Metal-Free Catalyst for Visible-Light-Induced Oxidation of Unactivated

Letter Cite This: ACS Catal. 2018, 8, 5425−5430

pubs.acs.org/acscatalysis

Metal-Free Catalyst for Visible-Light-Induced Oxidation of Unactivated Alcohols Using Air/Oxygen as an Oxidant Waldemar Schilling, Daniel Riemer, Yu Zhang, Nareh Hatami, and Shoubhik Das* Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Tammannstraße 2, 37077 Göttingen, Germany S Supporting Information *

ABSTRACT: 9-Fluorenone acts as a metal-free and additive-free photocatalyst for the selective oxidation of primary and secondary alcohols under visible light. With this photocatalyst, a plethora of alcohols such as aliphatic, heteroaromatic, aromatic, and alicyclic compounds has been converted to the corresponding carbonyl compounds using air/oxygen as an oxidant. In addition to these, several steroids have been oxidized to the corresponding carbonyl compounds. Detailed mechanistic studies have also been achieved to determine the role of the oxidant and the photocatalyst for this oxidation. KEYWORDS: O2, alcohols, photocatalysts, oxidations, metal-free

S

Inspired by this information, several metal-based photocatalysts have been reported, based on Ir, Cu, TiO2, Pt, Nb, Ptfunctionalized porphyrinic MOF, etc. (Figure 1).41−48 How-

ynthesis of carbonyl compounds via alcohol oxidation is one of the major important reactions in the pharmaceuticals, as well as in the fine chemical industries.1 So far, stoichiometric oxidants such as MnO2, hypochlorite, permanganate, osmium oxide, activated dimethylsulfoxide (DMSO), etc. prevail in this reaction.2−9 In comparison to these stoichiometric oxidants, O2 or air act as better oxidants, because of the avoidance of hazardous, toxic, and stoichiometric byproducts. Based on this approach, several homogeneous and heterogeneous transition-metal-based catalysts have been reported.10−20 However, the requirement of precious metals, expensive ligands, and costly and toxic additives always hinder their application in pharmaceutical industries. In contrast, transition-metal-free catalysts can be attractive, because of their less expensive price, nontoxicity, and easy separation from the reaction mixture. In fact, TEMPO-based and nitroxyl radicalbased transition-metal-free systems have already been reported. However, the reaction needed several co-catalysts, high temperature, and corrosive solvents.21−29 Compared to the thermal reactions, photocatalysis has a powerful impact for the requirement of clean energy and environmental applications. Over the past two decades, the number of applications based on photocatalysis has increased sharply, especially for hydrogen fuel generation and CO2 reduction.30−33 Significant efforts have also been paid for the development of visible-light-mediated reactions to achieve sustainable syntheses of chemicals.34−39 Therefore, there is always strong interest for the utilization of inexpensive and commercially available photocatalysts for the selective oxidation of alcohols. In fact, the O2 molecule can be activated by photocatalysts to transform it to reactive oxygen species (ROS) such as superoxide anion radical, hydrogen peroxide, singlet oxygen, and hydroxy radicals, which are the key oxidants in many organic reactions.40 © XXXX American Chemical Society

Figure 1. Visible-light-mediated photocatalysts for the oxidation of alcohols.

ever, utilization of precious metals and limited reactivity of these photocatalysts forced scientists toward the development of organic semiconductor-based photocatalysts. Advantageously, these photocatalysts are nonmetal in nature and exhibit high tunable physicochemical properties. In fact, small molecular- or macromolecular-based organic semiconductors such as porous carbon nitrides, graphene/carbon nitride, or thiophene-based covalent triazine framework have also shown reactivity for the oxidation of alcohols to the carbonyl compounds.50−54 However, often these organic semiconductors needed high temperatures, tedious syntheses of the catalysts, Received: March 16, 2018 Revised: April 30, 2018 Published: May 2, 2018 5425

DOI: 10.1021/acscatal.8b01067 ACS Catal. 2018, 8, 5425−5430

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ACS Catalysis and, more importantly, only showed reactivity toward activated alcohols such as benzylic and allylic alcohols. Therefore, there is a strong requirement for an alternative metal-free photocatalyst, which shows wide substrate scope and reactivity toward aliphatic, heteroaromatic, and alicyclic alcohols. Small organic molecules have also shown their activity toward the activation of O2 molecule under visible light, and this concept has already shown powerful applications in organic syntheses via C−H bond activation and others.55−59 These small organic molecules show unique reactivity and unparalleled selectivity in organic reactions. Advantageously, various commercially available structures of these molecules enhance easy optimization of the desired reaction. Based on all this information and our own interest to explore metal-free catalysis, we became interested to find a mild photocatalytic system for the selective oxidation of alcohols.60−65 At the outset of the project, several organic photocatalysts were investigated for the reaction of benzyl alcohol (1a) with O2 as the oxidant for the model system to identify and optimize potential reaction parameters (see Table 1). To our delight, in

Scheme 1. General Substrate Scope for the Oxidations of Primary and Secondary Alcoholsa

