Letter Cite This: Org. Lett. 2017, 19, 6072-6075
pubs.acs.org/OrgLett
Self-Supported BINOL-Derived Phosphoric Acid Based on a Chiral Carbazolic Porous Framework Xiang Zhang,† Attila Kormos,†,‡ and Jian Zhang*,† †
Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States Department Chemical Biology Research Group, Institute of Organic Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar tudósok krt. 2, H-1117 Budapest, Hungary
‡
S Supporting Information *
ABSTRACT: The facile synthesis of a porous heterogeneous BINOL-derived chiral phosphoric acid BiCz-POF-1 using the mild, FeCl3-promoted oxidative polymerization is reported. For the first time, carbazole is introduced at the 3,3′-positions of the chiral BINOL-derived phosphoric acid to (1) offer steric hindrance for achieving a high enantioselectivity and (2) serve as a cross-linker for the construction of the porous solid catalyst. BiCz-POF-1 exhibits remarkable catalytic activity and enantioselectivity toward transfer hydrogenation of 1,4-benzoxazine, 1,4-benzoxazinone, and 2-phenylquinolone. Combined with its facile synthesis and excellent recyclability, BiCz-POF-1 represents a new class of heterogeneous chiral phosphoric acid that has wide potential utility in enantioselective organocatalysis.
T
remote site of the bulky 3,3′-substituents.15 Although such structural modification can result in a good retention of the enantioselectivity, it is a tedious process and often requires a multistep synthesis involving C−C coupling chemistry and precious-metal-catalysts. It is therefore highly desirable to develop a simple synthesis to construct a self-supported chiral phosphoric acid catalyst with high activity and enantioselectivity. Here we describe the synthesis of a carbazolic porous organic framework, BiCz-POF-1, as a self-supported BINOL-derived chiral phosphoric acid for heterogeneous enantioselective organocatalysis. We use simple Ullman C−N coupling chemistry and, for the first time, introduce carbazole as the bulky substituent to provide a large steric hindrance at the 3,3′positions of the BINOL-derived chiral phosphoric acid, BiCz-1. More importantly, the two carbazole moieties in BiCz-1 serve as the connecting group in the subsequent facile FeCl3-promoted oxidative polymerization that affords the highly porous solid chiral phosphoric acid, BiCz-POF-1. Remarkably, BiCz-POF-1 exhibits excellent activity and enantioselectivity for asymmetric hydrogenation of 3-phenyl-1,4-benzoxazine, 3-phenyl-1,4-benzoxazinone, and 2-phenylquinoline. Interestingly, enhanced catalytic activity was observed for BiCz-POF-1 compared to BiCz-1, which was attributed to the change of electronic effect of carbazole upon polymerization. The synthetic routes of the 3,3′-carbazole-substituted BINOLderived chiral phosphoric acid (BiCz-1) and its corresponding cross-linked polymer (BiCz-POF-1) are outlined in Scheme 1. The Ullman coupling of 3,3′-dibromodimethylated (R)-BINOL 1 with carbazole led to compound 2, which underwent
he past decade has witnessed tremendous growth of enantioselective organocatalysis.1 Nontoxic, robust, and easily accessible organocatalysts and mild reaction conditions have warranted their wide application.2 However, high loading (up to 20 mol %) and high cost, difficult separation, and poor recyclability have limited their utilities in large-scale synthesis. Therefore, immobilization of chiral organocatalysts onto solid support represents a logical strategy to overcome these drawbacks.3 While grafting molecular catalysts on inorganic materials4 and organic polymers5 has been utilized, using metal− organic frameworks (MOFs),6 covalent−organic frameworks (COFs),7 and microporous polymers8 as self-supported heterogeneous catalysts has recently become an emerging approach to increase the catalysts’ recyclability and reusability. Thanks to the versatile modular synthesis of these porous materials, the catalytically active chiral groups can be facilely incorporated as building blocks (i.e., ligand or monomer) with high density and accessibility, uniform distribution, and undisturbed geometry.9 Importantly, the catalytic activity and selectivity of the self-supported organocatalysts can be largely retained and are comparable to their homogeneous counterparts. Among commonly used