Highly Enantioselective Oxidation of Spirocyclic Hydrocarbons by

Research Article Cite This: ACS Catal. 2018, 8, 2479−2487

pubs.acs.org/acscatalysis

Highly Enantioselective Oxidation of Spirocyclic Hydrocarbons by Bioinspired Manganese Catalysts and Hydrogen Peroxide Bin Qiu,†,‡ Daqian Xu,† Qiangsheng Sun,† Chengxia Miao,† Yong-Min Lee,§ Xiao-Xi Li,§ Wonwoo Nam,*,†,§ and Wei Sun*,† †

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Center for Excellence in Molecular Synthesis, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea S Supporting Information *

ABSTRACT: Bioinspired manganese complexes have emerged as attractive catalysts for a number of selective oxidation reactions over the past several decades. In the present study, we report the enantioselective oxidation of spirocyclic compounds with manganese complexes bearing tetradentate N4 ligands as catalysts and aqueous H2O2 as a terminal oxidant under mild conditions; spirocyclic tetralone (1a) and its derivatives bearing electron-donating and -withdrawing substituents are converted to their corresponding chiral spirocyclic β,β′-diketones with high yields and enantioselectivities. Spirocyclic indanones are also converted to the β,β′-spirobiindanones with high enantioselectivities. Indeed, the reaction expands the diversity of chiral spirocyclic diketones via a late-stage oxidative process. In addition, it is of importance to note that the catalytic reaction can be easily scaled up and the chiral spirocyclic β,β′-diketones can be transformed into diol products. In mechanistic studies, we have shown that (1) ketones were yielded as products via the initial formation of alcohols, followed by the further oxidation of the alcohols to ketones, (2) hydrogen atom (H atom) abstraction from the methylene C−H bonds of 1a by a putative Mn(V)-oxo intermediate was proposed to be the rate-determining step, and (3) the C−H bond hydroxylation of 1a by the Mn(V)-oxo species was proposed to occur via oxygen rebound mechanism. On the basis of these results, we have proposed a plausible mechanism for the selective C−H bond oxidation of hydrocarbons by bioinspired manganese catalysts and hydrogen peroxide. KEYWORDS: C−H oxidation, manganese catalyst, asymmetric oxidation, hydrogen peroxide, mechanism, manganese-oxo

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purpose of elucidating enzymatic reactions in biology.6 One notable example is the synthetic metalloporphyrins used as catalysts in the (enantio)selective oxidation of organic substrates as well as chemical models of Cytochromes P450.3b,7 For example, Groves and co-workers observed a moderate level of enantioselectivity in the asymmetric hydroxylation of ethylbenzene by a chiral iron porphyrin complex.8 Although significant progress has been made in the aliphatic C−H bond oxidation catalysis over the past several decades,9−12 asymmetric oxidation of aliphatic C−H bonds by chiral metal catalysts has been reported only in few cases.13−16 To develop efficient asymmetric C−H bond oxidation catalysis, two major challenges should be overcome, such as (1) the C−H oxidation of hydrocarbons by catalysts yields chiral alcohol products with a poor to moderate enantioselectivity and (2) the chiral alcohol products are

INTRODUCTION The selective oxidation of hydrocarbon C−H bonds is a highly challenging reaction in synthetic organic chemistry, but common in enzymatic reactions.1 For example, metalloenzymes (e.g., Cytochromes P450 and methane monooxygenases) generate highly reactive intermediates (e.g., high-valent metaloxo species) for the stereo-, regio-, and enantioselective oxidation of organic substrates.2 Therefore, development of efficient and selective oxidation of hydrocarbons, especially for the enantioselective oxidation of C−H bonds, using bioinspired metal catalysts and environmentally benign oxidants has been a long-standing goal in the communities of synthetic organic, oxidation, and biomimetic/bioinorganic chemistry.3 In biomimetic studies, tremendous efforts have been devoted to developing artificial catalysts that mimic the reactivities of metalloenzymes in catalytic oxidation reactions.3−5 In addition, bioinspired metal complexes have been used in the synthesis and characterization of metal−oxygen intermediates (e.g., highvalent metal-oxo species), and reactivities and mechanisms of the synthetic metal−oxygen intermediates have been intensively investigated in various oxidation reactions, with the © 2018 American Chemical Society