Table 1. Optimization for the Visible-Light-Induced Oxidation of Benzyl Alcohol to Benzaldehydea

entry

catalyst

solventb

atmosphere

yieldc [%]

1 2 3 4 5 6 7 8 9 10 11

9,10-dicyano-anthracene rose bengal riboflavine fluorescein Rhodamine 6G 9-fluorenone 9-fluorenone 9-fluorenone 9-fluorenone 9-fluorenone 9-fluorenone

DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMA DMF THF toluene

oxygen oxygen oxygen oxygen oxygen oxygen air air air air air

0 1 7 22 23 99 98 0 4 3 0

a

Reaction conditions: benzyl alcohol (0.29 mmol), photocatalysts (3 mol %), solvent (1 mL), O2 (balloon) or air, room temperature (rt), 18 h. bDMSO = dimethyl sulfoxide; DMA = N,N-dimethylacetamide; DMF = N,N-dimethylformamide; and THF = tetrahydrofuran. cYield determined by GC, using n-dodecane as an internal standard.

the presence of 3 mol % of 9-fluorenone, corresponding benzaldehyde (1b) was obtained in 99% yield within 18 h. Among other photocatalysts, Rhodamine 6G and fluorescein (Table 1, entries 4 and 5) showed activity under our reaction conditions. We attributed the high activity of 9-fluorenone to its high lifetime for the excited state, which was reported with up to 17.8 ns, with an additional stabilizing effect of DMSO in our case.66 The reaction was also investigated under air and to our delight, in the presence of 3 mol % of 9-fluorenone, 98% of the desired benzaldehyde was obtained. The reaction yield was suppressed in DMF, DMA, toluene, and THF (Table 1, entries 8−11). In addition, no formation of overoxidized carboxylic acid product was observed after the reaction. With these optimized conditions in hand, the scope of this alcohol oxidation reaction was explored (see Scheme 1). Several alcohols including aromatic, heteroaromatic, alicyclic, and aliphatic were smoothly oxidized with high yields up to 99%. Different benzylic primary alcohols reacted faster under

a Reaction conditions: Substrates (0.25 mmol), 9-fluorenone (3−6 mol %), DMSO (1 mL), 16−72 h. All are isolated yields except 23a and 27a. Entries 19a−25a and 27a−33a were performed under an O2 atmosphere. 13a* 10 mmol scale; see the Supporting Information for more information.

the optimized conditions (see Scheme 1; entries 2a, 6a, 7a, 10a, and 16a). To our delight, reaction also showed high selectivity in the presence of aldehyde functional group in the aromatic ring (see Scheme 1; entry 10a). In fact, attachment of aliphatic, heteroaromatic, aromatic, and alicyclic rings in the benzylic or allylic position did not hamper the reaction yield (see Scheme 1; entries 3a−5a, 8a, 9a, 11a, 12a, 14a, 15a, 17a, and 18a). After successfully investigating different benzylic and allylic alcohols, we became interested in determining the potential of 9-fluorenone toward unactivated alcohols. For this purpose, 5426


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ACS Catalysis aliphatic, alicyclic, and heteroaromatic alcohols were applied under our optimized reactions conditions (see Scheme 1; entries 21a−33a). To our delight, different aliphatic secondary alcohols showed excellent reactivity under our optimized reaction conditions. In fact, the desired oxidized products were easily isolated for both the internal and terminal aliphatic secondary alcohols, in good to excellent yield (see Scheme 1; entries 21a−25a). Notably, unactivated cyclic alcohols were also oxidized to the corresponding ketones with an excellent yield up to 83% (see Scheme 1; entries 26a−29a). In addition to these, both primary and secondary heteroaromatic alcohols reacted excellently under our conditions (see Scheme 1; entries 30a−33a). The sole byproduct for this oxidation reaction was DMSO2, which was generated after the oxidation of DMSO by the in-situ-generated H2O2. The byproduct was removed by aqueous workup, followed by column chromatography. Furthermore, we decided to apply our catalytic oxidation strategy to more complex steroids, as selective oxidation of saturated steroidal alcohols is highly important in steroid chemistry. 67 Major steroidal hormones contain ketone functionality and the oxidation of Δ5−3β-alcohols to the corresponding Δ4−3-ketones is highly important for the commercial synthesis of hormones.68 Based on this information, we applied our photocatalyst on testosterone, stanelone, androsterone, and, to our delight, all three of these steroids showed high formation of the desired ketones (Scheme 2). To

Table 2. Control Experiments for the Oxidation of Benzyl Alcohola entry

controlled parameter

yieldb [%]

1 2 3 4

standard conditions N2 atmosphere no light no catalyst

98 0 1 0

a Reaction conditions: Substrates (0.29 mmol), 9-fluorenone (3 mol %), DMSO (1 mL), 18 h. bYield determined by GC using n-dodecane as an internal standard.

