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Metal-Free Catalyst for Visible-Light-Induced Oxidation of Unactivated Alcohols Using Air/Oxygen as Oxidant Waldemar Schilling, Daniel Riemer, Yu Zhang, Nareh Hatami, and Shoubhik Das ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01067 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018
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ACS Catalysis
Metal-Free Catalyst for Visible-Light-Induced Oxidation of Unactivated Alcohols Using Air/Oxygen as 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 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 have 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 find out role of the oxidant and the photocatalyst for this oxidation.
KEYWORDS: O2 • alcohols • photocatalysts • oxidations • metal-free
Syntheses 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 DMSO etc. prevail in this reaction.2-9 In comparison to these stoichiometric oxidants, O2 or air acts as better oxidants due to the avoidance of hazardous, toxic and stoichiometric by-products. Based on this approach, several homogeneous and heterogeneous transition metal-based catalysts have been reported.10-20 However, requirement of precious metals, expensive ligands, costly and toxic additives always hinder their application in the pharmaceutical industries. In contrast, transition metal-free catalysts can be attractive due to their cheaper price, non-toxicity and easy separation from the reaction mixture. In fact, TEMPO-based and nitroxyl radical based 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 powerful impact for the requirement of clean energy and environmental applications. Over the past two decades, number of applications based on photocatalysis has been increased sharply especially for the 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 findings of cheap and commercially available photocatalyst for the selective oxidation of alcohols. In fact, O2 molecule can be activated by photocatalysts to transform it into reactive oxygen species (ROS) such as superoxide anion radical, hydrogen peroxide, singlet oxygen and hydroxy radical which are the key oxidants in many organic reactions.40
Ir, Cu, TiO2, Pt, Nbbased photocatalysts Limited to benzylic and allylic alcohols
(Ref. 38-46)
OH R1
Macromolecular based organic semiconductors
H
(Ref. 47-51)
This work Fluorenone (3 mol%) DMSO, rt
O R1
H
Limited to benzylic and allylic alcohols
Aliphatic, heteroaromatic and alicyclic alcohols
Figure 1: Visible-light mediated photocatalysts for the oxidation of alcohols Inspired by this information, several metal-based photocatalysts have been reported based on Ir, Cu, TiO2, Pt, Nb, Pt-functionalized porphyrinic MOF etc (Figure 1).41-48 However, utilization of precious metals and limited reactivity of these photocatalysts forced scientists towards the development of organic semiconductor-based photocatalysts. Advantageously, these photocatalysts are non-metal 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, 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 and more importantly, only showed reactivity towards activated alcohols such as benzylic and allylic alcohols. Therefore, there is a strong requirement for an alternative metal-free photocatalyst, which shows wide substrates scope and reactivity towards aliphatic, heteroaromatic and alicyclic alcohols.
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Small organic molecules have also shown their activity towards 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 these 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
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formation of over-oxidized carboxylic acid product was observed after the reaction.
Table 1. Optimization for the visible light induced oxidation of benzyl alcohol to benzaldehyde.[a-e]
Entry
Catalysts
Solvents
1c
9,10-dicyanoanthracene
Atmosphere Yield [%]
DMSO
oxygen
0
2
rose bengal
DMSO
oxygen
1
3
riboflavine
DMSO
oxygen
7
4
fluorescein
DMSO
oxygen
22
5
rhodamine 6G
DMSO
oxygen
23
6
9-fluorenone
DMSO
oxygen
99
7
9-fluorenone
DMSO
air
98
8d
9-fluorenone
DMA
air
0
9e
9-fluorenone
DMF
air
4
10f
9-fluorenone
THF
air
3
11
9-fluorenone
Toluene
air
0
[a] Reaction conditions: Benzyl alcohol (0.29 mmol), photocatalysts (3 mol%), solvent (1 mL), O2 (balloon) or air, rt, 18 h. [b] Yield determined by GC using n-dodecane as an internal standard. [c] DMSO = Dimethylsulfoxide. [d] DMA = N,N-dimethylacetamide. [e] DMF = N,Ndimethylformamide. [f] THF = Tetrahydrofuran.
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 (Table 1). To our delight, in 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 4-5) showed activity under our reaction conditions. We attributed the high activity of 9-fluorenone to its high life-time 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 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
Reaction conditions: Substrates (0.25 mmol), 9-fluorenone (3-6 mol%), DMSO (1 mL), 16-72 h. All are isolated yields except 22a and 27a. Entries 19-25a, 27-33a were performed under O2 atmosphere. 13a* 10 mmol scale see supporting info for more information.
Scheme 1. General substrates scope for the oxidations of primary and secondary alcohols. With these optimized conditions in hand, the scope of this alcohol oxidation reaction was explored (Scheme 1). A number of alcohols including aromatic, heteroaromatic, alicyclic, and aliphatic were smoothly oxidized with high yields up to 99%.
