Highly Enantioselective Oxidation of Spirocyclic Hydrocarbons byBioinspired Manganese Catalysts and Hydrogen PeroxideBin 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 ResearchInstitute 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 emergedas attractive catalysts for a number of selective oxidationreactions over the past several decades. In the present study, wereport the enantioselective oxidation of spirocyclic compoundswith manganese complexes bearing tetradentate N4 ligands ascatalysts and aqueous H2O2 as a terminal oxidant under mildconditions; spirocyclic tetralone (1a) and its derivatives bearingelectron-donating and -withdrawing substituents are convertedto their corresponding chiral spirocyclic β,β′-diketones withhigh yields and enantioselectivities. Spirocyclic indanones arealso converted to the β,β′-spirobiindanones with high enantioselectivities. Indeed, the reaction expands the diversity of chiralspirocyclic diketones via a late-stage oxidative process. In addition, it is of importance to note that the catalytic reaction can beeasily scaled up and the chiral spirocyclic β,β′-diketones can be transformed into diol products. In mechanistic studies, we haveshown that (1) ketones were yielded as products via the initial formation of alcohols, followed by the further oxidation of thealcohols to ketones, (2) hydrogen atom (H atom) abstraction from the methylene C−H bonds of 1a by a putative Mn(V)-oxointermediate was proposed to be the rate-determining step, and (3) the C−H bond hydroxylation of 1a by the Mn(V)-oxospecies was proposed to occur via oxygen rebound mechanism. On the basis of these results, we have proposed a plausiblemechanism 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

■ INTRODUCTION

The selective oxidation of hydrocarbon C−H bonds is a highlychallenging reaction in synthetic organic chemistry, butcommon in enzymatic reactions.1 For example, metalloenzymes(e.g., Cytochromes P450 and methane monooxygenases)generate highly reactive intermediates (e.g., high-valent metal-oxo species) for the stereo-, regio-, and enantioselectiveoxidation of organic substrates.2 Therefore, development ofefficient and selective oxidation of hydrocarbons, especially forthe enantioselective oxidation of C−H bonds, using bioinspiredmetal catalysts and environmentally benign oxidants has been along-standing goal in the communities of synthetic organic,oxidation, and biomimetic/bioinorganic chemistry.3 In bio-mimetic studies, tremendous efforts have been devoted todeveloping artificial catalysts that mimic the reactivities ofmetalloenzymes in catalytic oxidation reactions.3−5 In addition,bioinspired metal complexes have been used in the synthesisand characterization of metal−oxygen intermediates (e.g., high-valent metal-oxo species), and reactivities and mechanisms ofthe synthetic metal−oxygen intermediates have been inten-sively investigated in various oxidation reactions, with the

purpose of elucidating enzymatic reactions in biology.6 Onenotable example is the synthetic metalloporphyrins used ascatalysts in the (enantio)selective oxidation of organicsubstrates as well as chemical models of CytochromesP450.3b,7 For example, Groves and co-workers observed amoderate level of enantioselectivity in the asymmetrichydroxylation of ethylbenzene by a chiral iron porphyrincomplex.8

Although significant progress has been made in the aliphaticC−H bond oxidation catalysis over the past severaldecades,9−12 asymmetric oxidation of aliphatic C−H bondsby chiral metal catalysts has been reported only in fewcases.13−16 To develop efficient asymmetric C−H bondoxidation catalysis, two major challenges should be overcome,such as (1) the C−H oxidation of hydrocarbons by catalystsyields chiral alcohol products with a poor to moderateenantioselectivity and (2) the chiral alcohol products are

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

Research Article

pubs.acs.org/acscatalysisCite This: ACS Catal. 2018, 8, 2479−2487

© XXXX American Chemical Society 2479 DOI: 10.1021/acscatal.7b03601ACS Catal. 2018, 8, 2479−2487

further oxidized rapidly to ketones, thereby losing thestereogenic center (Scheme 1A). Fortunately, the problem of

the overoxidation of alcohols to the corresponding ketones hasthe potential application in enantioselective desymmetrizationreactions.17 For example, Bach and co-workers reported anelegant study for the oxidative desymmetrization of spirocyclicoxindoles by a chiral ruthenium porphyrin catalyst and 2,6-dichloropyridine N-oxide, in which the methylene C−H bondswere oxidized to yield ketone products with high enantiose-lectivities (Scheme 1B).18 In the study, enantioselectivities wereachieved by building hydrogen bonding between substrates andthe porphyrin ligand of the ruthenium complex (Scheme 1B).18

