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. 2015 Aug 1;6(8):4674-4680.
doi: 10.1039/c5sc01044f. Epub 2015 May 19.

Reaction-activated palladium catalyst for dehydrogenation of substituted cyclohexanones to phenols and H2 without oxidants and hydrogen acceptors

Jingwu Zhang  1 , Qiangqiang Jiang  1 , Dejun Yang  1 , Xiaomei Zhao  1 , Yanli Dong  1 , Renhua Liu  1
Affiliations

Reaction-activated palladium catalyst for dehydrogenation of substituted cyclohexanones to phenols and H2 without oxidants and hydrogen acceptors

Jingwu Zhang et al. Chem Sci. .

Abstract

It is widely believed that the dehydrogenation of organic compounds is a thermodynamically unfavorable process, and thus requires stoichiometric oxidants such as dioxygen and metal oxides or sacrificial hydrogen acceptors to remove the hydrogen from the reaction mixture to drive the equilibrium towards the products. Here we report a previously unappreciated combination of common commercial Pd/C and H2 which dehydrogenates a wide range of substituted cyclohexanones and 2-cyclohexenones to their corresponding phenols with high isolated yields, with H2 as the only byproduct. The reaction requires no oxidants or hydrogen acceptors because instead of removing the generated hydrogen with oxidants or hydrogen acceptors, we demonstrated it can be used as a cocatalyst to help power the reaction. This method for phenol synthesis manifests a high atom economy, and is inherently devoid of the complications normally associated with oxidative dehydrogenations.

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Figures

Scheme 1
Scheme 1. Dehydrogenation of substituted cyclohexanones to phenols. The reactions of both (a) and (b) proceeded with Pd(ii) complex catalysts requiring molecular oxygen as the oxidant. (c) The palladium(0)-catalyzed reaction for dehydrogenation of cyclohexanones to phenols under 1 atm of the gas mixture atmosphere (70–80 vol% N2 and 30–20 vol% H2) which requires no oxidants and hydrogen acceptors, and H2 is the only side product.
Fig. 1
Fig. 1. Reaction time course of the dehydrogenation of 3-isobutyl-5-phenyl-cyclohexanone to 3-isobutyl-5-phenyl-phenol under variable gas atmosphere conditions. The reaction conditions are as follows: 3-isobutyl-5-phenyl-cyclohexanone (2 mmol), DMA (4 mL), Pd/C (0.1 mmol Pd), K2CO3 (0.4 mmol), 150 °C. The yields were determined by GC analysis using n-dodecane as the internal standard. Curve ■ (black): the reaction was performed under 1 atm of the gas mixture atmosphere (70 vol% N2 and 30 vol% H2). Curve (blue): the Pd/C catalyst was pretreated with 1 atm H2 at room temperature for 1 h, and the dehydrogenation reaction with the pretreated Pd/C was performed under 1 atm N2 atmosphere. Curve ▲ (red): the reaction was performed under 1 atm N2 atmosphere. Curve ○ (green): the reaction was performed under 1 atm O2 atmosphere.
Fig. 2
Fig. 2. GC H2 detection for the dehydrogenation of cyclohexanone. The reactions were under N2 atmosphere. The gas sample was sampled from the reaction gas atmosphere (reaction conditions: 1 mmol cyclohexanone, 5% mol Pd/C, 150 °C, 1.5 h, 2 mL DMA under 1 atm N2 atmosphere). The H2 and N2 were confirmed by comparing with standard H2 and N2 GC spectra.
Scheme 2
Scheme 2. Proposed mechanistic pathway of palladium-catalyzed dehydrogenation of cyclohexanone to phenol and H2.
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