordinates with the metal center, an initial insertion of the Cu atom into the C–I bond of iodobenzene takes place via TS1c/2c, forming the Cu–C and Cu–I bonds, respectively. Subsequent step corresponds to formation of the C–N bond with concomitant cleavage of the Cu–C and Cu–N bonds, generating IM3c. Finally, dissociation of IM3c liberates the coupling product 1-phenyl-1H-imidazole and regenerates Cat. A or B. For this stepwise C–N coupling process, insertion of the Cu atom into the C–I bond serves as the rate limiting step. In addition, both Cat. A and B are capable of bringing about a coupling process proceeding via pathway c.

For C–N(3) coupling, once again, the same mechanisms for reactions promoted by Cat. A and B were addressed, and two energetically most favorable pathways were found. Likewise, elucidation of only the more preferable pathway, e is to be presented. As shown in Fig. 2, the transformations starts from an encounter complex IM1e in which the N(3) atom coordinates with the metal center, and the initial step involves insertion of the Cu atom into the C–I bond of iodobenzene via TS1e/2e. Next, the C–N(3) coupling is achieved via TS2e/3e, forming the product complex IM3e. Finally, release of 1-phenyl-1H-imidazol-3-ium iodide regenerates Cat. A. Although energy barrier of the rate limiting step of pathway e, i.e. IM1e → IM2e, is comparable with that of pathway c, the much lower stability of encounter complex IM1e as compared to IM1c makes formation of the former much less efficient, so that pathway e is bypassed. Thus, only C–N(1) coupling is available according to our calculation.

To make it clear if the effect of ligands is just to increase the solubility of Cu or more than that, the reaction of bare CuI catalyzed C–N coupling process was also calculated; PES and relevant structural sketches of the most favorable pathway (f) are shown in Fig. 3, and more detailed structural information is given in the ESI.† Alternative mechanisms were addressed for bare CuI catalyzed reaction. Initially, a σ-bond metathesis process takes place via cleavage of the Cu–I and N–H bonds to form IM5, with concomitant formation of the Cu–N and I–H bonds. Next, another σ-bond metathesis process is involved in subsequent reaction of intermediate IM6 with iodobenzene to generate IM8, that is, formation of C–N and the new Cu–I bonds is accomplished under cleavage of the Cu–N and C–I bonds. Finally, liberation of 1-phenyl-1H-imidazole regenerates the bare CuI. The initial activation of the Cu–I and N–H bonds corresponds to the rate limiting step; here, the energy barrier (212 kJ mol⁻¹) is much higher as compared to pathway c (85 kJ mol⁻¹).

To be concluded from the computational results, D-glucosamine has considerably promotive effect on the C–N coupling reaction in that, alternative pathways are available such that much lower activation energy is needed; the oxidative addition/reductive elimination mechanisms are addressed; only C–N(1) coupling is available; both Cat. A and B can serve as the active complex that directly promotes a catalytic cycle.

For C–N coupling reaction, L1 and L2 have been reported, but L3 was not involved in this reaction. Thus, L3 selected for further examination, and detailed results can be found in Table 2. Comparison of different copper sources, i.e. CuI, CuCl, CuCl₂, CuSO₄ and Cu(OAc)₂, showed that CuI was the best copper source (Table 2, Entry 1–5). Next, a series of solvents were tested and it was found that polar aprotic solvent favors this coupling reaction (Table 1, Entry 6–9). While water is not an ideal solvent, it serves to increase the solubility of ligand in the mixed solvents, DMSO–water (1 : 1) and DMF–water (1 : 1), so as to increase the reaction efficiencies (Table 2, Entry 1 and 10). Further, Cs₂CO₃ outperforms all the other bases employed (Table 1, Entry 11–14). With regard to the reaction temperature, the reactions at 110 °C proceeds most efficiently (Table 1, Entry 15–17). Thus, the optimum conditions are obtained: Cs₂CO₃ as the acid-binding reagent, DMSO–water (1 : 1) as solvent, 110 °C. In addition, CuI and DMSO were selected as catalyst precursor and solvent, respectively for subsequent theoretical investigation.

