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Highly Chemoselective and Enantioselective Catalytic Oxidation of Heteroaromatic Sulfides via High-Valent Manganese (IV)-Oxo Cation Radical Oxidizing Intermediates Wen Dai, Sensen Shang, Ying Lv, Guosong Li, Chunsen Li, and Shuang Gao ACS Catal., Just Accepted Manuscript • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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Highly Chemoselective and Enantioselective Catalytic Oxidation of Heteroaromatic Sulfides via High-Valent Manganese (IV)-Oxo Cation Radical Oxidizing Intermediates ⊥

Wen Dai,*,†,‡ Sensen Shang,†,‡ Ying Lv,†,‡ Guosong Li,†,‡ Chunsen Li,*,§, and Shuang Gao*,†,‡ †

Dalian Institute of Chemical Physics, the Chinese Academy of Sciences, Dalian 116023, China Dalian National Laboratory for Clean Energy, Dalian 116023, China § State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian 350002, China ‡

⊥

Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Xiamen, Fujian 361005, China

ABSTRACT: A manganese complex with porphyrin-like ligand that catalyzes the highly chemoselective and enantioselective oxidation of heteroaromatic sulfides, including imidazole, benzimidazole, indole, pyridine, pyrimidine, pyrazine , sym-triazine, thiophene, thiazole, benzothiazole, benzoxazole with hydrogen peroxide is described, furnishing the corresponding sulfoxides in good to excellent yields and enantioselectivities (up to 90% yield, and up to >99% ee ) within a short reaction time (0.5 h). The practical utility of the method has been demonstrated in gram-scale synthesis of chiral sulfoxide. Mechanistic studies, performed with 18O-labeled water (H218O), hydrogen peroxide (H218O2) and cumyl hydroperoxide reveal that a high-valent manganese-oxo species is generated as the oxygen atom delivering agent via carboxylic acid-assisted heterolysis of OO bond. Density functional theory (DFT) calculations were also carried out to give further insight into the mechanism of manganese-catalyzed sulfoxidation. On the basis of the theoretical study, the coupled high-valent manganese (IV)-oxo cation radical species which bears obvious similarities with that of reactive intermediates in the catalytic oxygenation reactions based on the cytochromes P450 and metalloporphyrin models, has been proposed as reactive oxidant in non-heme manganese catalyst system. KEYWORDS: asymmetric sulfoxidation, manganese, heteroaromatic sulfides, porphyrin-like, cation radical

INTRODUCTION Chiral sulfoxides are widely used as intermediates, auxiliaries, and ligands in modern organic chemistry and constitute important biologically active compounds, including several marketed pharmaceuticals.1 Among the various methods developed for construction of enantioenriched sulfoxides, asymmetric sulfoxidation undoubtedly serves as one of the most straightforward and reliable methods. Since the initial breakthrough accomplished in asymmetric sulfoxidation using titanium/tartrate catalysts together with alky hydroperoxides as oxidant by the groups of Kagan and Modena in 1984,2 other transition-metal catalysts,3 organocatalysts4 and chemoenzymatic methods5 have been extensively developed, and high enantioselectivity has already been achieved in oxidation of simple substrates such as phenyl alkyl and alkyl alky sulfides. However, asymmetric oxidation of heteroaromatic sulfides has been slow to develop, despite the special importance of enantiomerically enriched heteroaromatic sulfoxides in the synthesis of chiral drugs and natural products.1f,1g We postulated that the main issues were as follows: First, oxidation reactions involving this substrate have been plagued by catalyst deactivation owing to the strong coordinating ability of the sub-

strates and products. Second, the sulfur- or/and nitrogencontaining heteroaromatic rings moieties are potentially oxidizable (Scheme 1). Therefore, extremely high chemoselectivity and stereorecognition of the catalysts are essential to solve this challenging objective. To date, only limited examples of asymmetric Scheme 1. Challenges in Asymmetric Oxidation of Heteroaromatic sulfides.

