The first chiral diene-based metal–organic frameworks for highly enantioselective carbon–carbon bond formation reactions

Abstract

We have designed the first chiral diene-based metal–organic framework (MOF), E₂-MOF, and postsynthetically metalated E₂-MOF with Rh(I) complexes to afford highly active and enantioselective single-site solid catalysts for C–C bond formation reactions. Treatment of E₂-MOF with [RhCl(C₂H₄)₂]₂ led to a highly enantioselective catalyst for 1,4-additions of arylboronic acids to α,β-unsaturated ketones, whereas treatment of E₂-MOF with Rh(acac)(C₂H₄)₂ afforded a highly efficient catalyst for the asymmetric 1,2-additions of arylboronic acids to aldimines. Interestingly, E₂-MOF·Rh(acac) showed higher activity and enantioselectivity than the homogeneous control catalyst, likely due to the formation of a true single-site catalyst in the MOF. E₂-MOF·Rh(acac) was also successfully recycled and reused at least seven times without loss of yield and enantioselectivity.

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Introduction

In the past 15 years, metal–organic frameworks (MOFs) have emerged as a novel class of highly porous molecular materials with great potential for many applications, including gas storage,¹ chemical sensing,² biomedical imaging,^2a,3 drug delivery,⁴ nonlinear optics,⁵ and catalysis.⁶ Although a number of excellent MOF-based catalytic systems have been developed recently,⁷ examples of highly enantioselective asymmetric reactions catalyzed by MOFs are still limited despite their potential utility in the synthesis of high-value fine chemicals. Asymmetric MOF catalysts not only enable the recycling and reuse of expensive chiral ligands and precious metals, but also prevent the leaching of toxic metals into organic products which can be a significant issue for the pharmaceutical industry.

Since the first report of a MOF-based asymmetric catalyst with modest enantioselectivity in 2000,⁸ a number of highly enantioselective MOF catalysts with Lewis acid reactivity have been designed, including Ti(IV)-BINOL-based MOFs⁹ and Mn(III)- and Co(III)-salen-based MOFs.¹⁰ To expand the scope of MOF-catalyzed asymmetric reactions, Lin and coworkers recently developed BINAP-based MOFs for a number of important asymmetric catalytic reactions.^7h However, the bulky BINAP and derivatives reduce the channel/cavity sizes of BINAP-MOFs and hinder their applications in asymmetric reactions involving sterically demanding transition states/intermediates. Herein we report the design and synthesis of the first chiral diene-based MOFs and their use in asymmetric C–C bond formation reactions (Fig. 1). With less steric demand than their BINAP predecessors, the diene-MOFs metalated with Rh(I) complexes are excellent catalysts with high activities and enantioselectivities for 1,4-additions of arylboronic acids to α,β-unsaturated ketones and 1,2-additions of arylboronic acids to aldimines.¹¹

Fig. 1 Postsynthetically Rh-metalated E₂-MOF catalysts for asymmetric reactions.

Chiral dienes have been applied to a broad range of asymmetric reactions since being reported independently by Hayashi¹² and Carreira.^13,14 In particular, asymmetric Rh-diene complexes provide powerful methods to construct chiral centers in C–C bond formations. For example, 1,4-additions of electron-deficient olefins and 1,2-addition of imines with arylboronic acids in the presence of rhodium and a chiral diene provide highly desirable synthetic methods to obtain the addition products with high yields and enantioselectivities.^15,16 In these reactions, Rh complexes of chiral dienes typically afford higher yields and enantiomeric excesses (ee's) than the corresponding Rh-BINAP complexes.

