Copper Nanocrystal Plane Effect on Stereoselectivity of Catalytic Deoxygenation of Aromatic EpoxidesJ. Am. Chem. Soc.


Bin Xiao, Zhiqiang Niu, Yang-Gang Wang, Wei Jia, Jian Shang, Lan Zhang, Dingsheng Wang, Yao Fu, Jie Zeng, Wei He, Kai Wu, Jun Li, Jinlong Yang, Lei Liu, Yadong Li
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Copper Nanocrystal Plane Effect on Stereoselectivity of Catalytic

Deoxygenation of Aromatic Epoxides

Bin Xiao,†,‡,§,# Zhiqiang Niu,‡,§,# Yang-Gang Wang,‡,§ Wei Jia,‡,§ Jian Shang,∥ Lan Zhang,†

Dingsheng Wang,‡,§ Yao Fu,† Jie Zeng,† Wei He,§,⊥ Kai Wu,∥ Jun Li,‡,§ Jinlong Yang,† Lei Liu,*,†,‡,§,⊥ and Yadong Li*,†,‡,§ †Center of Advanced Nanocatalysis (CAN), University of Science and Technology of China, Hefei 230026, China ‡Department of Chemistry, Tsinghua University, Beijing 100084, China §Collaborative Innovation Center for Nanomaterial Science and Engineering, Tsinghua University, Beijing 100084, China ∥College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ⊥Tsinghua-Peking Center for Life Sciences, Tsinghua University, Beijing 100084, China *S Supporting Information

ABSTRACT: Previous studies have shown that crystal planes of heterogeneous catalysts could display enhanced activity, such that higher turnover or chemoselectivity could be achieved. Here we report an example where the reaction stereoselectivity was significantly affected by the catalyst crystal planes. In copper-catalyzed deoxygenation reaction of aromatic epoxides, copper cubes, wires, and plates gave the olefin products with different cis/trans selectivities, whereas homogeneous copper catalysts showed poor selectivity. Scanning tunneling microscope and density functional theory studies revealed that the different adsorption mode and higher adsorption strength of epoxide oxygen on Cu{100} plane were responsible for the observed variation of selectivity. The copper-catalyzed deoxygenation reaction provided new practical access to cis-olefins from readily available aromatic epoxides. Our work also indicated that nanocrystal catalysts may provide useful stereochemical control in organic reactions.

The discovery of catalyst single crystal plane effects on solidsurface-catalyzed reactions, marked by the observation that

Fe{111} planes exhibit higher activity than Fe{100} and Fe{110} in catalytic ammonia synthesis,1,2 has greatly promoted the advance of surface science and catalysis. With aspiration to develop practical catalysts that could fully capitalize the advantage of such crystal plane effects, recent research on single-crystalline nanocrystal catalysts with large “effective” surface planes has witnessed remarkable advances.3 For instance, single-crystalline CeO2 nanorods were shown to have a higher

CO oxidation activity with CeO2{100} and {110} than with

CeO2{111}. 4,5 Somorjai et al. discovered that in the catalytic hydrogenation of benzene, the exposed {100} surface of Pt cubic nanoparticles was more active than the {100} and {111} mixture surfaces of Pt cuboctahedral nanoparticles.6 For the methane combustion, it was shown that the catalytic activity of Co3O4 nanocrystals followed the order of {112} > {011}≫ {001}.7 The oxidation of CO was achieved at temperatures as low as −77 °C on Co3O4{110} planes that remained stable in a moist stream of normal feed gas.8 These examples illustrated the potential of single-crystalline nanocrystals as highly active catalysts. The advantages manifested by the crystal plane effect, however, are confined to reactivity enhancement. As catalysis concerns both reactivity and selectivity,9 it remains a desirable goal to elicit crystal plane effect on reaction selectivity, especially stereoselectivity.10

Here we report a surprising finding of crystal plane-dependent stereoselectivity changes in a copper nanocrystal (wires, plates, and cubes) catalyzed deoxygenation reaction of aromatic epoxides. This reaction is not only a new catalytic transformation useful to organic synthesis but also provides an interesting example for the variation of reaction stereoselectivity with differently exposed catalyst crystal planes. Our finding suggested the potential of using well-defined nanocrystals as practical catalysts to catalyze organic reactions with both high activity and high stereoselectivity. Moreover, this reaction provides a new convenient and practical access to some synthetically interesting cis-olefins11 from readily available epoxides.

Our study began with the finding of an unprecedented deoxygenation reaction of trans-stilbene oxide (1a) with a Si−B reagent12 Me2PhSi-Bpin ((dimethylphenylsilyl)boronic acid pinacol ester).13 The yields and the ratio of cis/trans-stilbene (2a/3a) were determined by 1H NMR using 1,2,4,5tetramethylbenzene as an internal standard.Without any catalyst, the reaction could take place at 60 °C in dioxane generating cisand trans-stilbenes in a ratio of 1.0:2.1 (individual yield = 22% and 45%) after 1 h. The low yield and poor selectivity called for the finding of catalysts to improve the performance. Many homogeneous or heterogeneous catalysts (e.g., CuCl2, CuO,

CuI, Fe2O3, Au) failed to induce stereoselectivity to favor the production of cis-olefin. Nonetheless, copper in charcoal (3 wt %) as a commercially available catalyst (Aldrich 709107) was found to favor the formation of cis-stilbene, but the cis/trans selectivity varied from 4:1 to 1:1 among different experiments.

Hypothesizing that the variation of stereoselectivity may arise from the heterogeneous nature of the exposed copper crystal planes in copper on charcoal, we conducted morphologyReceived: February 8, 2015

Published: March 17, 2015

Communication © 2015 American Chemical Society 3791 DOI: 10.1021/jacs.5b01391

J. Am. Chem. Soc. 2015, 137, 3791−3794 controlled synthesis of different nanocrystal copper catalysts and tested their activities.

Three general types of copper nanocrystals with well-defined shapes including cubes, wires, and plates were prepared by colloidal chemical synthesis.14,15 The nanocrystals structure and phase purity were determined by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and electron diffraction. The obtained data established the exposed planes in the cubes, wires, and plates to be {100}, {100}/{111}, and {111} planes (Figure 1). The three types of nanocrystals were then loaded on charcoal and used as the catalysts after the capping agent was removed by extensive washing (for detailed procedures, please see Supporting Information).