organocatalysts, the axially chiral 1,1′bi-2-naphthol (BINOL)-derived phosphoric acid is of particular interest since it is highly effective for a wide range of enantioselective transformations.10 It is known that the substituents at the 3,3′-positions of BINOL have a considerable effect on the stereochemical outcome of many reactions, and a large steric bulk is usually required for a high enantioselectivity.11 Therefore, most BINOL-derived supported12 and self-supported13 phosphoric acids are designed to avoid direct modification at the 3,3′-positions of BINOL by placing the linkage for immobilization at either the 6,6′-positions14 or the © 2017 American Chemical Society
Received: September 14, 2017 Published: October 31, 2017 6072
DOI: 10.1021/acs.orglett.7b02887 Org. Lett. 2017, 19, 6072−6075
Letter
Organic Letters
(Table 1, entries 1−4), indicating the sufficient steric bulk provided by the carbazole moiety. The best selectivities (92−
Scheme 1. Preparation of BiCz-1 and BiCz-POF-1
Table 1. Chiral Phosphoric Acid Catalyzed Transfer Hydrogenation of 3-Phenyl Benzoxazine 5a
subsequent cleavage of the methyl ether by BBr3 to afford carbazole-based BINOL 3. Note that no racemization was observed, judged by the ee value of 3 (see Supporting Information S-5). Phosphoric acid monomer BiCz-1 was obtained by treating 3 with phosphorus oxychloride in pyridine followed by hydrolysis. BiCz-POF-1, however, was synthesized via the FeCl3 induced oxidative polymerization via homocoupling16 of the phosphoric acid chloride 4 (at the 3,6-positions of carbazole) followed by hydrolysis. The resulting solid was washed with aqueous HCl, ethanol, THF, and CHCl3 to afford the target polymer BiCz-POF-1 as a light-yellow powder. BiCz-POF-1 is insoluble in common organic solvents such as THF, CH2Cl2, CHCl3, methanol, acetone, and DMF, indicating the nature of a highly cross-linked network. Thermogravimetric analysis (Supporting Information, Figure S1) reveals its excellent thermal stability with the decomposition temperature of 550 °C. Solid state CP/MAS 13C NMR spectroscopy (Supporting Information S-5) shows five broad resonance peaks at 142, 132, 127, 120, and 111 ppm. Specifically, the peak at 142 ppm corresponds to the aryl carbons next to the oxygen and nitrogen atoms. The peaks at 132 and 127 ppm can be attributed to other substituted aryl carbons, and the peaks at 120 and 111 ppm are due to the unsubstituted aryl carbons. The FT-IR spectra of BiCz-POF-1 and BiCz-1 also indicate successful polymerization (Figure S2). Upon polymerization, the absorption peak at ∼722 cm−1 (assigned to the bisubstituted phenyl ring in carbazole monomer) decreases, and the absorption peak at ∼802 cm−1 (assigned to the trisubstituted phenyl ring in carbazole polymer) increases.17 The nitrogen adsorption/desorption isotherms (Figure S3) confirm the microporosity of BiCz-POF-1, including a Brunauer−Emmett−Teller surface area of 555 m2/g (and a Langmuir surface area of 631 m2/g) and a pore volume of 0.32 cm3 g−1 with a significant portion (52%) contributed by mesosized pores (>2 nm). Such porosity parameters indicate that BiCz-POF-1, especially when present in solvents, can allow for fast mass transport and easy accessibility of the catalytic centers, two beneficial factors for efficient heterogeneous catalytic reactions that involve relatively large organic molecules. The catalytic properties of phosphoric acid BiCz-1 and BiCzPOF-1 were evaluated first with asymmetric transfer hydrogenation of 3-phenyl-2H-1,4-benzoxazine 5 with the Hantzsch dihydropyridine 6,18 a prototypic reaction that was used by Blechert and Thomas et al. in the performance assessment of polymeric phosphoric acids.13a,15 With 5 mol % of BiCz-1, the asymmetric reduction of 5 proceeded in four solvents with excellent yields (93−95%) and enantioselectivity (89−94% ee)
entry
catalyst
solvent
yield (%)b
ee (%)c
1 2 3 4 5 6 7 8 9
BiCz-1 BiCz-1 BiCz-1 BiCz-1 BiCz-POF-1 BiCz-POF-1 BiCz-POF-1 BiCz-POF-1 BiCz-POF-1e
benzened toluened CHCl3d THFd benzene toluene CHCl3 THF THF
93 95 95 94 93 94 95 95 93
92 94 93 89 84 90 88 94 89
a
Reaction conditions: 5, 6 (1.25 equiv), and catalyst (5 mol %). Isolated yields. cDetermined by HPLC analysis on chiral stationary phase. dReaction conducted at 40 °C. e1 mol % catalyst.