Table 2 The coupling reaction of 4-iodoanisole and imidazole catalyzed by copper/L3^a

Entry	Copper salt	Base	Solvent	Temp/°C	Yield^b/%
a Reaction conditions: 4-iodoanisole 1a (1.0 mmol), imidazole 2a (1.2 mmol), base (2.0 mmol), copper salt (0.1 mmol) and L3 (0.1 mmol) in the solvent (5 mL) for 24 h.b Isolated yield.
1	CuI	K2CO3	DMSO–water (1 : 1)	110	84
2	CuCl	K2CO3	DMSO–water (1 : 1)	110	37
3	CuCl2	K2CO3	DMSO–water (1 : 1)	110	24
4	CuSO4	K2CO3	DMSO–water (1 : 1)	110	71
5	Cu(OAc)2	K2CO3	DMSO–water (1 : 1)	110	76
6	CuI	K2CO3	DMF	110	32
7	CuI	K2CO3	DMSO	110	61
8	CuI	K2CO3	Methanol	110	21
9	CuI	K2CO3	Water	110	Trace
10	CuI	K2CO3	DMF–water (1 : 1)	110	69
11	CuI	Cs2CO3	DMSO–water (1 : 1)	110	91
12	CuI	KOH	DMSO–water (1 : 1)	110	57
13	CuI	NaOH	DMSO–water (1 : 1)	110	65
14	CuI	Na2CO3	DMSO–water (1 : 1)	110	60
15	CuI	Cs2CO3	DMSO–water (1 : 1)	80	40
16	CuI	Cs2CO3	DMSO–water (1 : 1)	100	68
17	CuI	Cs2CO3	DMSO–water (1 : 1)	120	75

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Further extensive study focused on expanding the substrate scope for CuI/L3 catalyzed Ullmann coupling reaction; more details are shown in Table 3. To our delight, both electron-withdrawing and electron-donating aryl iodides react efficiently with imidazole and the targeted products were obtained with good to excellent yields (Table 3, Entry 1–6). In addition, neither electron withdrawing nor donating groups substituted on the phenyl ring has obvious influence on the yields. Next, we examined the influences of different halides(Table 3, Entry 6–8), and the corresponding reactivities with imidazole decrease in the order PhI > PhBr > PhCl. Considering the increasing BDE(C₆H₅–X):²² BDE(C₆H₅–I) = 272.0 kJ mol⁻¹ < BDE(C₆H₅–Br) = 336.4 kJ mol⁻¹ < BDE(C₆H₅–Cl) = 397.9 kJ mol⁻¹, the experimental result is in line with our theoretical conclusion that insertion of the C–X bond serves as the rate limiting step. Further, experiments with the different nitrogen nucleophiles were performed, and several N–H heterocycles including 1H-benzimidazole, morpholine, pyrrolidine give the corresponding products in moderate to excellent yields.

Table 3 CuI/L3 catalyzed C–N coupling reaction^a

Entry	Aryl halides 1	Nitrogen compound	Product	Yield^b/%
a Reaction conditions: aryl halides 1 (1.0 mmol), nitrogen nucleophiles 2 (1.2 mmol), Cs2CO3 (2.0 mmol) and CuI (0.1 mmol) in the DMSO–water (5 mL, 1 : 1/v/v) at 110 °C for 24 h.b Isolated yield.
1				91 (3a)
2				94 (3b)
3				95 (3c)
4				87 (3d)
5				89 (3e)
6				86 (3f)
7				81 (3f)
8				Trace
9				76 (3g)
10				69 (3h)
11				57 (3i)
12				99 (3j)
13				71 (3k)

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As the final part of this work, we present a tricky application of the C–N coupling strategy in the synthesis of 1-(4-methoxyphenyl)indoline, which is abundant in nature and widely employed in biological and pharmaceutical chemistry.²³ As shown in Scheme 3, 1-(4-methoxyphenyl)indoline was obtained from 2-bromo-phenethylamine stepwisely via intramolecular cyclization and subsequent coupling with 4-iodoanisole in one pot; the yield amounts to 69%. All the above extensive reactions indicate the wide applicability of the C–N coupling strategy exploited in this work.

Scheme 3 Application of the Cu/L3-catalyzed C–N coupling reaction in the one-pot synthesis of 1-(4-methoxyphenyl)indoline.