Received: October 23, 2017 Revised: December 27, 2017 Published: February 7, 2018 2479

DOI: 10.1021/acscatal.7b03601 ACS Catal. 2018, 8, 2479−2487

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of these chiral spirocyclic compounds remain significantly underdeveloped.22

further oxidized rapidly to ketones, thereby losing the stereogenic center (Scheme 1A). Fortunately, the problem of Scheme 1. Strategy for the Enantioselective Oxidation of C− H Bonds in This Study

Figure 1. Examples of chiral ligands and natural product containing the spirocyclic structural motif.

Recently, a great advance has been achieved in the enantioselective epoxidation of olefins by bioinspired manganese complexes bearing nonheme ligands and H2O2 in the presence of additives (e.g., carboxylic acids or H2SO4);5,23,24 the asymmetric epoxidation reactions are highly efficient in terms of product yields and enantioselectivities. In contrast, there are only a couple of reports for the enantioselective oxidation of aliphatic C−H bonds by nonheme manganese catalysts and H2O2.14,15 Since we have reported an efficient manganese catalytic system using H2O2 as a terminal oxidant for the oxidation of benzylic and aliphatic methylene C−H bonds to ketones under mild reaction conditions,13 we attempted to examine the previously reported catalytic systems (i.e., using a bioinspired manganese catalyst and an environmentally benign oxidant) in the enantioselective synthesis of spirocyclic compounds. Herein, we report an enantioselective oxidation of benzylic methylene C−H bonds in spirocyclic precursors by a manganese catalyst bearing a tetradentate N4 ligand and aqueous H2O2, affording a vast array of chiral spirocyclic β,β′diketones in high yields with excellent enantioselectivities (Scheme 1C). Mechanistic aspects for the manganese-catalyzed enantioselective C−H bond activation reaction have also been discussed in the present study.

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RESULTS AND DISCUSSION To examine the feasibility of our proposed strategy, we conducted the oxidation of methylene C−H bonds of spirocyclic compounds by nonheme manganese complexes and aqueous H2O2 in the presence of carboxylic acids (see Table 1 for the structures of manganese complexes and carboxylic acids and the reaction scheme). First, the oxidation of spirocyclic tetralone (1a) by [(S-PEB)Mn] (3a)13 (2.0 mol %) and H2O2 (7.0 equiv to 1a) at 0 °C afforded spirocyclic diketone (2a) in 80% yield with 87% enantiomeric excess (ee) in the presence of 14 equiv of 2,2-dimethylbutanoic acid (DMBA) at 0 °C (Table 1, entry 1). To optimize the reaction conditions, the oxidation of 1a was carried out at different reaction temperatures with different amounts of catalyst loading and carboxylic acid additive, and the catalytic activity and enantioselectivity of several manganese catalysts were also examined (Table 1). When the reaction was carried out at −30 °C, the ee value of 2a increased to 94% (Table 1, entry 3). Reducing the catalyst loading (0.20 mol % catalyst) also afforded a high enantioselectivity albeit with a moderate yield (Table 1, entry 5). In the presence of 0.50 mol % catalyst 3a, the amount of oxidant could be reduced to 5.0 equiv while maintaining both reactivity and enantioselectivity (Table 1, entry 6). Changing the N-substituent of benzimidazole motif in catalyst 3 to methyl (3b) and isopropyl (3c) slightly decreased

the overoxidation of alcohols to the corresponding ketones has the potential application in enantioselective desymmetrization reactions.17 For example, Bach and co-workers reported an elegant study for the oxidative desymmetrization of spirocyclic oxindoles by a chiral ruthenium porphyrin catalyst and 2,6dichloropyridine N-oxide, in which the methylene C−H bonds were oxidized to yield ketone products with high enantioselectivities (Scheme 1B).18 In the study, enantioselectivities were achieved by building hydrogen bonding between substrates and the porphyrin ligand of the ruthenium complex (Scheme 1B).18 If such an oxidative strategy can be expanded to all-carbon spirocyclic precursors, a catalytic enantioselective oxidation of spirocyclic compounds to their corresponding spiro β,β′diketone derivatives can be successfully established (Scheme 1C). Although these optically pure spirocyclic compounds are important structural motifs that are found not only in many important chiral ligands of asymmetric catalysis but also in biologically active natural products (Figure 1),19−21 catalytic enantioselective methods for the synthesis and functionalization 2480


