Chiral Bifunctional Metalloporphyrin Catalysts for Kinetic Resolution of

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Chiral Bifunctional Metalloporphyrin Catalysts for Kinetic Resolution of Epoxides with Carbon Dioxide Chihiro Maeda,* Mayato Mitsuzane, and Tadashi Ema* Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Okayama 700-8530, Japan

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S Supporting Information *

ABSTRACT: Chiral binaphthyl-strapped Zn(II) porphyrins with triazolium halide units were synthesized as bifunctional catalysts for kinetic resolution of epoxides with CO2. Several catalysts were screened by changing the linker length and nucleophilic counteranions, and the optimized catalyst accelerated the enantioselective reaction at ambient temperature to produce optically active cyclic carbonates and epoxides.

arbon dioxide (CO2) fixations have attracted considerable attention during the last decades, from the viewpoint of green and sustainable chemistry.1,2 Synthesis of cyclic carbonates from epoxides and CO2 is one of the valuable CO2 fixations with high atom efficiency.3−11 Various catalysts including metal complexes4−6 and organocatalysts7 have been developed to accelerate the reaction. This reaction is applicable to the kinetic resolution of epoxides using chiral catalysts, although most catalysts reported so far are based on salen derivatives (Scheme 1).8−10 It has been revealed that the

C

Scheme 1. Kinetic Resolution of Epoxides

Figure 1. Chiral strapped porphyrin catalysts for the kinetic resolution of epoxides with CO2.

However, the doubly strapped porphyrins suffer from relatively low catalytic activity and narrow substrate scope. On the other hand, we have developed highly active bifunctional metalloporphyrin catalysts bearing quaternary ammonium halide groups for the synthesis of cyclic carbonates from epoxides and CO2.6 The metal center and halide anion acted as the Lewis acid and nucleophile, respectively, and the cooperative effect accelerated the reaction. In this work, we designed a series of chiral bifunctional porphyrins 1 (Figure 1). Triazolium salt was chosen as the nucleophilic moiety, because of the synthetic accessibility: click reaction of alkyne with azide, followed by the alkylation. In addition, the counter-

efficient kinetic resolution of epoxides with CO2 is quite difficult,8−10 and the development of effective chiral catalysts for this purpose remains a big challenge. We focused on strapped porphyrins, which may allow molecular recognition and catalysis in the unique molecular space.12 Several chiral strapped porphyrins have been used as catalysts for asymmetric synthesis,13 while kinetic resolution of epoxides with CO2 using chiral porphyrins had not been reported until 2018. Very recently, two-component catalytic systems composed of chiral strapped porphyrins and quaternary ammonium salts for enantioselective cycloaddition of epoxides to CO2 were reported by Jing and our group independently (Figure 1).9 In our previous study, the double straps were necessary to achieve the kinetic resolution. © XXXX American Chemical Society

Received: February 2, 2019

A

DOI: 10.1021/acs.orglett.9b00447 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

First, we used 1a−1e as catalysts for the kinetic resolution of styrene oxide (2a) to examine the linker length (Table 1,

anions near the binaphthyl moiety will enable the reaction to occur mainly at the bridged site (chiral space), so that the single strap is expected to be sufficient in this case. The total yields of the catalysts may be much higher than those in the previous work, and both the reaction conditions and substrate scope can be investigated in detail. Here, we report chiral bifunctional binaphthyl-strapped Zn(II) porphyrins with triazolium salts 1 (Figure 1). The bifunctional catalysts, which were prepared efficiently, catalyzed the kinetic resolution at ambient temperature to produce optically active cyclic carbonates and epoxides. The synthetic route to bifunctional catalysts 1 is shown in Scheme 2. Reaction of bromoalkoxybenzaldehyde 4 with

Table 1. Kinetic Resolution of Styrene Oxidea

% yieldc (% ee)d

Scheme 2. Synthesis of 1

entry

catalyst

time (h)

cb (%)

1 2 3 4 5 6 7 8f 9 10h 11i

1a 1b 1c 1d 1e 1f 1g 9c 1h 1c 1c

6 6 6 4 4 6 6 6 6 1 36

19 47 53 42 55 16 23 3.8g 0.7g 27 24

3a 17 45 49 40 53 11 21 1.7 1.0 30 23

(31) (31) (30) (30) (25) (33) (27) (6.7) (6.1) (27) (35)

75 25 39 37 33 71 71 83 71 57 50

2a

se

(7.4) (27) (34) (22) (31) (6.2) (8.0) (3.9) (1.2) (10) (11)

2.0 2.4 2.5 2.3 2.2 2.1 1.9 1.2 1.1 1.9 2.3

a

Reaction conditions: 2a (3.0 mmol), catalyst (0.2 mol %). Conversion calculated from c = ee(2a)/(ee(2a) + ee(3a)). cIsolated yield. dEnantiomeric excess determined by HPLC or GC analysis. e Calculated from s = ln[1 − c(1 + ee(3a))]/ln[1 − c(1 − ee(3a))]. f 0.4 mol % of TBAI was used as a co-catalyst. gNMR conversion. hAt 50 °C. iAt 10 °C. b

