• Amphiphilic catalysts: Under the spotlight

Anne Pichon

Introduction

© (2007) RSC

Under suitable conditions surfactants, which lower the interfacial tension between liquids, can enable two immiscible liquids to form a single phase called an emulsion. Can Li and co-workers at the Chinese Academy of Sciences in Dalian have synthesized a surfactant using a polyoxometalate (POM) substituted with europium¹. The surfactant is bright red under photoexcitation at a wavelength of 615 nm owing to the strong charge transfer from oxygen to europium, and this characteristic was exploited to observe the surfactant dispersion directly using fluorescence microscopy.

The researchers assembled surfactant catalysts, formed by quaternary ammonium cations and POM anions, into emulsion droplets for the oxidation of organic molecules in aqueous media. Until this study, the dispersion of the catalysts at the interface of the two liquids had only been characterized by indirect methods.

Surfactants avoid the traditional diffusion problems associated with multiphasic systems. The catalyst, which was first entirely dissolved in water, assembled into circular domains when immiscible dodecane was added, showing the presence of the surfactant at the water–dodecane interface of the emulsion droplets.

When tested for the oxidation of five different alcohols into ketones, the surfactant catalyst showed excellent properties in emulsion, including 100% activity and selectivity in the oxidation of organic molecules and a turnover number (the number of substrates converted before de-activation) that is 25 times higher than that of biphasic systems.

The authors of this work are from:
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China.

Reference

1. Gao, J. et al. A direct imaging of amphiphilic catalysts assembled at the interface of emulsion droplets using fluorescence microscopy. Chem. Commun. doi: 10.1039/b713831h (2007). | Article |

   我室乳液催化研究工作被NATURE CHINA网站评为亮点文章

                日前，李灿院士研究组的乳液催化研究工作在Chem. Commun.（高金波、蒋宗轩、李灿等2008，DOI: 10.1039/b713831h）上发表后被NATURE CHINA网 站评为最新研究亮点。该研究工作通过荧光标记成像方法观测到双亲型催化剂在水油两相界面上自组装形成的乳液微球。这种乳液催化体系使催化活性中心和极性氧 化剂在油相中高度分散，从而显示出较高的催化氧化活性。该体系可以通过简单破乳方法实现催化剂的分离回收，兼具均相催化的高活性、高选择性以及多相催化体 系易于分离的特点。

   
   

          近年来，李灿院士领导的研究组将乳液体系与多相催化相结合，发展了乳液催化并取得研究进展。该研究组的乳液催化氧化用于油品超深度脱硫可将柴油中几百个ppm的硫降低至几个ppm。该研究工作2004年发表于Chemistry-A European Journal并被英国皇家化学会Green Chemistry杂志评价为绿色化学研究的亮点；利用分子氧（O₂）乳液催化氧化超深度脱硫的工作（Chem. Commun. 150-152, 2007）入选美国化学会（ACS）“Heart Cut” 文章。最近乳液催化超深度脱硫工作与中石化合作进行的工业中试中也取得重要进展。该研究组不仅将乳液催化新概念应用于油品超深度脱硫等催化过程，还将其应 用范畴拓展到有机分子的乳液催化选择氧化和手性乳液酸催化等过程，显示出良好的效果。李灿院士应邀在第四届亚太催化大会（新加坡，2006年）作了题为“Emulsion Catalysis: an Environmental Benign & Green Chemistry Approach”的大会报告，这一方向的研究工作又被邀请为本年度北美催化大会的特邀报告。

   　

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   Published online: 31 January 2007 | doi:10.1038/nchina.2007.070131-3

   • Catalysis: Mirror, mirror, who's the most selective?

   Kaspar Mossman

   Abstract

   Researchers improve geometric selectivity of an organic catalyst by attaching electron-withdrawing groups

   Original article citation

   Tang, Z. et al. A Highly Efficient Organocatalyst for Direct Aldol Reactions of Ketones with Aldedydes. J. Am. Chem. Soc. 125, 9285–9289 (2005).

   Introduction

   Among the amino-acid building blocks of proteins, proline is unique for its ring-like structure. Proline also has the ability to catalyse the aldol reaction, which synthetic chemists use to form carbon—carbon bonds. Liu-Zhu Gong and others at the Chengdu Institute of Chemistry, Chinese Academy of Sciences developed an efficient catalyst derived from proline that is particularly picky about the geometry of the molecules it produces¹.