If such an oxidative strategy can be expanded to all-carbonspirocyclic precursors, a catalytic enantioselective oxidation ofspirocyclic compounds to their corresponding spiro β,β′-diketone derivatives can be successfully established (Scheme1C). Although these optically pure spirocyclic compounds areimportant structural motifs that are found not only in manyimportant chiral ligands of asymmetric catalysis but also inbiologically active natural products (Figure 1),19−21 catalyticenantioselective methods for the synthesis and functionalization

of these chiral spirocyclic compounds remain significantlyunderdeveloped.22

Recently, a great advance has been achieved in theenantioselective epoxidation of olefins by bioinspired man-ganese complexes bearing nonheme ligands and H2O2 in thepresence of additives (e.g., carboxylic acids or H2SO4);

5,23,24 theasymmetric epoxidation reactions are highly efficient in termsof product yields and enantioselectivities. In contrast, there areonly a couple of reports for the enantioselective oxidation ofaliphatic C−H bonds by nonheme manganese catalysts andH2O2.

14,15 Since we have reported an efficient manganesecatalytic system using H2O2 as a terminal oxidant for theoxidation of benzylic and aliphatic methylene C−H bonds toketones under mild reaction conditions,13 we attempted toexamine the previously reported catalytic systems (i.e., using abioinspired manganese catalyst and an environmentally benignoxidant) in the enantioselective synthesis of spirocycliccompounds. Herein, we report an enantioselective oxidationof benzylic methylene C−H bonds in spirocyclic precursors bya manganese catalyst bearing a tetradentate N4 ligand andaqueous H2O2, affording a vast array of chiral spirocyclic β,β′-diketones in high yields with excellent enantioselectivities(Scheme 1C). Mechanistic aspects for the manganese-catalyzedenantioselective C−H bond activation reaction have also beendiscussed in the present study.

■ RESULTS AND DISCUSSIONTo examine the feasibility of our proposed strategy, weconducted the oxidation of methylene C−H bonds ofspirocyclic compounds by nonheme manganese complexesand aqueous H2O2 in the presence of carboxylic acids (seeTable 1 for the structures of manganese complexes andcarboxylic acids and the reaction scheme). First, the oxidationof spirocyclic tetralone (1a) by [(S-PEB)Mn] (3a)13 (2.0 mol%) and H2O2 (7.0 equiv to 1a) at 0 °C afforded spirocyclicdiketone (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 reactionconditions, the oxidation of 1a was carried out at differentreaction temperatures with different amounts of catalyst loadingand carboxylic acid additive, and the catalytic activity andenantioselectivity of several manganese catalysts were alsoexamined (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) alsoafforded 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 whilemaintaining both reactivity and enantioselectivity (Table 1,entry 6). Changing the N-substituent of benzimidazole motif incatalyst 3 to methyl (3b) and isopropyl (3c) slightly decreased

Scheme 1. Strategy for the Enantioselective Oxidation of C−H Bonds in This Study

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

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the ee value of 2a (Table 1, entries 9 and 10). Compared withthe catalyst 3b, the catalyst 4a (R,R-MCMB-Mn)23d bearing aC2-symmetric cyclohexane-1,2-diamine backbone afforded alow enantioselectivity (Table 1, entry 11 (75% ee) versus entry9 (92% ee)). Similarly, the ee value of the product 2a was lowin the reaction of the catalyst 4b (R,R-PDMB-Mn) (Table 1,entry 12).23c Because it has been well documented that thereactivity and enantioselectivity of nonheme metal complexesare affected significantly by carboxylic acid additives in thecatalytic asymmetric epoxidation of olefins by H2O2,

24−27 othercarboxylic acids, such as 2-ethylhexanoic acid (EHA) and aceticacid (AcOH), were examined. In these reactions, we found thatthe ee value of 2a was decreased slightly (e.g., ∼ 90% ee)(Table 1, entries 13 and 14). Finally, it is notable that only traceamounts of products were formed from the methylene C−Hbonds of six-membered ring in 1a, demonstrating that the

oxidation of spirocyclic compounds in this manganese catalyticsystem 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 ofthe spirocyclic monoketone substrates), showing that spirocy-clic diketones were formed with moderate to good yields in allof the reactions (Table 2). More importantly, enantioselectiv-

ities were excellent in these reactions (up to 94% ee; see 2a and2e in Table 2). Only the substrate 1d bearing a methoxy groupat the C5 position afforded a low product yield; it was probablydue 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 caseof 2g. Interestingly, catalyst 4b with a bipyrrolidine backbone(see the Mn structure in Table 1) afforded a highenantioselectivity for this substrate (89% ee). Furthermore,spirocyclic chromanone 1i was readily converted to thediketone product (2i) with a high enantioselectivity (90%ee). The structures of the diketone products, 2g and 2i, were