Conclusions

In summary, we have presented the combined experimental/theoretical investigation on the D-glucosamine derivatives promoted Ullmann type C–N coupling reactions catalyzed by copper. In the first part, by finely tuning and designing the ligands, it was found that the hydroxyls on the C-3, C-4 and C-6 position may have significant influence on the catalysis process, while the hydroxyl on C-1 and the amino do not count. Subsequently, by using DFT calculation, not only the catalytic role of D-glucosamine was addressed, but also the inactiveness of bare CuI was explained. Here, the oxidative addition/reductive elimination mechanism works, and experimental results are in good agreement with theoretical findings. Next, extensive work was performed by employing various phenyl halide and amines as substrates, and moderate to excellent yields of corresponding coupling products were obtained. Finally, the wide applicability of the coupling strategy was further proved by a tricky application in the efficient preparation of 1-(4-methoxyphenyl)indoline starting from 2-bromo-phenethylamine.

General methods

Melting points were determined on an X4-Data microscopic melting point apparatus and were uncorrected. Nuclear magnetic resonance (NMR) spectra were measured at 400 MHz (¹H) or at 100 MHz (¹³C) on a Bruker Avance DRX-400 spectrometer. All reactions were monitored by analytical thin-layer chromatography (TLC) from Merck with detection by spraying with 5% (w/v) phosphomolybdic acid in ethanol and subsequent heating or UV. The products were purified by column chromatography through silica gel (300–400 mesh). D-Glucose, D-glucosamine and N-acetyl-D-glucosamine were purchased in Aladdin Industrial Corporation. All reagents and solvents were general reagent grade unless otherwise stated.

General procedure of CuI@ligand catalyzed coupling reaction of N–H heterocycles and aryl halides

To a stirred solution of DMSO–water (1 : 1, 5 mL) were added aryl halide (1.0 mmol), N–H heterocycle (1.2 mmol), CuI (0.1 mmol, 19 mg), ligand (0.1 mmol) and Cs₂CO₃ (2 mmol) at room temperature. Then the reaction mixture was heated to 110 °C under air and stirred for 24 h. After cooling to room temperature, the reaction mixture was partitioned by adding the ethyl acetate (20 mL) and water (20 mL). Subsequently, the organic phase was separated and the aqueous phase was extracted with ethyl acetate (20 mL) twice. The combined organic phases were washed with saturated brine, dried over MgSO₄, and concentrated in vacuo. Then the crude product was purified by column chromatography through silica gel, eluting with ethyl acetate/petroleum ether solvent mixture, to give the pure 3.

General procedure for one-pot synthesize of 1-(4-methoxyphenyl)indoline

To a stirred solution of DMSO–water (1 : 1, 5 mL) were added 2-bromo-phenethylamine (1.0 mmol), CuI (0.1 mmol, 19 mg), L3 (0.1 mmol) and Cs₂CO₃ (4 mmol) at room temperature. Then the reaction mixture was heated to 110 °C under air and stirred for 24 h. Subsequently, 4-iodoanisole (1.0 mmol) was added to this solution and continued to stir for 24 h. After cooling to room temperature, the reaction mixture was partitioned by adding the ethyl acetate (20 mL) and water (20 mL). Subsequently, the organic phase was separated and the aqueous phase was extracted with ethyl acetate (20 mL) twice. The combined organic phases were washed with saturated brine, dried over MgSO₄, and concentrated in vacuo. Then the crude product was purified by column chromatography through silica gel, eluting with ethyl acetate/petroleum ether solvent mixture, to give the pure product.

Computational details

The theoretical work was performed using the Gaussian 09 program.²⁴ The def2-TZVP basis set²⁵ with the ECP-28 pseudopotential²⁶ were used for the I atom, and the def2-TZVP all-electron basis set²⁵ was chosen for the Cu atoms; for all the other atoms, the def2-SVP all-electron basis set²⁵ was used for optimization and frequency analysis (BSI), and the def2-TZVP all-electron basis set²⁵ (BSII) for calculating the single-point energies. The Polarizable Continuum Model using the integral equation formalism variant (IEFPCM)²⁷ was employed to evaluate the solvation energies. The B97-1 functional²⁸ was employed for geometry optimizations and frequency analysis, which has previously been proven to be reliable for calculating Cu complexes.²⁹ Stationary points were optimized without symmetry constraint, and their nature confirmed by vibrational frequency analysis. Intrinsic reaction coordinate³⁰ calculations were performed to link transition structures with the respective intermediates. Unscaled vibrational frequencies were used to correct the relative energies for zero-point energy (ZPE) contributions.

Acknowledgements

The authors are grateful for the financial support from the Natural Science Foundation of China (21376213, 21476194), Zhejiang Provincial Public Technology Research of China (2014C31123, 2015C31038) and the Fundamental Research Funds for the Central Universities (JUSRP115A05).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03015g

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