Table 3. Quenching Experiments for the Oxidation of Benzyl Alcohola

Scheme 2. Oxidation of Alcohol Moiety in Steroidsa

quencher

equivalent

yieldb [%]

notes

BHT BHT TEMPO TEMPO tert-butanol CuCl2 sodium azide catalase benzoquinone

0.5 1.0 0.5 1.0 1.0 1.0 1.0 100 mg 1.0

37 9 31 11 81 4 22 0 5

radical scav. radical scav. radical scav. radical scav. hydroxide radical scav. electron scav. singlet oxygen scav. peroxide radical scav. superoxide radical anion scav.

a Reaction conditions: Substrates (0.29 mmol), 9-fluorenone (3 mol %), DMSO (1 mL), quenchers, 18 h. Data taken from ref 54. bYield determined by GC using n-dodecane as an internal standard.

exhibited lower yield that clearly suggested the interplay of a single electron in this photocatalytic system. We presume that this electron could be generated from the photocatalyst to generate the real oxidant such as singlet oxygen radical and superoxide radical anion from O2 molecule. Finally, reaction with catalase detected the presence of peroxide species in the reaction. However, direct introduction of H2O2 into oxidation reaction resulted in lower activity, indicating that H2O2 was not responsible oxidant in our reaction system but a peroxo species could be involved in the reaction. Further investigation by Stern−Volmer fluorescence quenching experiments revealed that the excited state of the photocatalyst was not quenched by oxygen but rather by benzyl alcohol (see Figure 2).72 Fluorescence intensity was dramatically decreased with the increase of benzyl alcohol concentration and no change was observed with saturated

a Reaction conditions: Substrates (0.25 mmol), 9-fluorenone (6 mol %), DMSO (1 mL), 16−72 h. All are isolated yields.

the best of our knowledge, currently there is no metal-free catalyst known for the oxidation of steroids using oxygen as the oxidant. In most of the cases, stoichiometric oxidants and/or metal-based catalysts have been used.69−71 Therefore, because of the interest of pharmaceutical companies, this catalyst can find strong interest in steroid chemistry. After substrate scope evaluation, we became interested in determining the actual role of the catalyst and the light source. Control experiments proved no formation of the product in the absence of light or photocatalyst (Table 2). In addition to these, no formation of product was observed under N2 atmosphere, which clearly suggested the significant role of O2. These control experiments led us to examine the effect of different quenchers to recognize the reactive oxygen species (see Table 3).54 In fact, the reaction showed a slight decrease in the yield in the presence of tertiary butanol, which ruled out the presence of hydroxide radical. However, using sodium azide and benzoquinone as quenchers revealed the presence of singlet oxygen radical and superoxide radical anion in the reaction system. Further application of CuCl2 in the reaction

Figure 2. Stern−Volmer plot for the oxidation of benzyl alcohol. 5427


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believe this methodology could find interest in the syntheses of pharmaceuticals and natural products.

oxygen/air solution. In addition to this, the KH/KD value of 3.0 in the kinetic isotope effect (KIE) experiment strongly suggested the C−H bond cleavage in the rate-determining step (see Scheme 3).

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ASSOCIATED CONTENT

S Supporting Information *

Scheme 3. Mechanistic Experiments and Proposed Reaction Mechanism

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b01067. Experimental details, characterization data, and spectra for the compounds of the synthesized compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shoubhik Das: 0000-0001-9102-3707 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

We thank Fonds der Chemischen Industrie (FCI, LiebigFellowship, to S.D.) and Chinese Scholarship Council (CSC, to Y.Z.) for the financial support. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are highly thankful to Prof. Dr. Lutz Ackermann for his kind support behind our work.

Combining all these mechanistic information, we rationalized that, at first, the photocatalyst reached the excited state by irradiation of visible light. The excited state of the photocatalyst rendered single electron transfer (SET) to benzyl alcohol and transformed itself to the corresponding radical anion. This fluorenone radical anion generated the real oxidants singlet oxygen radical and superoxide radical anion from O2. The activated benzyl alcohol then reacted with the superoxide anion to generate peroxide radical and further abstraction of one more hydrogen atom by the peroxide radical generated the final desired product. The reported value for the reduction potential of excited-state 9-fluorenone resides at −0.61 V vs SCE,54 which is sufficient for the reduction of molecular oxygen to its superoxide radical form (·O2/O2−) with the reduction potential residing at −0.56 V vs SCE.54,56,73 The fact that, without a substrate, no singlet oxygen radical nor superoxide radical anion could be detected indicates the pivotal role of the substrate, in combination with the photocatalyst. The postulated radical intermediates should be short-lived and are only effective due to the direct and close generation of the oxidant by the activated catalyst. Final experiment under 18O2 atmosphere confirmed this mechanism via the formation of DMSO18O (recall Scheme 3). In conclusion, we have demonstrated 9-fluorenone as a commercially available and inexpensive metal-free photocatalyst for the selective oxidation of alcohols to the corresponding carbonyl compounds. Notably, our photocatalyst showed wide substrate scope and especially high reactivity toward aliphatic, alicyclic, and heteroaromatic alcohols. In fact, this catalyst can also be applied to the oxidation of steroids, which is the key step for many pharmaceuticals. Finally, detailed mechanistic studies clearly demonstrated the role of O2 in the reaction. We

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Metal-Free Catalyst for Visible-Light-Induced Oxidation of Unactivated

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