1. J.-L. Wang, J.-Q. Wang, L.-N. He,* X.-Y. Dou, F. Wu, A CO₂/H₂O₂-tunable reaction: direct conversion of styrene into styene carbonate catalyzed by sodium phosphotungstate/n-Bu₄NBr, Green Chem., 2008, 10, 1218-1223.[link]

Abstract: A facile synthesis of styrene carbonate was realized directly from styrene and CO₂ in an environmentally benign manner with an inorganic base as a “deprotonation reagent”, and a recyclable catalyst system, and mild reaction conditions. Selective formation of carbonate 1b and preferentially producing benzoate 1a could be controlled by subtly tuning the quantities of CO₂ and H₂O₂.

2. Y Du, Y Wu, A.-H. Liu, L.-N. He*, Quaternary Ammonium Bromide Functionalized- Polyethylene Glycol: A Highly Efficient and Recyclable Catalyst for Selective Synthesis of 5-Aryl Oxazolidinones from Carbon Dioxide and Aziridines Under Solvent-Free Conditions, J. Org. Chem., 2008, 73(12), 4709-4712. [link]

(Selected by the Editorial Board of SYNFACTS for its important insights, SYNFACTS, Y08908SF)

Abstract: A quaternary ammonium bromide covalently bound to polyethylene glycol (PEG, MW=6000), i.e. PEG₆₀₀₀(NBu₃Br)₂ was found to be an efficient and recyclable catalyst for the cycloaddition reaction of aziridines to CO₂ under mild conditions without utilization of additional organic solvents or co-catalysts. As a result, 5-aryl-2-oxazolidinone was obtained in high yield with excellent regioselectivity. The catalyst worked well for a wide variety of 1-alkyl-2-arylaziridines. Besides, the catalyst could be recovered by centrifugation and reused without significant loss of catalytic activity and selectivity.

3. C.-X. Miao, J.-Q. Wang, L.-N. He*, Catalytic process for chemical conversion of carbon dioxide into cyclic carbonates and polycarbonates, The Open Org. Chem. J., 2008, 2, 68-82. [link]

Abstract: The development of catalytic methods for chemical transformation of CO₂ into useful compounds is of paramount importance from a standpoint of C1 chemistry and so-called green chemistry. The kinetic and thermodynamic stability of CO₂ molecule presents significant challenges in designing efficient chemical transformations based on this potential feedstock. In this context, efforts to convert CO₂ to useful chemicals will inevitably rely on its activation through molecular catalysts, particularly transition-metal catalysts. Two preparative processes employing solid catalyst or CO₂-philic homogeneous catalyst were devised for environmentally benign synthesis of organic carbonates and oxazolidinones under solvent-free conditions. Those processes represent pathways for greener chemical fixations of CO₂ to afford industrial useful materials such as organic carbonates and oxazolidinones with great potential applications.

4. C.-X. Miao, J.-Q. Wang, Y. Wu, Y. Du, and L.-N. He*, Bifunctional Metal-Salen Complexes as Efficient Catalysts for the Fixation of CO₂ with Epoxides under Solvent-Free Conditions, ChemSusChem, 2008, 1(3), 236-241. (one most cited articles published in 2008)[link]

Abstract: A bifunctional cobalt-salen complex containing a Lewis acidic metal center and a quaternary phosphonium salt unit anchored on the ligand (Co-salen-R₃P⁺X^-) was used to effectively synthesize cyclic carbonates from CO₂ and epoxides under mild conditions without the utilization of additional organic solvents or co-catalysts. The effects of various reaction variables on the catalytic performance were also discussed in detail. The results indicate that the optimized reaction temperature is ca. 100 ^oC. The maximum appeared in the yield versus pressure curves at around 4 MPa, whereas the reaction could smoothly be operated even under the pressure as low as 2 MPa. Interestingly, the axial ligands have a great effect on the catalyst performance. In this work, the catalyst was found to be applicable to a variety of epoxides, producing the corresponding cyclic carbonates. Furthermore, the catalyst can be easily recovered and reused at least four times without significant loss of its catalytic activity. In addition, this process represents a greener pathway for the environmentally benign chemical fixation of CO₂ to produce cyclic carbonate using a single-component bifunctional metal-salen complex as an effective catalyst.

5. Y. Du, L.-N. He*, D.-L. Kong, Magnesium-catalyzed synthesis of organic carbonate from 1, 2-diol/alcohol and carbon dioxide, Catal. Commun., 2008, 9(8), 1754-1758.[link]

Abstract: Low toxic magnesium and its oxide are proved to be a catalyst for highly selective synthesis of propylene carbonate (PC) through the carbonylation of propylene glycol (PG) with CO₂ without any organic solvents or additives. Interestingly, the catalyst was demonstrated to be applicable to a variety of 1, 2-diols such as glycol, phenyl glycol, and methanol, forming the corresponding carbonates in 100% selectivity. As a result, employing a low toxic and cheap catalyst could make PC synthesis much environmentally friendlier. A mechanistic insight into the Sn/Mg-catalyzed reaction of “PG+CO₂” was also studied herein.

6. Y. Zhao, L.-N. He, Y. Y. Zhuang, J. Q. Wang, Dimethyl carbonate synthesis via transesterification catalyzed by quaternary ammonium salt functionalized chitosan, Chin. Chem. Lett., 2008, 19(3), 286-290.[link]

Abstract: A quaternary ammonium salt covalently linked to chitosan was first used as a catalyst for dimethyl carbonate (DMC) synthesis by the transesterification of propylene carbonate (PC) and methanol. The effects of various reaction variables like reaction time, temperature and pressure on the catalytic performance were also investigated. 54% DMC yield and 71% PC conversion has been obtained under the optimal reaction conditions. Notably, the catalyst was able to be reused with retention of high catalytic activity and selectivity. Consequently, the process presented here has great potential for industrial application due to its advantages such as stability, easy preparation from renewable biopolymer, and simple separation from products.

7. J.-S. Tian, F. Cai, J.-Q. Wang, Y. Du, L.-N. He*, Environmentally Benign Chemical Conversion of CO₂ into Organic Carbonates Catalyzed by Phosphonium Salts, Phosphorus, Sulfur, and Silicon and the Related Elements, 2008, 183(2-3), 494-498.[link]

Abstract: Quaternary phosphonium salts were proved to be efficient homogeneous catalysts for solvent-free synthesis of cyclic carbonates from carbon dioxide and epoxides. Propylene carbonate was produced in quantitative yield and excellent selectivity. Furthermore, a PEG-supported phosphonium salt was solidified by adding ether and cooling after reaction and recovered by a simple filtration. Hence immobilization of a phosphonium salt on PEG provides an alternative pathway for realizing homogeneous catalyst recycling.

8. H. Tang, A Lu, Z. Zhou*, P. Gao, L.-N. He*, C. Tang, Chiral tertiary amine/L-proline cocatalyzed enantioselective Morita-Baylis-Hilliman (MBH) reaction, Eur. J. Org. Chem. 2008, 126-135.[link]

9. H. Tang, G. Zhao, Z. Zhou*, G.-F. Zhao, L.-N. He*, C. Tang, Chiral Phosphoric Acid Catalyzed Asymmetric Friedel–Crafts Alkylation of Indoles with Simple α, β-Unsaturated Aromatic Ketones，Eur. J. Org. Chem. 2008, 1406–1410.[link]