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oxidation of spirocyclic compounds in this manganese catalytic system is highly site-selective (Scheme 1C). With the optimized reaction conditions (Table 1, entry 6), we investigated the oxidation of spirocyclic tetralone derivatives (1a− 1i; see Supporting Information (SI) for the syntheses of the spirocyclic monoketone substrates), showing that spirocyclic diketones were formed with moderate to good yields in all of the reactions (Table 2). More importantly, enantioselectiv-

Table 1. Optimization of Site- and Enantiotopos-Selective Oxidation of Methylene C−H Bonds of Spirocyclic Tetralonea

Table 2. Products Formed in the Oxidation of Spirocyclic Tetralone Derivatives (1a− 1i)a,b

entry

catalyst (mol %)

H2O2 (equiv)

temp. (°C)

conv.b (%)

yieldc (%)

eed (%)

1 2 3 4 5 6 7 8e 9 10 11 12 13f 14g

3a (2.0) 3a (2.0) 3a (2.0) 3a (0.50) 3a (0.20) 3a (0.50) 3a (0.50) 3a (0.50) 3b (0.50) 3c (0.50) 4a (0.50) 4b (0.50) 3a (0.50) 3a (0.50)

7.0 7.0 7.0 7.0 7.0 5.0 3.5 5.0 5.0 5.0 5.0 5.0 5.0 5.0

0 −20 −30 −30 −30 −30 −30 −30 −30 −30 −30 −30 −30 −30

94 88 86 88 60 90 86 78 86 80 92 92 80 92

80 77 76 74 54 80 74 56 74 67 75 78 64 83

87 92 94 94 93 94 93 92 92 86 75h 80h 89 90

a Reaction conditions: Substrate 1a (0.20 mmol), manganese catalyst, and DMBA (14 equiv) were dissolved in CH2Cl2 (1.0 mL) at −30 °C under an Ar atmosphere. Then, H2O2 (30% aqueous solution diluted in 1.0 mL of MeCN) was added to the solution dropwise over 2 h using a syringe pump, and the reaction solution was stirred for additional 2 h. bConversion was based on the amount of the recovered 1a. cIsolated yield. dThe ee values were determined by HPLC using chiral stationary phases. eDMBA (7 equiv) was used. fEHA (14 equiv) was used instead of DMBA. gAcOH (14 equiv) was used instead of DMBA. hThe configuration of 2a was opposite due to the use of (R,R) manganese catalyst.

a Reaction conditions: Substrate 1 (0.20 mmol), manganese catalyst 3a (0.50 mol %), and DMBA (14 equiv) were dissolved in CH2Cl2 (1.0 mL) at −30 °C under an Ar atmosphere. Then, 5.0 equiv of H2O2 (30% aqueous solution diluted in 1.0 mL of MeCN) was added dropwise over 2 h using a syringe pump, and the reaction solution was stirred for additional 2 h. The ee values were determined by HPLC using chiral stationary phases. bSee SI for the product analyses for 2a− 2i (see also SI, CIF files for the X-ray structures of 2g and 2i). c4b was used as a catalyst, and the configuration of 2a was opposite due to the use of (R,R) manganese catalyst.

the ee value of 2a (Table 1, entries 9 and 10). Compared with the catalyst 3b, the catalyst 4a (R,R-MCMB-Mn)23d bearing a C2-symmetric cyclohexane-1,2-diamine backbone afforded a low enantioselectivity (Table 1, entry 11 (75% ee) versus entry 9 (92% ee)). Similarly, the ee value of the product 2a was low in the reaction of the catalyst 4b (R,R-PDMB-Mn) (Table 1, entry 12).23c Because it has been well documented that the reactivity and enantioselectivity of nonheme metal complexes are affected significantly by carboxylic acid additives in the catalytic asymmetric epoxidation of olefins by H2O2,24−27 other carboxylic acids, such as 2-ethylhexanoic acid (EHA) and acetic acid (AcOH), were examined. In these reactions, we found that the ee value of 2a was decreased slightly (e.g., ∼ 90% ee) (Table 1, entries 13 and 14). Finally, it is notable that only trace amounts of products were formed from the methylene C−H bonds of six-membered ring in 1a, demonstrating that the