Table 1. Optimization of Site- and Enantiotopos-SelectiveOxidation of Methylene C−H Bonds of SpirocyclicTetralonea

entrycatalyst(mol %)

H2O2(equiv)

temp.(°C)

conv.b

(%)yieldc

(%)eed

(%)

1 3a (2.0) 7.0 0 94 80 872 3a (2.0) 7.0 −20 88 77 923 3a (2.0) 7.0 −30 86 76 944 3a (0.50) 7.0 −30 88 74 945 3a (0.20) 7.0 −30 60 54 936 3a (0.50) 5.0 −30 90 80 947 3a (0.50) 3.5 −30 86 74 938e 3a (0.50) 5.0 −30 78 56 929 3b (0.50) 5.0 −30 86 74 9210 3c (0.50) 5.0 −30 80 67 8611 4a (0.50) 5.0 −30 92 75 75h

12 4b (0.50) 5.0 −30 92 78 80h

13f 3a (0.50) 5.0 −30 80 64 8914g 3a (0.50) 5.0 −30 92 83 90

aReaction conditions: Substrate 1a (0.20 mmol), manganese catalyst,and DMBA (14 equiv) were dissolved in CH2Cl2 (1.0 mL) at −30 °Cunder an Ar atmosphere. Then, H2O2 (30% aqueous solution dilutedin 1.0 mL of MeCN) was added to the solution dropwise over 2 husing a syringe pump, and the reaction solution was stirred foradditional 2 h. bConversion was based on the amount of the recovered1a. cIsolated yield. dThe ee values were determined by HPLC usingchiral stationary phases. eDMBA (7 equiv) was used. fEHA (14 equiv)was used instead of DMBA. gAcOH (14 equiv) was used instead ofDMBA. hThe configuration of 2a was opposite due to the use of (R,R)manganese catalyst.

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

aReaction conditions: Substrate 1 (0.20 mmol), manganese catalyst 3a(0.50 mol %), and DMBA (14 equiv) were dissolved in CH2Cl2 (1.0mL) at −30 °C under an Ar atmosphere. Then, 5.0 equiv of H2O2(30% aqueous solution diluted in 1.0 mL of MeCN) was addeddropwise over 2 h using a syringe pump, and the reaction solution wasstirred for additional 2 h. The ee values were determined by HPLCusing 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 wasused as a catalyst, and the configuration of 2a was opposite due to theuse of (R,R) manganese catalyst.

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determined by single-crystal X-ray diffraction analysis (Table 2;also see SI).To examine the substrate scope of the catalytic system,

oxidation of spirocyclic indanone derivatives by H2O2 in thepresence of catalyst 3a and DMBA as additive was investigated(see the reaction scheme in Table 3). To our delight, the

oxidation of spirocyclic indanones (5a−5k; see SI for thesyntheses of the spirocyclic monoketone substrates) yielded thecorresponding spirocyclic β,β′-spirobiindanones with good toexcellent enantioselectivities (Table 3, 68%−98% ee). Interest-ingly, spirocyclic compounds containing two methoxy groups inindanone side (5f) and in indane ring (5k) were converted tothe same product (6f and 6k) but with different ee values (92%and 74% ees for 5f and 5k, respectively). The absoluteconfigurations of these spirobiindanone products (6a−6k)were confirmed to be R-configuration by comparing with thereported compounds (see SI for the product analyses).21e