Results and discussion

We targeted the synthesis of the chiral diene-based MOF, E₂-MOF, based on the linear dicarboxylate ligand containing an orthogonal chiral diene group and the Zr₆(μ₃-O)₄(μ₃-OH)₄ secondary building unit (SBU). Zr MOFs of the UiO structure not only provide a highly tunable platform for designing functional materials but also are stable under a broad range of reaction conditions.¹⁷ The diene ligand (LH₂) was synthesized from a known chiral diene-carboxylic acid compound (Scheme 1).¹⁸ Upon treatment with oxalyl chloride, the corresponding acid chloride was reacted with the dicarboxylic ester possessing an orthogonal amino group to afford the methyl ester of the chiral diene (LMe₂) in 57% yield. Subsequent saponification of LMe₂ afforded enantiopure LH₂ in 77% yield. E₂-MOF was synthesized as colorless crystals in 42% yield by treating ZrCl₄ with 1 equiv. of LH₂ and a small amount of trifluoroacetic acid (TFA) in dimethylformamide (DMF) at 70 °C for 5 d.

Scheme 1 Synthesis of LH₂. (i) oxalyl chloride, CH₂Cl₂; (ii) TEA, THF, 57% yield over 2 steps; (iii) NaOH, THF, EtOH, 77% yield; (iv) ZrCl₄, TFA, DMF, 70 °C, 5 d, 42% yield.

Single crystal X-ray diffraction revealed that E₂-MOF crystallizes in the Fm3̄m space group and adopts the UiO structure. The chiral diene moieties are randomly distributed in the framework and could not be located in the electron density map. The ¹H NMR spectra of digested E₂-MOF confirms that the chiral diene groups remain intact during the MOF crystal growth (Fig. S1 and S3, ESI†) and E₂-MOF has a formula of Zr₆(μ₃-O)₄(μ₃-OH)₄(L)₆·143DMF·109H₂O. Thermogravimetric analysis indicated that E₂-MOF contains 73% solvent (Fig. S2, ESI†), suggesting a highly porous framework structure. However, nitrogen sorption measurements afforded negligible surface areas, presumably due to framework distortion upon solvent removal (Fig. S4 and S5, ESI†).^9c,19 The pore accessibility of E₂-MOF, E₂-MOF·RhCl, and E₂-MOF·Rh(acac) was demonstrated by dye absorption measurements which shows the uptake of 5.32 (112 wt%), 2.53 (98 wt%), and 4.47 (107 wt%) of Brilliant Blue R-250 per unit cell by E₂-MOF, E₂-MOF·RhCl, and E₂-MOF·Rh(acac), respectively (Fig. S7, ESI†).^9c

Postsynthetic metalation of E₂-MOF was carried out by treatment with 1 equiv. of [RhCl(C₂H₄)₂]₂ or 1 equiv. of Rh(acac)(C₂H₄)₂, (based on the Rh equivalent with respect to the L equivalents in E₂-MOF, ESI†; acac is acetylacetonate). Powder X-ray diffraction (PXRD) studies indicated that E₂-MOF·RhCl and E₂-MOF·Rh(acac) remained crystalline and adopted the same structure as the original E₂-MOF (Fig. 2a). Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the extent of metalation in E₂-MOF based on the Rh to Zr ratios. E₂-MOF·RhCl achieved 66% metalation whereas E₂-MOF·Rh(acac) only had 13% of the L ligands metalated.

Fig. 2 (a) PXRD patterns of pristine E₂-MOF (simulated from the CIF file, black; experimental, red), and freshly prepared E₂-MOF·RhCl (blue) and E₂-MOF·Rh(acac) (pink). (b) PXRD patterns of pristine E₂-MOF·Rh(acac) (black) and E₂-MOF·Rh(acac) recovered from 1,2-addition reactions (after 1st run (red) and 8th run (blue)). (c) EXAFS data (squares) and best fits(lines) for E₂-MOF·RhCl. Data are displayed in R-space containing both magnitude of Fourier transform and real components. An R-factor of 0.01 was obtained for the fit. (d) A comparison of EXAFS data for E₂-MOF·RhCl, RhCl(LMe₂), and the [RhCl(nbd)]₂ dimer. (e) EXAFS data (squares) and best fits (lines) for E₂-MOF·Rh(acac). An R-factor of 0.016 was obtained for the fit. (f) A comparison of EXAFS data for E₂-MOF·Rh(acac) and Rh(acac)-LMe₂.