ities were excellent in these reactions (up to 94% ee; see 2a and 2e in Table 2). Only the substrate 1d bearing a methoxy group at the C5 position afforded a low product yield; it was probably due to the steric effect of methoxy group at the C5 position (see the data of 2d in Table 2, such as 34% yield and 77% ee). In addition, a low ee value (67%) was observed only in the case of 2g. Interestingly, catalyst 4b with a bipyrrolidine backbone (see the Mn structure in Table 1) afforded a high enantioselectivity for this substrate (89% ee). Furthermore, spirocyclic chromanone 1i was readily converted to the diketone product (2i) with a high enantioselectivity (90% ee). The structures of the diketone products, 2g and 2i, were 2481


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Scheme 2. Gram-Scale Synthesis of Spirocyclic Diketone (2a) and Its Derivatization

Table 3. Spirocyclic Indanone Derivatives (5) as Substratea

transformed into bisphosphinite ligand,21e,28 we reduced 2a with the literature procedure and obtained the spirocyclic diol product (7a) without the loss of the optical purity (Scheme 2). The structure of the diol product, 7a, was determined by singlecrystal X-ray diffraction analysis (Scheme 2; also see SI, CIF file). To gain mechanistic insights into the oxidation of spirocyclic hydrocarbons catalyzed by manganese catalyst and H2O2, we first conducted the oxidation of 1a by reducing the amounts of the Mn catalyst (3a) and the oxidant (H2O2) (Scheme 3A). In this reaction, spirocyclic alcohol (8a) was isolated with high diastereoselectivity and enantioselectivity (13:1 diastereomer ratio (dr) and 97% ee; see SI, Figure S1) (Scheme 3A, eq 1), and further oxidation of 8a produced the corresponding spirocyclic diketone (2a) with 92% yield and 95% ee (Scheme 3A, eq 2). These results demonstrate unambiguously that alcohols were produced initially in the oxidation of spirocyclic compounds by 3a and H2O2, followed by the further oxidation of the alcohols to the spirocyclic diketones.14,29 Second, we determined a kinetic isotope effect (KIE) value of 3.0(2) by carrying out an intermolecular competitive oxidation of spirocyclic tetralone (1a) and its deuterated spirocyclic tetralone (1a-d4; SI, Figure S2 for the synthesis) (Scheme 3B), proposing that hydrogen atom (H atom) abstraction from the methylene C−H bonds of 1a by a putative Mn(V)-oxo intermediate is the rate-limiting step (vide infra); a KIE value of 3.9 was reported in the oxidation of cyclohexane and deuterated cyclohexane by 3b and H2O213 and KIE values of ∼3−4 were obtained in the hydroxylation of cumene, ethylbenzene, and cyclohexane by nonheme manganese catalysts and H2O2.11b,15 Further, when the catalytic oxidation of 1a by 3a and H2O2 was carried out using H216O2 under an 18O2 atmosphere, most (>99%) of oxygen atoms in 2a were found to derive from H216O2 (Scheme 3C; SI, Figure S3 for GC-MS spectrum). Furthermore, when the catalytic oxidation of 1a by 3a and H2O2 was carried out in the presence of CCl3Br, the ketone product (2a) was formed predominantly with a small amount of brominated product (99% ee) could be obtained after recrystallization in ethanol. Further, since the reduction of spirobiindanone 6a was reported previously using DIBALH and t-BuLi in THF at −78 °C and the resulting chiral diol could be 2482


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presence of carboxylic acid additives,4,5 a Mn(V)-oxo species is formed via O−O bond heterolysis of a putative Mn(III)-OOH intermediate (Scheme 4, pathway a); the role of carboxylic acids for the formation of Mn(V)-oxo (and Fe(V)-oxo) has been extensively discussed as “carboxylic acid-assisted mechanism”.11,24−26 Interestingly, when H atom abstraction occurs by this Mn(V)-oxo species (Scheme 4, pathway c), the resultant Mn(IV)−OH and carbon radical species rebound at a fast rate in the cage to form a chiral alcohol product (Scheme 4, pathway d). Indirect evidence supporting the fast oxygen rebound process between the Mn(IV)−OH and carbon radical species was obtained in the 18O2 and CCl3Br experiments (Scheme 4, pathway e). Finally, the alcohol product is further oxidized by another molecule of Mn(V)-oxo species to yield the ketone product (Scheme 4, pathway f).

