Nepali young scientist's (Dr. Raghu Dhital-Oasis) work has recently been featured as Editors's choice in reputed journal Science. Here is the text.
"In the classic Ullmann coupling reaction reported more than a century ago, iodine substituted aromatic rings were coupled at high temperatures by using copper as a reducing agent to form a carbon-carbon bond. Further work extended this coupling reaction to more conventional chloroarenes by using coreductants and precious-metal catalysts, but reaction temperatures still tended to be high. Dhital et al. now show that a wide variety of chloroarenes can be coupled at ambient temperatures (25° to 45°C). The reactions proceeded under basic conditions (in a mixed organic-aqueous solvent) in the presence of gold-palladium nanocluster catalysts. Neither pure gold nor pure palladium clusters could catalyze the reaction, indicating that alloying of the metals was critical for reactivity. Density functional theory calculations indicated that the critical difference for the alloy clusters is that they favor activation of the carbon-chlorine bond through the dissociative adsorption of the arene chlorides."
Dr. Dhital's work has also been featured in JACS's spotlight. See below.
TWO METALS ARE BETTER THAN ONE
Heterogeneous catalysis using nanoclusters consisting of more than one metal is seeing more widespread application in chemistry, because these types of catalysts can be highly efficient or may have unusual properties. Particularly attractive are the gold/palladium bimetallic cluster alloys, due to their high catalytic turnover. Masahiro Ehara, Hidehiro Sakurai, and co-workers have used this bimetallic alloy to activate the high-energy carbon−chlorine bond (DOI: 10.1021/ja309606k). They discover that the Au/ Pd nanocluster can catalyze Ullmann coupling of chloroarenes at room temperature, a reaction that otherwise requires harsh conditions. The reaction does not progress with either gold or palladium single-metal nanoclusters alone, nor with a macroscopic mixture of the two metals. Through computational analysis, the researchers find that the high activity of the bimetallic nanocluster is due to the substrate being adsorbed onto the alloy surface, which is unlikely to occur with single metal catalysts. In addition, the nanocluster is found to have higher activity toward chloroarenes than their bromo equivalents. This straightforward method to activate the robust C−Cl bond may enable the design and synthesis of new multimetallic catalyst for similar activation energy. (JACS spotlight, J. Am. Chem. Soc. 2013, 135, 949)
Dr. Dhital completed his PhD last year from Institute for Molecular Science, The Graduate University for Advanced Studies (SOKENDAI) with Prof Hidehiro Sakurai. He is now working as post-doctoral fellow. He has MSc degree from CDC, Tribhuvan university, Nepal.
Following are some of his recent publications.
1. R. N. Dhital, A. Murugadoss and H. Sakurai, “Dual Roles of Polyhydroxy Matrices in the Homocoupling of Arylboronic Acids Catalyzed by Gold Nanoclusters under Acidic Conditions”, Chem. Asian J. 2012, 7, 55. (Inside cover picture)
2. R. N. Dhital and H. Sakurai, “Anomalous Efficacy of Bimetallic Au/Pd Nanoclusters in C-Cl Bond Activation and Formal Metathesis-type C-B Bond Activation at Room Temperature”, Chem. Lett. 2012, 41, 630. (Editor’s choice)
3. R. N. Dhital, C. Kamonsatikul, E. Somsook, K. Bobuatong, M. Ehara, S. Karanjit and H. Sakurai, “Low-temperature Carbon–Chlorine Bond Activation by Bimetallic Gold/Palladium Alloy Nanoclusters: An Application to Ullmann Coupling”, J. Am. Chem. Soc. 2012, 134, 20250. (Highlight in Science, 2013, 339, January 4 and Highlight in JACS spotlight, J. Am. Chem. Soc. 2013,135, 949).
4. R. N. Dhital, C. Kamonsatikul, E. Somsook Y. Sato and H. Sakurai, “Aryl Iodide as a Strong Inhibitor for the Gold and Gold-based Bimetallic quasi-Homogeneous Catalysis”, Chem.Comm. DOI: 10.1039/C3CC39019E, in press.