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Scheme 2. Representative Examples of Asymmetric Oxidation of Heteroaromatic sulfides, and limitations of these catalyst systems.

oxidation of heteroaromatic sulfides have been documented. For instance, enzymatic methods offer a partial solution to this problem, but there are some inherent disadvantages associated with such transformation such as instability, limited availability, high cost of the catalysts and narrow substrate tolerance (Scheme 2, eq 1).5b,5e Although the titanium/tartrate systems have also been applied to the oxidation of heteroaromatic sulfides, only sulfides bearing imidazole substituents can achieve a high enantioselectivity and environmentally unfriendly alky hydroperoxides are used as terminal oxidant (Scheme 2, eq 2).6 Given these limitations, development of a highly efficient and practical method that enables enantioselective oxidation of heteroaromatic sulfides is highly desirable and of great significance. Inspired by porphyrin ligand, we recently developed a new type of tetradentate N4 ligands (L) bearing relatively long conjugation and two N-H moieties exhibiting strong σdonation that fulfilled the structural requirements of the porphyrin ligand in some way, which has a highly conjugated planar with strong donor moiety.7 The porphyrin-like ligands L exhibit extraordinary chemoselectivity and enantioselectivity in various manganese-catalyzed oxidation reactions, including electron-rich olefins epoxidation and alkyl phenyl sulfides oxidation.7a,7b Scheme 3. Strategy for the development of Asymmetric Oxidation of Heteroaromatic sulfides.

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Therefore, we reasoned that the chiral manganese complexes with ligands L might be extended to enantioselective oxidation of heteroaromatic sulfides. Herein, we report a highly efficient enantioselective oxidation of a remarkable broad range of heteroaromatic sulfides by an in situ formed porphyrin-like chiral manganese complex with hydrogen peroxide, providing the corresponding sulfoxides in good to excellent yields and enantioselectivities (up to 90% yield, and >99% ee), under mild conditions and in short reaction time as well as application of the method to the gram-scale synthesis of optically pure sulfoxide. Detailed mechanism studies evidence that a highvalent manganese-oxo species is generated as the reactive oxidizing intermediate via carboxylic acid-assisted O-O bond heterolysis. Specifically, on the basis of DFT analysis, the coupled high-valent manganese (IV)-oxo cation radical species (L+•)MnIVO, which bear obvious similarities with that of oxidizing intermediates in the catalytic oxygenation reactions based on the cytochromes P450 and metalloporphyrin models, has been proposed as the reactive oxidant in non-heme manganese catalyst system (Scheme 3).8 RESULTS AND DISCUSSION Optimization of catalytic activity with regard to identity of carboxylic acid and oxidants. At the outset, 2(methylsulfinyl)pyridine (1a) was chosen as a model substrate to optimize the reaction conditions. In the presence of 1.0 mol% loading of chiral manganese complex generated in situ from Mn(OTf)2 and L4, 50% mol carboxylic acid as additive and 1.3 equiv 45% H2O2 as the oxidant, the asymmetric oxidation of 1a was conducted in acetonitrile at –20 oC (see Table S1 in SI). Among these aliphatic carboxylic acids, the adamantane carboxylic acid (aca) provided the best result (entry 6, Table S1). It is noteworthy that the aca loading could be reduced to 20 mol% without erosion of the yield and enantioselectivity (entry 1, Table 1). A deleterious effect in the yield and enantioselectivity was observed when the aca loading was further lowered to 10% mol (entry 2, Table 1). Then oxidants t-BuOOH and Cumyl-OOH were investigated in the presence of aca (entries 3 and 4, Table 1). Interestingly, the ee values were virtually the same irrespective of the type of oxidants, suggesting that a common oxidizing intermediate was generated in the reactions of H2O2, t-BuOOH and Cumyl-OOH in the presence of carboxylic acid. Table 1. Optimization of Catalytic Activity with Regard to Identity of Oxidants.

entry

Oxidant

RCO2H

% yielda

% eeb

1

H2O2

aca

95

81

2c

H2O2

aca

90

79

3

t-BuOOH

aca

91

82

4

Cumyl-OOH

aca

89

80

a

Isolated yield. Determined by chiral HPLC analysis. caca (10 mol%).