To demonstrate the synthetic utility of the present catalyticsystem, a gram-scale reaction was performed as shown inScheme 2. The oxidative desymmetrization of 1a (1.24 g)yielded the desired spirocyclic diketone S-2a in 70% isolatedyield (0.91 g) with a high enantioselectivity (92% ee) (Scheme2), and the enantiopure S-2a (>99% ee) could be obtained afterrecrystallization in ethanol. Further, since the reduction ofspirobiindanone 6a was reported previously using DIBALH andt-BuLi in THF at −78 °C and the resulting chiral diol could be

transformed into bisphosphinite ligand,21e,28 we reduced 2awith the literature procedure and obtained the spirocyclic diolproduct (7a) without the loss of the optical purity (Scheme 2).The structure of the diol product, 7a, was determined by single-crystal X-ray diffraction analysis (Scheme 2; also see SI, CIFfile).To gain mechanistic insights into the oxidation of spirocyclic

hydrocarbons catalyzed by manganese catalyst and H2O2, wefirst conducted the oxidation of 1a by reducing the amounts ofthe Mn catalyst (3a) and the oxidant (H2O2) (Scheme 3A). Inthis reaction, spirocyclic alcohol (8a) was isolated with highdiastereoselectivity and enantioselectivity (13:1 diastereomerratio (dr) and 97% ee; see SI, Figure S1) (Scheme 3A, eq 1),and further oxidation of 8a produced the correspondingspirocyclic diketone (2a) with 92% yield and 95% ee (Scheme3A, eq 2). These results demonstrate unambiguously thatalcohols were produced initially in the oxidation of spirocycliccompounds by 3a and H2O2, followed by the further oxidationof the alcohols to the spirocyclic diketones.14,29 Second, wedetermined a kinetic isotope effect (KIE) value of 3.0(2) bycarrying out an intermolecular competitive oxidation ofspirocyclic tetralone (1a) and its deuterated spirocyclictetralone (1a-d4; SI, Figure S2 for the synthesis) (Scheme3B), proposing that hydrogen atom (H atom) abstraction fromthe methylene C−H bonds of 1a by a putative Mn(V)-oxointermediate is the rate-limiting step (vide infra); a KIE value of3.9 was reported in the oxidation of cyclohexane and deuteratedcyclohexane by 3b and H2O2

13 and KIE values of ∼3−4 wereobtained in the hydroxylation of cumene, ethylbenzene, andcyclohexane by nonheme manganese catalysts and H2O2.

11b,15

Further, when the catalytic oxidation of 1a by 3a and H2O2 wascarried out using H2

16O2 under an 18O2 atmosphere, most(>99%) of oxygen atoms in 2a were found to derive fromH2

16O2 (Scheme 3C; SI, Figure S3 for GC-MS spectrum).Furthermore, when the catalytic oxidation of 1a by 3a andH2O2 was carried out in the presence of CCl3Br, the ketoneproduct (2a) was formed predominantly with a small amountof brominated product (<2% yield, based on GC-MSanalysis).30 Thus, the results of the 18O2 and CCl3Brexperiments lead us to propose that the C−H bond activationreaction by 3a and H2O2 occurs via oxygen reboundmechanism; it has been proposed previously that C−H bondactivation by mononuclear nonheme metal(IV)-oxo complexes,

Table 3. Spirocyclic Indanone Derivatives (5) as Substratea

aReaction conditions: Substrate 5 (0.20 mmol), manganese catalyst 3a(0.50 mol %), and DMBA (14 equiv) were dissolved in CH2Cl2 (1.0mL) at −30 °C under an Ar atmosphere. Then, H2O2 (5.0 equiv; 30%aqueous solution diluted in 1.0 mL of MeCN) was added to thesolution dropwise over 2 h using a syringe pump, and the reactionsolution was stirred for additional 2 h. The ee values were measured byHPLC analysis using chiral stationary phases.

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

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including Mn(IV)-oxo complexes, occurs via oxygen nonre-bound mechanism.31,32

On the basis of the results discussed above and thosereported previously,4,5,10−12,14,15,23a,24−26 we propose thefollowing mechanism for the C−H bond oxidation ofhydrocarbons catalyzed by nonheme Mn catalysts and H2O2in the presence of carboxylic acid additives (Scheme 4). Asproposed in numerous reports of nonheme iron and manganesecomplex-catalyzed oxidation of hydrocarbons by H2O2 in the

presence of carboxylic acid additives,4,5 a Mn(V)-oxo species isformed via O−O bond heterolysis of a putative Mn(III)-OOHintermediate (Scheme 4, pathway a); the role of carboxylicacids for the formation of Mn(V)-oxo (and Fe(V)-oxo) hasbeen extensively discussed as “carboxylic acid-assisted mecha-nism”.11,24−26 Interestingly, when H atom abstraction occurs bythis Mn(V)-oxo species (Scheme 4, pathway c), the resultantMn(IV)−OH and carbon radical species rebound at a fast ratein the cage to form a chiral alcohol product (Scheme 4,pathway d). Indirect evidence supporting the fast oxygenrebound process between the Mn(IV)−OH and carbon radicalspecies was obtained in the 18O2 and CCl3Br experiments(Scheme 4, pathway e). Finally, the alcohol product is furtheroxidized by another molecule of Mn(V)-oxo species to yieldthe ketone product (Scheme 4, pathway f).