Scheme 3. Summary of Mechanistic Studies in the Oxidation of Spirocyclic Tetralone (1a)

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CONCLUSIONS In summary, we have reported an example of site- and enantioselective oxidation of benzylic methylene of spirocyclic compounds using bioinspired manganese complexes as catalysts and aqueous H2O2 as an oxidant. The product yields and enantioselectivities of chiral spirocyclic β,β′-diketones were high, and the catalytic system was demonstrated to be applicable to the gram-scale synthesis of the chiral spirocyclic diketones. Further, the chiral diketones were easily converted to the corresponding alcohols. We have also discussed mechanistic insights into the C−H bond activation of hydrocarbons catalyzed by the manganese catalyst and H2O2, such as the nature of Mn-oxo intermediate and the C−H bond activation mechanisms. Future studies will be focused on developing highly efficient enantioselective oxidation of unactivated C−H bonds by bioinspired nonheme iron and manganese catalysts as well as elucidating chemical and physical properties of the putative metal(V)-oxo species in nonheme systems.

including Mn(IV)-oxo complexes, occurs via oxygen nonrebound mechanism.31,32 On the basis of the results discussed above and those reported previously,4,5,10−12,14,15,23a,24−26 we propose the following mechanism for the C−H bond oxidation of hydrocarbons catalyzed by nonheme Mn catalysts and H2O2 in the presence of carboxylic acid additives (Scheme 4). As proposed in numerous reports of nonheme iron and manganese complex-catalyzed oxidation of hydrocarbons by H2O2 in the

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EXPERIMENTAL SECTION Materials. All chemicals were purchased from Aldrich, Alfa Aesar, and TCI and used as received unless otherwise indicated. Solvents were dried according to published procedures and distilled under argon prior to use.33 All reactions were performed under an Ar atmosphere using dried solvents and standard Schlenk techniques unless otherwise noted. Ligands, (S)-PEB, (S)-PMB, (S)-PiPB, (R,R)-MCMB, and (R,R)-PDMB and their manganese complexes were prepared according to the literature procedures.13,23 Instrumentation. 1H and 13C NMR spectra were recorded on a Bruker AVANCE III 400 MHz spectrometer. All NMR spectra were recorded at room temperature and were indirectly referenced to TMS using residual solvent signals as internal standards. High resolution mass spectra (HRMS) were obtained on an Agilent 6530 Q-TOF mass spectrometer with an ESI source. X-ray crystallographic data were collected on a Bruker SMART CCD 1000 diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) at 296(2) K. GC-MS analysis was performed with Agilent 7890A/5975C GC-MS system with an HP-5 MS column. Optical rotation was recorded with a PerkinElmer 341 polarimeter (sodium lamp, 1dm cuvette, c in g/100 mL, 20 °C). High Performance Liquid Chromatography (HPLC) analysis for the ee values was performed on a SHIMADZU system (SHIMADZU LC-20AT pump, SHIMADZU LC-20A Absorbance Detector). Chiralpak OD-H and AD-H were purchased from Daicel Chemical

Scheme 4. Proposed Mechanism for the Methylene Oxidation by Mn/H2O2 Catalytic System