Due to the positional disorder and incomplete metalation of the diene moiety, the Rh coordination environments could not be determined by traditional crystallographic techniques. X-ray absorption fine structure (XAFS) spectroscopy at the Rh K-edge was used to investigate the local coordination environment of Rh in E₂-MOF·RhCl, E₂-MOF·Rh(acac), Rh-metalated LMe₂, and the dimeric [RhCl(nbd)]₂ standard. Data were processed and analyzed using the Athena and Artemis programs of the IFEFFIT package based on FEFF 6. E₂-MOF·RhCl was fitted with a monomeric model where the Rh coordination sphere is occupied by norbornadiene, chloride, and a THF molecule (Fig. 2c). The spectra for [RhCl(nbd)]₂ was fitted by the corresponding crystal structure (Fig. S13†). Compared to the spectra for E₂-MOF·RhCl, a significant peak was observed in R-space at ∼2 Å which is largely attributable to a second Rh–Cl direct scattering path; amplitude from Rh–Rh direct scattering paths can also be observed at ∼3.2 Å (Fig. 2d). The RhCl-LMe₂ system was best fitted with a combination of monomeric (∼85%) and dimeric (∼15%) models (Fig. S15, ESI†), which was confirmed by ¹H NMR spectroscopy (Fig. S18, ESI†). E₂-MOF·Rh(acac) and Rh(acac)-LMe₂ were fitted with a reported crystal structure where the Rh coordination sphere is occupied by a diene and an acac ligand.^18a There is little difference between E₂-MOF·Rh(acac) and Rh(acac)-LMe₂ in their EXAF spectra (Fig. 2f) presumably due to the similarity of one chelating diene and two bridging diene on each Rd center in EXAFS, but ¹H NMR of Rh(acac)-LMe₂ indicated that the Rh(acac)-LMe₂ contained a complex mixture including oligomeric/polymeric species in the homogeneous control (Fig. S19, ESI†). These results indicate that E₂-MOF·RhCl is a true single-site catalyst by prohibiting any such dimer formation owing to site isolation.^7a–j,20

E₂-MOF·RhCl is a highly effective catalyst for 1,4-additions of arylboronic acids to α,β-unsaturated ketones. The reaction of 2-cyclohexenone (1a) with phenylboronic acid (2a) in the presence of 0.01 mol% E₂-MOF·RhCl gave the addition product in 97% yield and 95% ee (Table 1, entry 1). At 0.005 mol% Rh loading, the reaction proceeded to give the addition product in 67% yield and 94% ee, leading to a high turnover number (TON) of 13 400 (entry 2). In comparison, the 1,4-addition reaction with 0.005 mol% Rh of E₂-MOF·Rh(acac) gave the addition product in 46% yield with 93% ee. These results are comparable with those of the homogeneous control catalyst (Table 1, entry 3). As expected, the catalytic activity of E₂-MOF·RhCl is much higher than BINAP-MOF·RhCl (Table 1, entries 2 vs. 4). E₂-MOF·RhCl catalyzed 1,4-addition reactions have a broad substrate scope for both arylboronic acids and α,β-unsaturated ketones. Both electron donating groups and electron withdrawing groups can be installed to the aromatic ring of arylboronic acids, giving the addition products in high yields and high ee's (Table 1, entries 5–8). The addition of arylboronic acids having a substituent at the meta and ortho position also proceeded (Table 1, entries 9 and 10). For α,β-unsaturated ketones, the reactions proceeded with five-membered ring and seven-membered ring substrates (Table 1, entries 11 and 12) as well as with a linear ketone (Table 1, entry 13). Heterogeneity of the 1,4-addition reaction was confirmed by ICP-MS, which indicates the leaching of only small amounts of Rh (1.3%) and Zr (<0.01%) into the solution. However, the recovered E₂-MOF·RhCl showed reduced catalytic activity (Scheme S1, ESI†), which might be due to the gradual loss of MOF crystallinity during the course of the reaction (Fig. S19, ESI†). Consistent with this, E₂-MOF soaked in 1 M HCl, water, or 1 M NaOH for 40 h lost crystallinity as judged by PXRD (Fig. S6, ESI†).