■ CONCLUSIONS

In summary, we have reported an example of site- andenantioselective oxidation of benzylic methylene of spirocycliccompounds using bioinspired manganese complexes as catalystsand aqueous H2O2 as an oxidant. The product yields andenantioselectivities of chiral spirocyclic β,β′-diketones werehigh, and the catalytic system was demonstrated to beapplicable to the gram-scale synthesis of the chiral spirocyclicdiketones. Further, the chiral diketones were easily converted tothe corresponding alcohols. We have also discussed mechanisticinsights into the C−H bond activation of hydrocarbonscatalyzed by the manganese catalyst and H2O2, such as thenature of Mn-oxo intermediate and the C−H bond activationmechanisms. Future studies will be focused on developinghighly efficient enantioselective oxidation of unactivated C−Hbonds by bioinspired nonheme iron and manganese catalysts aswell as elucidating chemical and physical properties of theputative metal(V)-oxo species in nonheme systems.

■ EXPERIMENTAL SECTION

Materials. All chemicals were purchased from Aldrich, AlfaAesar, and TCI and used as received unless otherwise indicated.Solvents were dried according to published procedures anddistilled under argon prior to use.33 All reactions wereperformed under an Ar atmosphere using dried solvents andstandard Schlenk techniques unless otherwise noted. Ligands,(S)-PEB, (S)-PMB, (S)-PiPB, (R,R)-MCMB, and (R,R)-PDMBand their manganese complexes were prepared according to theliterature procedures.13,23

Instrumentation. 1H and 13C NMR spectra were recordedon a Bruker AVANCE III 400 MHz spectrometer. All NMRspectra were recorded at room temperature and were indirectlyreferenced to TMS using residual solvent signals as internalstandards. High resolution mass spectra (HRMS) wereobtained on an Agilent 6530 Q-TOF mass spectrometer withan ESI source. X-ray crystallographic data were collected on aBruker SMART CCD 1000 diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 296(2)K. GC-MS analysis was performed with Agilent 7890A/5975CGC-MS system with an HP-5 MS column. Optical rotation wasrecorded with a PerkinElmer 341 polarimeter (sodium lamp, 1-dm cuvette, c in g/100 mL, 20 °C). High Performance LiquidChromatography (HPLC) analysis for the ee values wasperformed on a SHIMADZU system (SHIMADZU LC-20ATpump, SHIMADZU LC-20A Absorbance Detector). ChiralpakOD-H and AD-H were purchased from Daicel Chemical

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

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

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Industries, Ltd. Column chromatography was generallyperformed on silica gel (200−300 mesh) and TLC inspectionswere 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 anAr atmosphere, substrates 1a−1i and 5a−5k (0.20 mmol, 1.0equiv), manganese catalyst 3a (0.50 mol %) were added to adry 10 mL Schlenk tube, followed by 1.0 mL of CH2Cl2 and2,2-dimethylbutanoic acid (DMBA, 0.35 mL, 14 equiv). Thereaction tube was immersed into a −30 °C bath, H2O2 (1.0mmol, 5 equiv, 30% aqueous solution diluted in 1.0 mL ofMeCN) was added dropwise over 2 h using a syringe pump,and the reaction mixture was stirred at −30 °C for additional 2h. The reaction solution was then quenched with sodium sulfiteand NaHCO3. After the solvent was removed by theevaporation, the residue was purified by chromatography onsilica gel (petroleum ether/ethyl acetate = 20:1−10:1) to affordthe desired spirocyclic diketone compound 2 or 6.Gram-Scale Reaction for the Asymmetric Oxidation of