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ammonium chloride (5.0 mL) and stirred for 30 min. The precipitated aluminum salts were removed, and the biphasic mixtures were extracted by addition of chloroform (10 mL × 3 times). The combined organic extracts were dried over anhydrous Na2SO4 and filtered to remove Na2SO4 added. The solution phase was then purified by silica gel chromatography (petroleum ether/ethyl acetate = 10:1−5:1) to give the desired spirocyclic compound 7a as a white solid (114 mg, 86%, > 20:1 dr). Stepwise Oxidation of 1a. Substrate 1a (0.40 mmol), catalyst 3a (0.25 mol %), and DMBA (7.0 equiv) were dissolved in CH2Cl2 (2.0 mL) under an Ar atmosphere at −30 °C. H2O2 (1.0 mmol, 2.5 equiv; 30% aqueous solution diluted in 0.50 mL of MeCN) was added dropwise to this mixture over 30 min using a syringe pump. The reaction mixture was stirred for additional 10 min and then quenched with sodium sulfite. The final reaction mixture was purified by silica gel chromatography (petroleum ether/ethyl acetate = 20:1−10:1) to give the desired spirocyclic alcohol 8a and diketone 2a. On the basis of the 1H NMR, the dr of 8a was 13 (SI, Figure S1). Then, the catalytic oxidation of 8a was performed separately. Spirocyclic alcohol 8a (0.060 mmol), manganese catalyst 3a (0.50 mol %) and DMBA (14 equiv) were dissolved in CH2Cl2 (0.50 mL) under an Ar atmosphere at −30 °C. H2O2 (3.0 equiv; 30% aqueous solution diluted in 0.50 mL of MeCN) was added dropwise to this mixture over 30 min using a syringe pump. The reaction mixture was stirred for additional 10 min and then quenched with sodium sulfite. The final reaction mixture was purified by silica gel chromatography (petroleum ether/ethyl acetate = 20:1−10:1) to give the desired spirocyclic diketone 2a with the isolated yield of 92% and 95% ee. Deuterium Kinetic Isotope Effect in Intermolecular Competitive Reaction. The deuterated spirocyclic tetralone (1a-d4) was synthesized according to the same synthetic method of 1a with 1,2-bis(bromomethyl-d4)benzene (SI, Figure S2). Substrate 1a-h4 (0.10 mmol), 1a-d4 (0.10 mmol), catalyst 3a (0.50 mol %), and DMBA (14 equiv) were dissolved in CH2Cl2 (1.0 mL) under an Ar atmosphere at −30 °C. H2O2 (0.10 mmol, 0.50 equiv; 30% aqueous solution diluted in 0.50 mL of MeCN) was added dropwise to this mixture over 30 min using a syringe pump. The reaction mixture was stirred for additional 10 min and then quenched with sodium sulfite. The final reaction solution was purified by silica gel chromatography (petroleum ether/ethyl acetate = 20:1−10:1) to recover the unreacted substrate and ketone and alcohol. The KIE value (kH/kD = 3.0(2)) was determined on the basis of the 1H NMR of the isolated alcohol and ketone products (SI, Figures S4− S6).