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Different benzylic primary alcohols reacted faster under the optimized conditions (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 (Scheme 1; entry 10a). In fact, attachment of aliphatic, heteroaromatic, aromatic and alicyclic ring in the benzylic or allylic position did not hamper the reaction yield (Scheme 1; entries 3-5a, 8-9a, 1112a, 14-15a, and 17-18a). After successfully investigating different benzylic and allylic alcohols, we became interested to find out the potential of 9fluorenone towards unactivated alcohols. For this purpose, aliphatic, alicyclic and heteroaromatic alcohols were applied under our optimized reactions conditions (Scheme 1; entries 2133a). To our delight, different aliphatic secondary alcohols showed excellent reactivity under our optimized reaction conditions. In fact, the desired oxidized products were easily isolated in case of both the internal and terminal aliphatic secondary alcohols in good to excellent yield (Scheme 1; entries 2125a). Notably, unactivated cyclic alcohols were also oxidized to the corresponding ketones with an excellent yield up to 83% (Scheme 1; entries 26-29a). In addition to these, both primary and secondary heteroaromatic alcohols reacted excellently under our conditions (Scheme 1; entries 30-33a). The sole byproduct for this oxidation reaction was DMSO2, which was generated after the oxidation of DMSO by the in situ generated H2O2. The by-product was removed by aqueous work up followed by column chromatography.
Reaction conditions: Substrates (0.25 mmol), 9-fluorenone (6 mol%), DMSO (1 mL), 16-72 h. All are isolated yields.
Scheme 2: Oxidation of alcohol moiety in steroids. 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 these three steroids showed high formation of the desired ketones (Scheme 2). To 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, owing to the interest of pharmaceutical companies, this catalyst can find strong interest in steroid chemistry. Table 2: Control experiments for the oxidation of benzyl alcohol.[a,b] entry Controlled parameter yield [%] 1
Standard conditions
98
2
N2 atmosphere
0
3
No light
1
4
No catalyst
0
[a] Reaction conditions: Substrates (0.29 mmol), 9-fluorenone (3 mol%), DMSO (1 mL), 18 h. [b] Yield determined by GC using n-dodecane as an internal standard.
After substrates scope evaluation, we became interested to find out 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 find out the effect of different quenchers to recognize the reactive oxygen species (Table 3).54 In fact, reaction showed 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 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 excited state of the photocatalyst was not quenched by oxygen rather by benzyl alcohol (Figure 2).72 Fluorescence intensity was dramatically decreased with the increase of benzyl alcohol concentration and no change was observed with saturated oxygen/air solution. In addition to this, 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 (Scheme 3). Table 3: Quenching experiments for the oxidation of benzyl alcohol.[a,b]54
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experiment under 18O2 atmosphere confirmed this mechanism via the formation of DMSO18O (Scheme 3).
Quenchers
equivalents yields
BHT
0.5
notes
37%
radical scav.
BHT
1.0
9%
radical scav.
TEMPO
0.5
31%
radical scav.
TEMPO
1.0
11%
radical scav.
Tert-butanol
1.0
81%
hydroxide radical scav.
CuCl2
1.0
4%
electron scav.
Sodium azide
1.0
22%
singlet oxygen scav.
Catalase
100 mg
0%
peroxide radical scav.
Benzoquinone
1.0
5%
superoxide radical anion scav.
[a] Reaction conditions: Substrates (0.29 mmol), 9-fluorenone (3 mol%), DMSO (1 mL), quenchers, 18 h. [b] Yield determined by GC using ndodecane as an internal standard.
1,35
c(SM) saturation with O2
1,30
linear fit of c(SM) linear fit of oxygen
1,25
Scheme 3. Mechanistic experiments and proposed reaction mechanism.
1,20
I0 / I
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1,15 1,10 1,05 1,00 0,95 0,00
0,05
0,10
0,15
0,20
0,25
c(quencher) / mmol
Figure 2: Stern-Volmer plot for the oxidation of benzyl alcohol. 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 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
In conclusion, we have demonstrated 9-fluorenone as a commercially available and cheap metal-free photocatalyst for the selective oxidation of alcohols to the corresponding carbonyl compounds. Notably, our photocatalyst showed wide substrates scope especially high reactivity towards 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 believe this methodology could find interest in the syntheses of pharmaceuticals and in the syntheses of natural products.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS publications website. Experimental details, characterization data and spectra for the compounds of the synthesized compounds (PDF).
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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Funding Sources We thank Fonds der Chemischen Industrie (FCI, Liebig-Fellowship to S.D.) and Chinese Scholarship Council (CSC to Y.Z.) for the financial support. ACKNOWLEDGMENTS
We are highly thankful to Prof. Dr. Lutz Ackermann for his kind support behind our work.
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ACS Catalysis
Table of contents: O
OH R1
R1
H
H Metal-free catalyst Unactivated alcohols Commercially available Air/oxygen as the oxidant Visible light mediated Oxidation of steroids
air/oxygen Fluorenone (3 mol%)
Blue LED O
OH R1
R2
R1
R2
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