Table 1 Asymmetric 1,4-additions of arylboronic acids to α,β-unsaturated ketones with E₂-MOF·RhCl and homogeneous catalysts^a

Entry	Enone	Aryl boronic acid	Catalyst loading (mol% Rh)	Yield^b (%)	ee^c (%)	TON
a Reaction conditions: 1 (1 equiv.), 2 (1.2 equiv.), toluene, H2O at 100 °C for 40 h. b Isolated yield. c Determined by chiral HPLC. d [RhCl(C2H4)2]2 and LMe2 were used as catalyst. e BINAP-MOF·RhCl was used as catalyst. f Not determined. g 2.0 equiv. of PhB(OH)2.
1	1a	2a	0.01	97	95	9700
2	1a	2a	0.005	67	94	13 400
3^d	1a	2a	0.005	70	91	14 000
4^e	1a	2a	0.005	3	—^f	600
5	1a	2b	0.025	90	94	3600
6	1a	2c	0.05	80	91	1600
7	1a	2d	0.01	84	96	8400
8	1a	2e	0.05	87	95	1740
9	1a	2f	0.05	84	94	1680
10	1a	2g	0.05	82	74	1640
11	1b	2a	0.1	82	90	820
12^g	1c	2a	0.1	93	70	930
13^g	1d	2a	0.25	84	90	336

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Asymmetric 1,2-addition of arylboronic acids to aldimines proceeded in the presence of E₂-MOF·Rh(acac).²¹ At 0.2 mol% Rh loading, the reaction gave the addition product in 55% yield and 98% ee (Table 2, entry 1). Quantitative yield of the addition product was obtained at 3 mol% Rh loading (Table 2, entry 3). Interestingly, E₂-MOF·Rh(acac) performed better than the homogeneous control catalyst both in terms of yields and ee's. For the homogeneous control, the product yield can be increased by increasing the catalyst loading but at the expense of the obtained ee's. The ee's of the E₂-MOF·Rh(acac)-catalyzed reactions remained constant at different catalyst loadings. This striking difference can be attributed to the desirable site isolation provided by the MOF, which exclusively affords the desired monomeric Rh species; in contrast, the homogeneous control can form a dimeric/oligomeric species which might be less enantioselective. This monomer/dimer equilibrium was proved by EXAFS and ¹H NMR studies for the RhCl(LMe₂) system (Fig. 2d and 3a).

Table 2 Asymmetric 1,2-addition of aldimine 4a with E₂-MOF·Rh(acac) and homogeneous control catalyst^a

Entry	Catalyst	Catalyst loading	Yield^b (%)	ee^c (%)	TON
a 4a (1.0 equiv.), 2a (2.0 equiv.), catalyst, 1,4-dioxane, 100 °C, 20 h. b NMR yield based on internal standard. c Determined by chiral HPLC analysis. Ts = p-toluenesulfonyl.
1	E2-MOF·Rh(acac)	0.2 mol%	55	98	275
2	E2-MOF·Rh(acac)	0.6 mol%	71	98	118
3	E2-MOF·Rh(acac)	3 mol%	99	98	33
4	Rh(acac)/LMe2	0.2 mol%	11	94	55
5	Rh(acac)/LMe2	0.6 mol%	55	89	92
6	Rh(acac)/LMe2	3 mol%	87	83	29

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Fig. 3 (a) Structures of rhodium-coordinated diene complexes in MOF catalyst and the homogeneous control catalyst. (b) Plot of yield (%) and ee (%) of 1,2-addition product at various runs in the recycle and reuse of E₂-MOF·Rh(acac) (6 mol% Rh) for 1,2-addition of aldimine 4a with phenylboronic acid (2a).