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b

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Screening of the other reaction parameters. Due to the oxidative kinetic resolution process, the amount of oxidant has been previously shown to play a positive role in improving enantioselectivity in metal-catalyzed oxidation of sulfides.3e,7b Because of that, further studies were conducted to investigate the amount of oxidant. A critical improvement in enantioselectivity was attained by increasing the equivalents of hydrogen peroxide, although some loss in yield occurred upon switching from 1.3 equiv. to 1.5 equiv. and finally 1.8 equiv. of hydrogen peroxide (Table 2, entries 1 and 2). The use of 1.8 equiv. hydrogen peroxide was the most beneficial in terms of yield, enantioselectivity and cost (entry 2). A simple inspection on the reaction temperature revealed that decreasing the temperature to –30 oC can afford a better enantioselectivity (entry 3). Further lowering the temperature to –40 oC resulted in no improvement of enantioselectivity (entry 4). Subsequently, an enhancement in yield and enantioselectivity failed to achieve by prolonging the reaction time to 1 h (entry 5). From the evaluation of various ligands, ligand L3 bearing sec-Butyl– substituted oxazolines was identified as the ligand being the most favorable in view of yield and enantioselectivity (entries 6–9). It is worth mentioning that the catalyst loading could be further reduced to 0.5 mol% without loss of either reactivity or enantioselectivity (entry 10). Further lowering the catalyst loading to 0.25 mol% resulted in a marked decrease in yield, albeit with an almost identical enantioselectivity (entry 11). Substrate scope of asymmetric oxidation of heteroaromatic sulfides. With the optimized conditions in hand, we next to set out to explore the substrate scope, and the results were summarized in Scheme 4. A variety of pyridine derivatives could be efficiently oxidized within short time to generate the corresponding chiral sulfoxides in high yields with excellent enantioselectivities. Excellent enantioselectivity was preserved Table 2. Screening of the Other Reaction Parameters.

entry

ligand

H2O2 (equiv.)

% yielda

% eeb

1c

L4

1.5

93

84

c

L4

1.8

88

86

3

L4

1.8

87

88

d

L4

1.8

86

88

5e

L4

1.8

87

88

6

L1

1.8

79

80

2 4

7

L2

1.8

81

79

8

L3

1.8

88

90

L5

1.8

83

83

9 f

10

L3

1.8

89

90

11g

L3

1.8

85

89

a Isolated yield. bDetermined by chiral HPLC analysis. c– 20 oC. d–40 oC. e1.0 h. fMn(OTf)2 (0.5 mol%), L3 (0.5 mol%). gMn(OTf)2 (0.25 mol %), L3 (0.25 mol%).

even when methyl on sulfur atom was replaced by branched or longer alkyl or sterically hindered benzyl (entries 2a–2j). Notably, the substrate containing various functional groups such as halogen, hydroxyl, ester and trifluoromethyl were equally well tolerated. High yield and excellent enantioselectivity were obtained for oxidation of 2-(phenylthio)ethan-1-ol and no trace of aldehyde was observed which indicated that our catalyst system possessed high chemoselectivity of oxidation of sulfide over alcohol (entry 2e). Various pyrimidine derivatives were also suitable reaction partners, thus affording the corresponding sulfoxides in high yields and excellent enantioselectivities regardless of the electronic properties (entries 2k–2n). The oxidation reactions of pyrazine derivatives were also carried out. Good yields and excellent enantioselectivities were obtained (entries 2o and 2p). Subjecting the 1,3,5triazine derivative to the optimized reaction conditions provided the sulfoxide product in good yield and excellent enantioselectivity (entry 2q). Substrates bearing thiazole, benzothiazole, 1,3,4-thiadiazole or thiophene substituents were also compatible with the current catalyst system. No observation of oxidation of sulfur-containing heteroaromatic rings further indicated that the present sulfoxidation is highly chemoselective (entries 2r–2v). Benzoxazole Scheme 4. Substrate Scope of Asymmetric Oxidation of Heteroaromatic Sulfidesa,b,c

a Reaction conditions: substrate (0.4 mmol), Mn(OTf)2 (0.5 mmol%), L3 (0.5 mmol%), aca (20 mo%), 45% H2O2 (1.8

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equiv.), CH3CN (1.5 mL), –30 oC, 0.5 h. bIsolated yield. cDetermined by chiral HPLC analysis.

could also serve as a good substrate, offering the sulfoxide with satisfactory yield and excellent yield (entry 2w). When the sulfide with imidazole substituent, which is identified as an ideal model substrate in titanium/tartrate systems, was subjected to the oxidation, the desired sulfoxide product was obtained in good yield and enantioselectivity (entry 2x). Finally, the present sulfoxidation could also be successfully applied to pyrazole derivative, giving the corresponding sulfoxide in good yield and enantioselectivity (entry 2y). Gram-scale synthesis of chiral sulfoxides. To further evaluate the synthetic potential of the catalytic system, asymmetric sulfoxidation of benzothiazole derivative 1p and pyrazine derivative 1t was performed under the optimal conditions on a gram scale, giving the desired product 2p and 2t in 82% yield with 91% ee and 88% yield and 90% ee, respectively (Scheme 5). Scheme 5. The Gram-Scale Synthesis of 2p and 2t.

