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Visible Light Photocatalysis Employing Dye-Sensitized Semiconductor: Selective Aerobic Oxidation of Benzyl Ethers Li Ren, Ming-Meng Yang, Chen-Ho Tung, Li-Zhu Wu, and Huan Cong ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03029 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017
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ACS Catalysis
Visible Light Photocatalysis Employing Dye-Sensitized Semiconductor: Selective Aerobic Oxidation of Benzyl Ethers Li Ren,†,‡ Ming-Meng Yang, †,‡ Chen-Ho Tung,†,‡ Li-Zhu Wu,†,‡ and Huan Cong*,†,‡ †
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190 (China)
‡
School of Future Technology, University of Chinese Academy of Sciences, Beijing, 100190 (China)
ABSTRACT: The aerobic oxidation is an attractive approach toward environmentally benign synthesis of fine chemicals. In addition, dye-sensitized semiconductors are underdeveloped photocatalysts for selective organic synthesis. With the aid of catalytic eosin Y-sensitized titanium dioxide, we have developed efficient aerobic photooxidation of benzyl ethers to benzoates, featuring low cost, high atom economy, broad substrate scope, and user-friendly setup. Furthermore, preliminary mechanistic studies established that the reaction pathway likely entails a photo-induced, radical-based two-step process via an isolable peroxide intermediate.
KEYWORDS. photocatalysis, oxidation, aerobic, dye-sensitized semiconductor, radical reactions Environmentally friendly metal oxide semiconductors, such as titanium dioxide, are efficient heterogeneous photocatalysts.1 In most cases, these large band gap semiconductors only respond to UV light irradiation, leading to limited utility for organic synthesis. Although harvesting the power of visible light becomes viable with the help of dye sensitizers as demonstrated by the successful applications including solar energy conversion2 and pollutant degradation,3 the use of dye-sensitized semiconductors as visible light photocatalysts for selective chemical transformations remains underdeveloped.4 Compared to recent rapid progress in homogeneous photoredox catalysis,5 these cheap, non-toxic, heavy metal-free, and userfriendly alternatives should find broader applications which rival or even surpass existing synthetic protocols.
Scheme 1. Reaction Highlights
In order to achieve not only effective catalytic oxidation at the target site, but also good selectivity in the presence of competing C(sp3)-H bonds (e.g. those next to a heteroatom or an aromatic ring),9 our investigation began with the evaluation of reaction parameters for the oxidation of benzyl ether 1a. We were pleased to determine that the commercially available P25-type titanium dioxide,10 in the presence of catalytic eosin Y as a dye sensitizer, served as an effective photocatalyst for the desired transformation under visible light irradiation (Table 1, entry 1). Notably, the reaction can be setup easily,11 and proceed smoothly with reagent-grade acetone as solvent and balloon-pressure air as the oxidant of choice. In the absence of light, air, or either catalyst component, significantly less product formation was observed (entries 2-5). P25-type TiO2 and eosin Y became the optimal combination among the metal oxide semiconductors (entries
Here we report selective aerobic oxidation of benzyl ethers to benzoates facilitated by the combination of an eosin Y-sensitized TiO2 catalyst and visible light. Environmentally benign oxidation reactions represent an important direction toward the next-generation, sustainable synthesis of fine chemicals.6 The use of ambient air, instead of other stoichiometric oxidants, features zero cost, virtually no waste, high atom economy, and the most accessible oxidant source.7 Furthermore, compared to existing benzyl ether oxidation protocols,8 the energy imported from visible light enables high reaction efficiency with good functional group compatibility under mild and userfriendly conditions (Scheme 1).
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ACS Catalysis
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6-9) and dye sensitizers (entries 10-12) which had been evaluated (Table S2).
peroxide intermediate 3k was isolated, with substantial quantity, when starting material 1k was subjected to the
Table 1. Reaction Optimizationa
Scheme 2. Substrate Scope
H H Ph
O
O
2a
variation from the standard condition conv.(%) yield(%) none no light under N2 no TiO2(P25) no eosin Y TiO2(anatase) instead of TiO2(P25) TiO2(rutile) instead of TiO2(P25) ZnO instead of TiO2(P25) Cu2O instead of TiO2(P25) etythrosine B instead of eosin Y eosin B instead of eosin Y alizarin red S instead of eosin Y
1 2 3 4 5 6 7 8 9 10 11 12
Ph
O
acetone, r.t., 12 h blue LED Standard Condition
1a
entry
air (balloon) 2 mol% eosin Y TiO2(P25)
100