1a Catalyzed by Manganese Complex 3a. Under an argonatmosphere, substrate 1a (5.0 mmol), catalyst 3a (0.50 mol %)and DMBA (14 equiv, 70 mmol) were dissolved in 12 mL ofCH2Cl2 at −30 °C, and then H2O2 (25 mmol, 5.0 equiv; 30%aqueous solution diluted in 6.0 mL of MeCN) was addeddropwise over 3 h using a syringe pump and the reactionmixture was stirred at −30 °C for additional 2 h. The reactionsolution was then quenched with sodium sulfite and NaHCO3.After the solvent was removed by the evaporation, the residuewas purified by chromatography on silica gel (petroleum ether/ethyl acetate = 20:1−10:1) to give the desired spirocycliccompound 2a (0.91 g, 92% ee). The product was furtherrecrystallized 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]D20=

−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 AD-H, 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 tothe literature procedures.21e,28 Under an Ar atmosphere, tert-butyllithium (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 solutionwas stirred for 5 min and then warmed to room temperature.The solution was cooled again to −78 °C. To this solution, asuspension of the recrystallized 2a (131 mg, 0.50 mmol, 1.0equiv) in THF (3.0 mL) was added dropwise over 10 min(additional 1.0 mL of THF was used to rinse the syringe), andthe reaction mixture was stirred overnight at −78 °C.Afterward, saturated aqueous ammonium chloride was addedto quench the reaction at −78 °C, and then the mixture waswarmed to room temperature. The mixture was transferred to abeaker containing chloroform (10 mL) and saturated aqueous

ammonium chloride (5.0 mL) and stirred for 30 min. Theprecipitated aluminum salts were removed, and the biphasicmixtures were extracted by addition of chloroform (10 mL × 3times). The combined organic extracts were dried overanhydrous Na2SO4 and filtered to remove Na2SO4 added.The solution phase was then purified by silica gelchromatography (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) weredissolved in CH2Cl2 (2.0 mL) under an Ar atmosphere at −30°C. H2O2 (1.0 mmol, 2.5 equiv; 30% aqueous solution dilutedin 0.50 mL of MeCN) was added dropwise to this mixture over30 min using a syringe pump. The reaction mixture was stirredfor additional 10 min and then quenched with sodium sulfite.The final reaction mixture was purified by silica gelchromatography (petroleum ether/ethyl acetate = 20:1−10:1)to give the desired spirocyclic alcohol 8a and diketone 2a. Onthe 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 (3.0equiv; 30% aqueous solution diluted in 0.50 mL of MeCN) wasadded dropwise to this mixture over 30 min using a syringepump. The reaction mixture was stirred for additional 10 minand then quenched with sodium sulfite. The final reactionmixture was purified by silica gel chromatography (petroleumether/ethyl acetate = 20:1−10:1) to give the desired spirocyclicdiketone 2a with the isolated yield of 92% and 95% ee.

Deuterium Kinetic Isotope Effect in IntermolecularCompetitive Reaction. The deuterated spirocyclic tetralone(1a-d4) was synthesized according to the same syntheticmethod 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 dissolvedin CH2Cl2 (1.0 mL) under an Ar atmosphere at −30 °C. H2O2(0.10 mmol, 0.50 equiv; 30% aqueous solution diluted in 0.50mL of MeCN) was added dropwise to this mixture over 30 minusing a syringe pump. The reaction mixture was stirred foradditional 10 min and then quenched with sodium sulfite. Thefinal reaction solution was purified by silica gel chromatography(petroleum ether/ethyl acetate = 20:1−10:1) to recover theunreacted substrate and ketone and alcohol. The KIE value(kH/kD = 3.0(2)) was determined on the basis of the 1H NMRof the isolated alcohol and ketone products (SI, Figures S4−S6).

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acscatal.7b03601.

Syntheses of the spirocyclic monoketone substrates (1a−1i and 5a−5k), general procedure for the oxidation of1a−1i and 5a−5k catalyzed by manganese complex 3aand analyses of the products (2a−2i and 6a−6k),product analyses for 7a and 8a, and Figures S1−S5(PDF)

Accession CodesCCDC 1523258−1523259 and 1577809 contain the supple-mentary crystallographic data for this paper. These data can be

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

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: wsun@licp.cas.cn.*E-mail: wwnam@ewha.ac.kr.ORCIDQiangsheng Sun: 0000-0002-7721-1290Yong-Min Lee: 0000-0002-5553-1453Wonwoo Nam: 0000-0001-8592-4867Wei Sun: 0000-0003-4448-2390NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge financial support of this work from theNational Natural Science Foundation of China (21473226 and21773273 to W.S.) and the NRF of Korea through CRI (NRF-2012R1A3A2048842 to W.N.) and GRL (NRF-2010-00353 toW.N.).

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