Mechanistic studies. The original mechanism for the ironcatalyzed carboxylic acid-assisted epoxidation of olefins with H2O2 was proposed by Que, et al and the reaction involved a high-valent iron (V)-oxo species as oxygen-transfer reagent which is formed via acid-assisted heterolytic cleavage of O-O bond in a FeIII(OOH)(HOAc) precursor.9 Experimental spectroscopic evidence and computational support in favor of this “carboxylic-acid assisted” mechanism have been also recently provided.10 On the other hand, the porphyrin-inspired ligands used in the current catalyst system bear relatively long conjugation and two N-H moieties that exhibit strong σ-donation that fulfill the structural requirements of the porphyrin ligand in some way. In cytochrome P450 and iron porphyrin models, high valent porphyrin π-cation radicals ferryl-oxo, the socalled compound I (Por+•)FeIVO, have been widely accepted as reactive intermediates in the catalytic oxygenation reactions.8 Therefore, we Scheme 6. Proposed Mechanism.

envisioned that the electronic structure of this active intermediate in the current catalyst system might bear obvious similarities with that of compound I. On the basis of our speculation and literature precedence, we proposed a possible reaction pathway shown in Scheme 6. Likewise, some experiments and DFT calculations have been carried out to verify this mechanism. Isotopically labeled experiments were employed as mechanistic tools, providing insights into the mechanism of O-O lysis (Scheme 7). Oxidation of 1a using H218O2 as oxidant afforded the corresponding sulfoxide 2a 94% 18O labelled, and virtually no 18O–incorporation was observed when H216O2/H218O was used, demonstrating unambiguously that the exclusive source of oxygen atoms incorporated into the sulfoxide did stem from the hydrogen peroxide oxidant and also that the incorporation of oxygen from the water into the sulfoxide did not occur. The isotopic study argues against the implication of a water-assisted O-O lysis and instead points toward that the reaction should proceed via the carboxylic acidassisted pathway that leads to form a high-valent manganeseoxo species, which is consistent with Scheme 6. Scheme 7. Isotopic Analysis Studies.

H218O2 reagent is a 2% solution in H216O

The enantioselectivity was approximately the same virtually approximately common stereoselectivity irrespective of Scheme 8. (a) Heterolytic and Homolytic O-O Bond Cleavage Mechanisms in the Presence of Carboxylic Acid, Respectively, (b) Reaction of 1a with Cumyl-OOH as Oxidant and Analysis of the Decomposition of the Cumyl-OOH.

sis oly ter He

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O

O

oxidant employed constitutes strong evidence that a common oxygen atom transfer agent is formed with both Cumyl-OOH and H2O2 oxidant in the presence of carboxylic acids (Table 1, entries 3 and 4). To further probe the O−O bond cleavage mechanism in the current catalyst system, the oxidation of 1a with cumyl hydroperoxide was performed and products originated from the decomposition of the cumyl hydroperoxide were analyzed to distinguish homolytic vs heterolytic O−O bond cleavage pathways (Scheme 8). In this reaction, cumyl alcohol was yielded quantitatively in the presence of aca, indicating that a high-valent manganese-oxo species was generated from MnIII alkylperoxo species precursor via a heterolytic O-O bond cleavage. The theoretical study at DFT level was also performed to investigate the nature of the active oxidant and its mechanism of formation in the current catalyst system. Consistent with the experimental results, DFT calculations on the formation of coupled high-valent (L+•)MnIVO species (B) rather than (L)MnVO species (B') from its precursor (L)MnIII(OOH) complex (A), revealed that the conversion of A to B proceeds in a concerted heterolytic manner. Figure 1 displays the results for this conversion in its triplet and quintet spin states. In accord with previous experimental and theoretical results,11 the ground state of complex A is quintet state and the corresponding triplet state lies higher by 7.4 kcal/mol. During the O−O bond cleavage, the calculated transition state 3TS1 is lower than 5TS1 by 1.4 kcal/mol. As such, the triplet state crosses below the quintet state and hence mediates the conversion. The spin density on the leaving OH group in the Mn-O-OH moiety is only -0.15 (see Scheme S1 in SI) on 3TS1 and the transition state leads directly to complex 3B without additional steps. Thus, the overall O−O bond cleavage is effectively concerted heterolytic. The nascent manganese-oxo complex B has a triplet ground state and a low-lying excited-quintet state with a small energy difference of 2.5 kcal/mol as calculated with UB3LYP. Inspection of electronic configurations of complex B (Figure 2) shows that three unpaired spin-up electrons occupy the d-orbital block of manganese center and one single electron in the π orbital of the

