Man Kin Tse

Green and Efficient Synthesis of Sulfonamides Catalyzed by Nano-Ru/Fe3O4

The environmentally benign synthesis of carbon-nitrogen bonds continues to be an active and challenging field of chemical research. Here, a novel, environmentally benign method for the direct coupling of sulfonamides and alcohols is described. Despite the importance of sulfonamide derivatives as intermediates in drug synthesis, till now such transformations are rarely known. For the first time a domino dehydrogenation-condensation-hydrogenation sequence of alcohols and sulfonamides has been realized in the presence of a nanostructured catalyst. The magnetic property of the catalyst system allows for convenient isolation of the product and efficient recycling of the catalyst. A variety of coupling reactions of benzylic alcohols and sulfonamides including various heterocycles were successfully realized, often with >80% isolated yield. Advantageously, only one equivalent of the primary alcohol is consumed in the process. Mechanistic investigations of the competitive reactions of benzyl alcohol and d(7)-benzyl alcohol with p-toluenesulfonamide revealed a kinetic isotope effect (k(H)/k(D)) of 2.86 (+/-0.109) for the dehydrogenation of benzyl alcohol and 0.74 (+/-0.021) for the hydrogenation of N-benzylidene-p-toluenesulfonamide intermediate, which suggests dehydrogenation of the alcohol to be the rate determining step.

Copper‐Catalyzed Alkylation of Sulfonamides with Alcohols

the Chinese Academy of Sciences;the DFG (SPP 1118 and Leibniz Prize), and the BMBF. F. Shi thanks the Alexander-von-Humboldt-Stiftung for an AvH Fellowship

An efficient biomimetic Fe-catalyzed epoxidation of olefins using hydrogen peroxide.

A new, environmentally benign and practical epoxidation method was developed using inexpensive and efficient Fe catalysts. FeCl3.6H2O in combination with commercially available pyridine-2,6-dicarboxylic acid and amines showed excellent reactivity and selectivity towards aromatic olefins and moderate reactivity towards 1,3-cyclooctadiene utilizing H2O2 as the terminal oxidant.

A general gold-catalyzed direct oxidative coupling of non-activated arenes

A gold-catalyzed mild and general oxidative homo-coupling of arenes using PhI(OAc)2 as the oxidant is described (13 examples, 31-81% yield).

Controlling Iron‐Catalyzed Oxidation Reactions: From Non‐Selective Radical to Selective Non‐Radical Reactions

Among all elements widely distributed in nature, iron is probably the most versatile and important redox center for life and natural transformation processes. With respect to sustainable chemistry and following nature s principles, iron is the ideal metal for more environmentally benign catalyst generations in future because of availability, low toxicity, and price. Although iron plays wonderfully in nature, the difficulty to prevent non-selective radical reactions makes its usage on laboratory scale, but especially for industrial production, more difficult—except being directly used as Lewis acid. In order to apply iron catalysts for a specific reaction with sufficient selectivity and activity, so far the general concept is based on the design of a suitable ligand which is further on optimized. In recent years iron catalysis has become a “hot topic” and notable progress has been accomplished applying this approach. For instance, novel iron containing systems have been developed for hydroxylation, sulfide oxidation, cross-coupling reactions, heterolytic RO OH bond cleavage, hydroamination, allylic alkylation or amination, epoxidation, and alcohol oxidation. However, the intrinsic problem of nonselective radical side-reactions in the redox chemistry of iron has not been generally solved. This is especially evident for Fentonor Gif-type reactions, that is, Fe with H2O2, which constitute one of the most famous iron salt-dependent systems. Its high efficiency for oxidations of alkanes to ketones, benzene to phenol, and for environmental pollutant treatment has been extensively explored and has significant industrial potential. Notably, after more than a century of the first report, detailed investigations of the Fenton reaction still reveal two possible mechanisms, that is, the classical free radical mechanism and the oxygenated Fenton chemistry. Until today there is no concrete evidence to exclude each other completely, because both mechanisms have their own supporting evidence. In former reports, the importance of acids in Fenton chemistry has been studied, however a detailed study on the crucial function of pH variation on the catalyst activity and selectivity has been ignored. Earlier on, we discovered the pH dependency of the selectivity in osmium-catalyzed dihydroxylations of olefins applying oxygen as the terminal oxidant. Based on our recent experience in iron catalysis, we supposed that the proton concentration in the reaction system might be also important for Fe-dependant oxidation reactions. Indeed, studying the oxidation of benzyl alcohol to benzaldehyde as a typical model reaction, a remarkable influence of the pH is observed (Table 1).

Design of and mechanistic studies on a biomimetic iron-imidazole catalyst system for epoxidation of olefins with hydrogen peroxide.

Novel iron catalysts, both defined and in situ generated, for the epoxidation of aromatic and aliphatic olefins with hydrogen peroxide as terminal oxidant are described. Our catalyst approach is based on bio-inspired 1-aryl-substituted imidazoles in combination with cheap and abundant iron trichloride hexahydrate. We show that the free 2-position of the imidazole ligand motif plays a key role for catalytic activity, as substitution leads to a dramatic depletion of yield and conversion. X-ray studies, UV/Vis titrations, and NMR studies were carried out to clarify the mechanism.

Iron-Catalyzed Epoxidations with Hydrogen Peroxide

Review: Previous work with Ru: M. Beller and coworkers Chem. Eur. J. 2006, 12, 1855. Comment: To date, the combination of iron catalysts and hydrogen peroxide gave only moderate results in epoxidation reactions. Using the new method, chiral oxiranes can be obtained in high yields and enantioselectivities vary with steric bulk in the substrate. Small ortho substituents still give good results (only one example). Increasing steric bulk in the meta-position diminishes the selectivity and unsymmetrically substituted alkenes can be converted in moderate enantioselectivities. The stereochemistry of the products was proven by derivatization to known compounds. R R * * O R R FeCl3⋅H2O (5 mol%) H2pydic (5 mol%) diamine ligand (12 mol%) H2O2 (2 equiv)