b
94% ee) were achieved in nonpolar aromatic solvents (Table 1, entries 1−2), followed by polar THF (89% ee). This trend is consistent with previously observed “solvent effects” in the chiral phosphoric acid catalyzed hydrogenation reactions.19 To our delight, BiCz-POF-1 exhibited similar reactivity (93−95%) and enantioselectivity (84−94% ee) (Table 1, entries 5−8) as BiCz-1. The highest enantioselectivity (94% ee) was obtained, however, in the polar solvent THF (entry 8). Notably, the loading of BiCzPOF-1 can be further decreased to 1 mol % to complete the reaction (93% yield) at 40 °C, with a slightly lower enantioselectivity (entry 9, 89% ee). To further probe the difference of the catalytic activities of BiCz-1 and BiCz-POF-1, we monitored the reaction rate (defined by the rate of conversion of 5) using 1H NMR spectroscopy in CDCl3 at room temperature.20 To our surprise, BiCz-1 showed a slower reaction rate compared to BiCz-POF-1. After 2.5 h, a conversion of 84% was achieved for BiCz-POF-1, while BiCz-1 only resulted in a conversion of 35% (Figure 1a). Such acceleration effect is also present in THF, benzene, and toluene.20 This is in sharp contrast with previous reports where polymer-based catalysts generally exhibit similar or slower reaction rates than their molecular counterparts due to inferior
Figure 1. Reaction kinetics on catalytic asymmetric transfer hydrogenation of 5 in CDCl3 monitored by 1H NMR (catalyst loading: 0.2 mol %20). Recycle of BiCz-POF-1 for the asymmetric transfer hydrogenation of 1 (reaction conducted in THF, catalyst loading = 5 mol %, 24 h, rt). 6073
DOI: 10.1021/acs.orglett.7b02887 Org. Lett. 2017, 19, 6072−6075
Letter
Organic Letters mass transport.21 Further, since the enantioselectivities of BiCz-1 and BiCz-POF-1 are comparable, which indicates a similar steric effect, the enhanced activity of BiCz-POF-1 is likely due to an electronic effect. It is known that a small change of pKa of chiral phosphoric acid could lead to a significant change of reaction rate.22 We therefore hypothesized that upon polymerization via homocoupling at the 3,6-positions, the carbazole becomes more electron deficient, which results in a slight increase of the Brønsted acidity of the phosphoric acid that eventually leads to the enhanced catalytic activity. As a support, a dimerized phosphoric acid BiCz-1 (see Supporting Information for synthetic details) was tested in the same kinetic study, which also exhibited an increased reaction rate (Figure S3) compared to BiCz-1, suggesting that the change of the electronic effect in BiCz-POF-1 is likely an important factor accounting for the enhanced catalytic activity. It is noted, however, other factors such as substrate-catalyst affinity and substrate size can also play a role in the accelerated effect, which warrants further investigation that is currently underway in our laboratory. The most advantageous feature of heterogeneous catalysis is the recyclability and reusability of the catalysts. Indeed, the porous solid catalyst BiCz-POF-1 can be easily separated from the reaction mixture through centrifugation and reused. After four additional runs, no loss of activity was observed (99% conversion for the fifth run), and the enantioselectivity only slightly decreased from 94% ee to 91% ee (Figure 1b). To demonstrate the versatility of BiCz-1 and BiCz-POF-1 as chiral phosphoric acid catalyst, we next tested their activity in the asymmetric transfer hydrogenation of benzoxazinones, a reaction that can afford dihydrobenzoxazinones as the important structural motif of clinically significant pharmaceuticals and biologically active molecules.23 At a 5 mol % loading, the hydrogenation of 3-phenyl-2H-1,4-benzoxazin-2-one 8 proceeded smoothly at 40 °C (Table 2). The activity and selectivity
is moderate (76% conversion), it is still significantly higher than that of BiCz-1 (36% conversion). Similar to the reduction of 5, BiCz-POF-1 exhibited an enhanced reaction rate compared to BiCz-1 (Figure S4) and can be reused for an additional four times without significant loss of performance (Table 2, entry 7). Finally, we evaluated the activity of BiCz-1 and BiCz-POF-1 in transfer hydrogenation of 2-phenyl-quinoline 10 (Scheme 2).24 Scheme 2. Chiral Phosphoric Acid Catalyzed Transfer Hydrogenation of 2-Phenylquinoline 10
Despite similar conversions (93−95%), BiCz-POF-1, in general, outperforms BiCz-1 in enantioselectivity with the highest 90% ee obtained in THF (Table S1). The reaction kinetics study also indicates that BiCz-POF-1 offers a faster reaction rate compared to its molecular counterpart (Figure S5). In conclusion, we have demonstrated the synthesis of a heterogeneous BINOL-derived chiral phosphoric acid, BiCzPOF-1. For the first time, the carbazole group is introduced into the 3,3′-positions of BINOL to not only provide the required steric hindrance to achieve high enantioselectivity but also serve as a convenient linkage to construct the porous solid catalyst. Three prototypic substrates including 3-phenyl-1,4-benzoxazine, 3-phenyl-1,4-benzoxazinone, and 2-phenylquinoline can be efficiently converted into optically active hydrogenated products in excellent yields and enantioselectivities. In all three reactions, an enhanced catalytic performance of BiCz-POF-1 compared to its molecular counterpart was observed, which was best attributed to the change of electronic effect of the carbazole group. Furthermore, BiCz-POF-1 is highly robust and recyclable and can be easily separated and reused for up to four additional times without a significant loss of performance. Overall, BiCzPOF-1 represents a new class of heterogeneous chiral phosphoric acid that has wide potential utilities in enantioselective organocatalysis.