entries 1−5). 2a was allowed to react at 30 °C in the presence of 0.2 mol % of 1 under a CO2 pressure of 1.7 MPa in an autoclave. Styrene carbonate (3a) and 2a were isolated, and their enantiomeric excesses were analyzed. The catalytic activity increased with an increase in the linker length (1a < 1b < 1c ≈ 1d < 1e), suggesting that the strap moieties of 1a− 1e acted as steric hindrance, to some degree. To our delight, they showed enantioselectivities, and 1c recorded the highest s value of 2.5 (1c > 1b > 1d > 1e > 1a). Since 1c showed the highest enantioselectivity, the counteranions of 1c were then changed to Cl− and Br− to give catalysts 1f and 1g, respectively. 1f and 1g showed lower activity and enantioselectivity than 1c, suggesting the importance of the leaving ability (entries 3, 6, and 7 in Table 1). In addition, we used free-base porphyrin 1h with triazolium moieties and the twocomponent catalytic system composed of 9c and TBAI as reference catalysts. 1h and 9c showed much lower activity with almost no enantioselectivity, which demonstrates the importance of the bifunctional system for catalytic activity and the enantioselectivity (entries 3, 8, and 9 in Table 1). Importantly, the low enantioselectivity of 9c suggests that the reaction mainly proceeds on the opposite side of the strap and that the reaction using 1c mainly proceeds at the bridged site. As shown above, 1c was found to be the best catalyst, and we then investigated the reaction conditions in detail using 1c. First, the reaction was performed at 50 and 10 °C. Curiously, the enantioselectivity decreased in both cases (Table 1, entries 3, 10, and 11), although selectivity is higher in most reactions at lower temperature. We anticipated that high viscosity under the solvent-free conditions might decrease the enantioselectivity at 10 °C. Therefore, we then performed the reactions in the presence of an additive solvent (Table 2). To our delight, we found that addition of CHCl3 or CH2Cl2 increased the enantioselectivity, while the use of other solvents resulted in

sodium azide provided azidoalkoxybenzaldehyde 5. Click reaction of 5 with propargyl-appended BINOL 6 gave dialdehyde 7 containing triazole and binaphthyl. Acidcatalyzed condensation of 7 with dipyrromethane formed free-base strapped porphyrin 8.14 Zinc metalation of 8 with Zn(OAc)2, and the subsequent reaction of 9 with MeI afforded bifunctional catalyst 1. Thus, 1a−1e were prepared in yields of 5.4%−11% from 4a−4e. The total yields were much improved, compared to those for the previously reported doubly strapped porphyrins (0.4%−1.4%; see Figure 1). The counteranions of 1c were changed to Cl− and Br− to give catalysts 1f and 1g, respectively. In addition, the reaction of 8c with MeI gave freebase porphyrin 1h with triazolium salt (Figure 1). B

DOI: 10.1021/acs.orglett.9b00447 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

the products exhibited an empirical rule as shown in Scheme 3: the enantiomers with the same stereochemistry reacted faster. Considering further modifications, 1c is a promising catalyst showing wide substrate scope with low catalyst loading (0.2− 0.5 mol %), compared to the catalysts reported so far. In summary, we developed singly binaphthyl-strapped chiral porphyrins 1a−1g as bifunctional catalysts for the kinetic resolution of epoxides with CO2. We performed the structural optimization of the catalysts by changing the alkyl chain length and counteranions, and 1c was found to be the best catalyst. The comparison with the two-component catalytic system revealed the importance of the bifunctional system. Reaction conditions were then optimized, and the addition of CHCl3 was found to increase the enantioselectivity. In addition, various epoxides reacted under the optimized condition to give optically active cyclic carbonates and epoxides. Further investigation on the development of chiral catalysts for kinetic resolution, as well as novel CO2 fixation reaction, is currently underway in our laboratory.

Table 2. Kinetic Resolution of Styrene Oxide with an Additive Solventa

% yieldc (% ee)d b

entry

solvent

time (h)

c (%)

1 2 3 4 5 6 7 8 9 10f

− CHCl3 CH2Cl2 CCl4 acetone THF CH3CN toluene hexane CHCl3

36 48 48 48 48 48 48 48 48 48

24 33 35 29 15 31 42 15 31 10

3a 23 31 34 26 15 34 38 13 29 9.3

(35) (41) (37) (34) (37) (34) (32) (31) (33) (59)

50 47 45 49 31 51 46 49 48 61

2a

se

(11) (20) (20) (14) (6.8) (15) (23) (5.6) (15) (6.4)

2.3 2.9 2.6 2.3 2.3 2.4 2.4 2.0 2.3 4.1

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a

Reaction conditions: 2a (3.0 mmol), catalyst (0.2 mol %), solvent (50 μL). bConversion calculated from c = ee(2a)/(ee(2a) + ee(3a)). c Isolated yield. dEnantiomeric excess determined by HPLC or GC analysis. eCalculated from s = ln[1 − c(1 + ee(3a))]/ln[1 − c(1 − ee(3a))]. f500 μL of CHCl3 was used.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00447. Experimental procedures and compound data (PDF)

unchanged or lower s values. Furthermore, the enantioselectivity increased with an increase of the amount of CHCl3, and the use of 500 μL reached the highest s value of 4.1 (Table 2, entry 10). The addition of further amounts of CHCl3 decreased the enantioselectivity (see Table S1 in the Supporting Information (SI)). Finally, we investigated the substrate scope of 1c using CHCl3 as an additive solvent (see Scheme 3 and Table S2 in

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C. Maeda). *E-mail: [email protected] (T. Ema). ORCID

Chihiro Maeda: 0000-0003-4370-3905 Tadashi Ema: 0000-0002-2160-6840

Scheme 3. Substrate Scope of 1c

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Grant No. JP16H01030) in Precisely Designed Catalysts with Customized Scaffolding and The Foundation for The Promotion of Ion Engineering. We thank Prof. H. Yorimitsu (Kyoto University) for mass measurements.

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REFERENCES

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DOI: 10.1021/acs.orglett.9b00447 Org. Lett. XXXX, XXX, XXX−XXX

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D

DOI: 10.1021/acs.orglett.9b00447 Org. Lett. XXXX, XXX, XXX−XXX