(L) MnIV O O-O bond heterolysis

LM nIII OOH

OCOR

R

B

H

H2O

or

A O

O

(L) MnV O

R

B'

Figure 1. Energy profiles (in kcal/mol) and geometries of key species (in Å) for the conversion of (L)MnIII(OOH)-(CH3COOH) (A) to (L+•)MnIV(O)-(CH3COO)-H2O (B). Energies were calculated at the level of UB3LYP/Def2-TZVPP//LACVP**. Zero-point energies (ZPE) and solvation were taken into account.

equatorial ligand is coupled to the quartet pair in either a ferromagnetic or antiferromagnetic manner, to give a triplet and quintet spin states. As such, the

Figure 2 Spin-natural orbitals along with their occupancies for (a) 3B and (b) 5B.

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the dedicated grant for new technology of methanol conversion from Dalian Institute of Chemical and Physics, Chinese Academy of Science is gratefully acknowledged.

oxidation number on manganese is IV and complex B is represented as (L+•)MnIVO. All attempts to optimize the structure with the electron-transferred (L)MnVO complex B' caused it to collapse to (L+•)MnIVO. Thus, as cytochrome P450 enzymes utilize coupled porphyrin π-cation radical ferryl-oxo, the socalled compound I (Por+•)FeIVO, to efficiently activate substrate, in this work the nascent manganese-oxo complex employs an analogous (L+•)MnIVO oxidant to catalyze the reactions. CONCLUSION In conclusion, the present work describes a highly efficient and practical method that enables enantioselective oxidation of a broad range of heteroaromatic sulfides with a manganese complex bearing porphyrin-like ligand. Reactions are performed in short reaction times and low catalyst loading, employing aqueous hydrogen peroxide as terminal oxidant and utilizing simple operations. These attributes make the current catalyst system an ideal alternative to enzymatic methods and the titanium/tartrate systems. The practical utility of the method has been demonstrated in gram-scale synthesis of chiral sulfoxides. The involvement of high valent Mn-oxo species as oxidizing intermediate, which was formed via carboxylic acid assisted heterolysis cleavage of the O−O bond, was evidenced experimentally. Particularly of note, based on the DFT analysis, the coupled high-valent manganese (IV)-oxo cation radical species as the reactive oxidizing intermediates which bear obvious similarities with that of reactive intermediates in the catalytic oxygenation reactions based on the cytochromes P450 and metalloporphyrin models have been proposed for the first time in nonheme manganese catalyst system. So, the present chiral manganese complex is an enzyme-like catalyst, leading to highly chemoselective and enantioselective catalytic oxidation of heteroaromatic sulfides. The above mechanistic study may provide some useful hints for chiral oxidation catalysis design and extend the scope of asymmetric oxidation catalyzed by the porphyrin-like manganese complex.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:xxxxx. Procedures and NMR, HRMS, and HPLC data

AUTHOR INFORMATION Corresponding Author *[email protected]. *[email protected] *[email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21502187 and 21573237), China Postdoctoral Science Foundation Funded Project (2015M581364) and Natural Science Foundation of Fujian Province (2015J01069) and

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M. Coord. Chem. Rev. 2011, 255, 2912-2932. (e) Shaik, S.; Hirao, H.; Kumar, D. Acc. Chem. Res. 2007, 40, 532-542. (9) Mas-Ballesté, R.; Que, L. J. Am. Chem. Soc. 2007, 129, 1596415972. (10) (a) Lyakin, O. Y.; Ottenbacher, R. V.; Bryliakov, K. P.; Talsi, E. P. ACS Catal. 2012, 2, 1196-1202. (b) Ansari, A.; Kaushik, A.;

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