E₂-MOF·Rh(acac)-catalyzed 1,2-addition reactions have a broad substrate scope for both arylboronic acids and aldimines to give addition products with excellent ee's (ranging from 97% to >99%). The reaction works with aldimines and arylboronic acids having electron donating groups or electron withdrawing groups (Table 3, entries 1–6).

Table 3 Asymmetric addition of arylboronic acids to N-tosylaldimines^a

Entry	R^1	R^2	Yield^b (%)	ee^c (%)	TON
a 4 (1.0 equiv.), 2 (2.0 equiv.), E2-MOF·Rh(acac) (3 mol% Rh), 1,4-dioxane, 100 °C, 20 h. b NMR yield based on internal standard. c Determined by chiral HPLC analysis.
1	Cl (4a)	H (2a)	99	98	33
2	Cl (4a)	F (2b)	95	99	32
3	Cl (4a)	OMe (2d)	80	97	27
4	H (4b)	F (2b)	97	99	32
5	H (4b)	OMe (2d)	96	97	32
6	OMe (4c)	H (2a)	98	99	33
7	OMe (4c)	F (2b)	99	>99	33

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Several experiments proved that E₂-MOF·Rh(acac) is a true heterogeneous and reusable catalyst. First, the MOF catalyst (6 mol% Rh) could be recycled and reused for at least 7 times without loss of yield and ee (Fig. 3b). Second, the crystallinity of the MOF catalyst recovered from the 1st and 8th runs was still maintained as the PXRD of the recovered catalyst remained the same as the freshly prepared E₂-MOF·Rh(acac) (Fig. 2b). Third, ICP-MS analysis showed negligible leaching of Rh (0.49%) and Zr (0.07%) during the reaction. Fourth, the progress of the reaction was stopped by removing the MOF catalyst from the reaction mixture, indicating that the supernatant is inactive in catalyzing the 1,2-addition reactions (Scheme S3, ESI†).

Conclusions

We have developed catalytically active chiral Rh-diene-based MOFs for asymmetric C–C formation reactions. The metalated MOFs catalyzed 1,4-addition of arylboronic acids to α,β-unsaturated ketones with a TON of 13 400 and 1,2-addition of arylboronic acids to aldimines with excellent enantioselectivity (up to >99% ee). E₂-MOF·Rh(acac) showed higher activity and enantioselectivity than the homogeneous control catalyst owing to the formation of a single-site catalyst in the MOF, and was reused for at least 7 times without loss of yield and ee. Our work thus establishes metalated diene-MOFs as highly active and enantioselective single-site solid catalysts for the construction of carbon–carbon bonds.

Acknowledgements

This work was supported by NSF (CHE-1464941). We thank C. Poon for help with ICP-MS analyses. Single crystal diffraction studies were performed at ChemMatCARS (Sector 15), APS, Argonne National Laboratory. ChemMatCARS is principally supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under grant number NSF/CHE-1346572. XAFS studies were performed at beamlines 9BM-B and 20BM-B at the APS. Use of the APS, an Office of Science User Facility operated for the U.S. DOE Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

Notes and references

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21. The 1,2-addition reaction did not proceed with E₂-MOF·RhCl.

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Footnote

† Electronic supplementary information (ESI) available: General experimental section; synthesis and characterization of ligand LH₂ and E₂-MOF; XAFS experiments and analyses; MOF-catalyzed asymmetric additions of arylboronic acids to α,β-unsaturated ketones and aldimines. CCDC 1064133. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5sc02100f

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