Industries, Ltd. Column chromatography was generally performed on silica gel (200−300 mesh) and TLC inspections were carried out on silica gel GF254 plates. General Procedure for the Site- and Enantioselective Oxidation of 1a−1i and 5a−5k Catalyzed by 3a. Under an Ar atmosphere, substrates 1a−1i and 5a−5k (0.20 mmol, 1.0 equiv), manganese catalyst 3a (0.50 mol %) were added to a dry 10 mL Schlenk tube, followed by 1.0 mL of CH2Cl2 and 2,2-dimethylbutanoic acid (DMBA, 0.35 mL, 14 equiv). The reaction tube was immersed into a −30 °C bath, H2O2 (1.0 mmol, 5 equiv, 30% aqueous solution diluted in 1.0 mL of MeCN) was added dropwise over 2 h using a syringe pump, and the reaction mixture was stirred at −30 °C for additional 2 h. The reaction solution was then quenched with sodium sulfite and NaHCO3. After the solvent was removed by the evaporation, the residue was purified by chromatography on silica gel (petroleum ether/ethyl acetate = 20:1−10:1) to afford the desired spirocyclic diketone compound 2 or 6. Gram-Scale Reaction for the Asymmetric Oxidation of 1a Catalyzed by Manganese Complex 3a. Under an argon atmosphere, substrate 1a (5.0 mmol), catalyst 3a (0.50 mol %) and DMBA (14 equiv, 70 mmol) were dissolved in 12 mL of CH2Cl2 at −30 °C, and then H2O2 (25 mmol, 5.0 equiv; 30% aqueous solution diluted in 6.0 mL of MeCN) was added dropwise over 3 h using a syringe pump and the reaction mixture was stirred at −30 °C for additional 2 h. The reaction solution was then quenched with sodium sulfite and NaHCO3. After the solvent was removed by the evaporation, the residue was purified by chromatography on silica gel (petroleum ether/ ethyl acetate = 20:1−10:1) to give the desired spirocyclic compound 2a (0.91 g, 92% ee). The product was further recrystallized in ethanol (8 mL) to provide a white solid (86% yield, > 99% ee). Product Analyses for (S)-3′,4′-Dihydro-1′H-spiro[indene-2,2′-naphthalene]-1,1′(3H)-dione (S-2a). [α]20 D= −125.5 (c = 0.20 in CHCl3), 1H NMR (400 MHz, CDCl3) in ppm: δ = 8.07 (d, J = 8.4 Hz, 1H), δ = 7.80 (d, J = 7.6 Hz, 1H), 7.67 (t, J = 7.2 Hz, 1H), 7.53−7.58 (m, 2H), 7.44 (t, J = 7.6 Hz, 1H), 7.31−7.39 (m, 2H), 3.88 (d, J = 17.2 Hz, 1H), 3.50−3.57 (m, 1H), 3.09 (d, J = 17.2 Hz, 1H), 3.01−3.09 (m, 1H), 2.57− 2.62 (m, 1H), 2.32−2.39 (m, 1H); 13C NMR (100 MHz, CDCl3) in ppm: δ = 204.1, 196.4, 153.0, 144.3, 135.2, 133.9, 131.5, 128.8, 128.2, 127.9, 126.8, 126.5, 124.7, 61.1, 38.0, 32.2, 25.5; HRMS [M + H]+: calculated for C18H15O2: 263.1071, found: 263.1066; HPLC-separation conditions: Chiralcel ADH, 20 °C, 210 nm, 90/10 hexane/iPrOH, 1.0 mL/min; tR = 11.62 and 15.24 min, 94% ee. Reduction of Chiral Spirocyclic Diketone 2a. Reduction of chiral spirocyclic diketone 2a was performed according to the literature procedures.21e,28 Under an Ar atmosphere, tertbutyllithium (1.6 M in pentane, 1.9 mL, 3.0 mmol, 6.0 equiv) was slowly added to a solution of DIBAL-H (1.0 M in hexane, 3.3 mL, 3.3 mmol, 6.6 equiv) at −78 °C. The colorless solution was stirred for 5 min and then warmed to room temperature. The solution was cooled again to −78 °C. To this solution, a suspension of the recrystallized 2a (131 mg, 0.50 mmol, 1.0 equiv) in THF (3.0 mL) was added dropwise over 10 min (additional 1.0 mL of THF was used to rinse the syringe), and the reaction mixture was stirred overnight at −78 °C. Afterward, saturated aqueous ammonium chloride was added to quench the reaction at −78 °C, and then the mixture was warmed to room temperature. The mixture was transferred to a beaker containing chloroform (10 mL) and saturated aqueous

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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/acscatal.7b03601. Syntheses of the spirocyclic monoketone substrates (1a− 1i and 5a−5k), general procedure for the oxidation of 1a−1i and 5a−5k catalyzed by manganese complex 3a and analyses of the products (2a−2i and 6a−6k), product analyses for 7a and 8a, and Figures S1−S5 (PDF) Accession Codes

CCDC 1523258−1523259 and 1577809 contain the supplementary crystallographic data for this paper. These data can be 2484


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ACS Catalysis

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obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Qiangsheng Sun: 0000-0002-7721-1290 Yong-Min Lee: 0000-0002-5553-1453 Wonwoo Nam: 0000-0001-8592-4867 Wei Sun: 0000-0003-4448-2390 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support of this work from the National Natural Science Foundation of China (21473226 and 21773273 to W.S.) and the NRF of Korea through CRI (NRF2012R1A3A2048842 to W.N.) and GRL (NRF-2010-00353 to W.N.).

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