   If a carbon atom bonds to four different groups, two mirror-image geometries are possible. Molecules that are mirror-image pairs are called enantiomers. Synthetic chemists value catalysts that, in speeding up a reaction, also generate the desired enantiomeric product.

   In previous work, Gong and co-workers were able to alter both the efficiency and selectivity of proline-based catalysts by attaching side groups. They observed that the more effective the side group was at withdrawing electrons, the more selective the catalyst's output became. In this paper, the researchers made a thorough investigation of proline alteration, and concluded that the greatest advantage was to be gained by adding esters as two electron-withdrawing groups. Cooling the reaction (to –25 °C) also increased the selectivity of the preferred enantiomeric candidate to 99%. Similar modifications may improve the properties of other organic catalysts.

   References

2. Tang, Z. et al. A Highly Efficient Organocatalyst for Direct Aldol Reactions of Ketones with Aldedydes. J. Am. Chem. Soc. 125, 9285–9289 (2005). | Article |

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   Published online: 21 March 2007 | doi:10.1038/nchina.2007.24

   • Catalytic reactions: As good as gold

   Vikki Allen

   Abstract

   A porphyrin molecule with a gold atom at the centre is a highly reactive catalyst for organic reactions

   Original article citation

   Zhou, C.-Y, Chan, P. W. H. & Che, C.-M. Gold(III) porphyrin-catalyzed cycloisomerization of allenones. Org. Lett. 8, 325–328 (2006).

   Introduction

   A gold–porphyrin complex has been developed as a practical catalyst for a carbon–oxygen bond-forming reaction. The catalyst, which is highly selective and reactive, is both easy to recover and re-use.

   Researchers are very interested in gold complexes as catalysts for bond-forming reactions. Although such compounds are reactive, the challenge remains to develop ones that are both robust and selective. Some studies have already looked at the physical and biological properties of gold–porphyrin complexes, but Chi Ming Che and co-workers¹ at the University of Hong Kong, are the first to use them for organic reactions.

   The reaction they studied forms oxygen containing carbon rings, known as furans, with almost complete conversion of reactants into products. No by-products were formed, which is a problem when using other catalysts. This recyclable catalytic system also has a remarkable turnover — the average number of times each molecule of catalyst can successfully produce a molecule of product — of 8,300.

   The unique catalytic properties of these complexes results from the bulky groups attached to the porphyrins. Importantly, these groups can be easily modified to fine tune the reactivity and selectivity of the complexes.

   References

3. Zhou, C.-Y, Chan, P. W. H. & Che, C.-M. Gold(III) porphyrin-catalyzed cycloisomerization of allenones. Org. Lett. 8, 325–328 (2006). | Article |

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   • Catalysis: A two-in-one converter

   Anne Pichon

   Published online: 30 May 2007 | doi:10.1038/nchina.2007.95

   Original article citation:

   Zhou, L., Wang, Z., Wei, S. & Sun, J. Evolution of chiral Lewis basic N-formamide as highly effective organocatalyst for asymmetric reduction of both ketones and ketimines with an unprecedented substrate scope. Chem. Comm. doi: 10.1039/b703307a (2007).

4. 【篇名】	Evolution of chiral Lewis basic N-formamide as highly effective organocatalyst for asymmetric reduction of both ketones and ketimines with an unprecedented substrate scopeElectronic supplementary information (ESI) available: Experimental section. See DOI: 10.1039/b703307a
   【刊名】	Chemical Communications
   【ISSN】	1359-7345
   【卷期】	卷 28期
   【出版日期】	20070710
   【页码】	从 2977 页到 2979 页共 3 页
   【DOI】	10.1039/b703307a
   【作者】	Li Zhou Zhouyu Wang Siyu Wei   Chengdu Institute of BiologyChinese Academy of Sciences Jian Sun   Chengdu Institute of BiologyChinese Academy of Sciences
   【文摘】 l-Pipecolinic acid derived Lewis basic N-formamide 5e has been developed as a first highly effective catalyst for the asymmetric reduction of aromatic and aliphatic ketones as well as aromatic and aliphatic ketimines in good to high enantioselectivity.