Table 2. Chiral Phosphoric Acid Catalyzed Transfer Hydrogenation of 3-Phenyl Benzoxazinone 8a
entry
catalyst
solvent
yield (%)b
ee (%)c
1 2 3 4 5 6 7
BiCz-1 BiCz-1 BiCz-1 BiCz-POF-1 BiCz-POF-1 BiCz-POF-1 BiCz-POF-1e
toluene CHCl3 THF toluene CHCl3 THF CHCl3
91 (78d) 87 36 90 95 (83d) 76 93 (80d)
99 98 88 85 96 96 93d
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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/acs.orglett.7b02887. Materials, general procedures, synthesis, physical measurements, NMR spectra, and chiral HPLC traces (PDF)
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Reaction conditions: 8, 6 (1.5 equiv), and catalyst (5 mol %), 40 °C. b Yields determined by 1H NMR. cDetermined by HPLC analysis on chiral stationary phase. dIsolated yields. eUsing BiCz-POF-1 that has been recycled four times. a
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jian Zhang: 0000-0003-0274-0814 Notes
of BiCz-1 exhibited a strong dependence on solvent polarity, and the nonpolar toluene appeared to be the better solvent (91% conversion and 99% ee, Table 2, entry 1) compared to CHCl3 and THF, which afforded a lower conversion of 87% and 36%, respectively (entries 2−3). Interestingly, such solvent dependence is much reduced for BiCz-POF-1 (Table 2, entries 4−6), with CHCl3 being the best solvent (95% conversion, 96% ee). It is noted that although the catalytic activity of BiCz-POF-1 in THF
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.Z. acknowledges a National Science Foundation CAREER Award (DMR-1554918) for support of this research. We thank Ms. Veronika Shoba (UNL) and Prof. Jim Takacs (UNL) for assistance in chiral HPLC. 6074
DOI: 10.1021/acs.orglett.7b02887 Org. Lett. 2017, 19, 6072−6075
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Organic Letters
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(18) Rueping, M.; Antonchick, A. P.; Theissmann, T. Angew. Chem., Int. Ed. 2006, 45, 6751. (19) Rueping, M.; Sugiono, E.; Azap, C.; Theissmann, T.; Bolte, M. Org. Lett. 2005, 7, 3781. (20) We chose 0.2 mol % loading in CDCl3 to monitor the reaction kinetics because at such loading in three hours, BiCz-POF-1 gave a conversion of 5 of 81%, 48%, 43%, and 34% in CHCl3, THF, benzene, and toluene, respectively. As a comparison, BiCz-1 gave a conversion of 44%, 25%, 18%, and 22%, respectively. (21) Schmidt, J.; Kundu, D. S.; Blechert, S.; Thomas, A. Chem. Commun. 2014, 50, 3347. (22) Kaupmees, K.; Tolstoluzhsky, N.; Raja, S.; Rueping, M.; Leito, I. Angew. Chem., Int. Ed. 2013, 52, 11569. (23) Macías, F. A.; Marín, D.; Oliveros-Bastidas, A.; Molinillo, J. M. G. Nat. Prod. Rep. 2009, 26, 478. (24) (a) Rueping, M.; Antonchick, A. P.; Theissmann, T. Angew. Chem., Int. Ed. 2006, 45, 3683. (b) Mitra, R.; Zhu, H.; Grimme, S.; Niemeyer, J. Angew. Chem., Int. Ed. 2017, 56, 11456.
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DOI: 10.1021/acs.orglett.7b02887 Org. Lett. 2017, 19, 6072−6075
