Chemoenzymatic dynamic kinetic resolution (DKR) constitutes a convenient and efficient method to access enantiomerically pure alcohol and amine derivatives. This Perspective highlights the work carried out within this field during the past two decades and pinpoints important avenues for future research. First, the Perspective will summarize the more developed area of alcohol DKR, by delineating the way from the earliest proof-of-concept protocols to the current state-of-the-art systems that allows for the highly efficient and selective preparation of a wide range of enantiomerically pure alcohol derivatives. Thereafter, the Perspective will focus on the more challenging DKR of amines, by presenting the currently available homogeneous and heterogeneous methods and their respective limitations. In these two parts, significant attention will be dedicated to the design of efficient racemization methods as an important means of developing milder DKR protocols. In the final part of the Perspective, a brief overview of the research that has been devoted toward improving enzymes as biocatalysts is presented.
12 November 2013 in Angewandte Chemie International Edition
Surpassing nature: A hybrid catalyst in which Candida antarctica lipase B and a nanopalladium species are co-immobilized into the compartments of mesoporous silica is presented. The metal nanoparticles and the enzyme are in close proximity to one another in the cavities of the support. The catalyst mimics a metalloenzyme and was used for dynamic kinetic resolution of a primary amine in high yield and excellent enantioselectivity.
Human society faces a fundamental challenge as energy consumption is projected to increase due to population and economic growth as fossil fuel resources decrease. Therefore the transition to alternative and sustainable energy sources is of the utmost importance. The conversion of solar energy into chemical energy, by splitting H2O to generate molecular O2 and H2, could contribute to solving the global energy problem. Developing such a system will require the combination of several complicated processes, such as light-harvesting, charge separation, electron transfer, H2O oxidation, and reduction of the generated protons. The primary processes of charge separation and catalysis, which occur in the natural photosynthetic machinery, provide us with an excellent blueprint for the design of such systems. This Account describes our efforts to construct supramolecular assemblies capable of carrying out photoinduced electron transfer and to develop artificial water oxidation catalysts (WOCs). Early work in our group focused on linking a ruthenium chromophore to a manganese-based oxidation catalyst. When we incorporated a tyrosine unit into these supramolecular assemblies, we could observe fast intramolecular electron transfer from the manganese centers, via the tyrosine moiety, to the photooxidized ruthenium center, which clearly resembles the processes occurring in the natural system. Although we demonstrated multi-electron transfer in our artificial systems, the bottleneck proved to be the stability of the WOCs. Researchers have developed a number of WOCs, but the majority can only catalyze H2O oxidation in the presence of strong oxidants such as CeIV, which is difficult to generate photochemically. By contrast, illumination of ruthenium(II) photosensitizers in the presence of a sacrificial acceptor generates [Ru(bpy)3]3+-type oxidants. Their oxidation potentials are significantly lower than that of CeIV, but our group recently showed that incorporating negatively charged groups into the ligand backbone could decrease the oxidation potential of the catalysts and, at the same time, decrease the potential for H2O oxidation. This permitted us to develop both ruthenium- and manganese-based WOCs that can operate under neutral conditions, driven by the mild oxidant [Ru(bpy)3]3+. Many hurdles to the development of viable systems for the production of solar fuels remain. However, the combination of important features from the natural photosynthetic machinery and novel artificial components adds insights into the complicated catalytic processes that are involved in splitting H2O.
21 July 2017 in Journal of the American Chemical Society
The development of new antimalarial therapeutics is necessary to address the increasing resistance to current drugs. Bicyclic azetidines targeting Plasmodium falciparum phenylalanyl-tRNA synthetase comprise one promising new class of antimalarials, especially due to their activities against three stages of the parasite’s life cycle, but a lengthy synthetic route to these compounds may affect the feasibility of delivering new therapeutic agents within the cost constraints of antimalarial drugs. Here, we report an efficient synthesis of antimalarial compound BRD3914 (EC50 = 15 nM) that hinges on a Pd-catalyzed, directed C(sp3)–H arylation of azetidines at the C3 position. This newly developed protocol exhibits a broad substrate scope and provides access to valuable, stereochemically defined building blocks. BRD3914 was evaluated in P. falciparum-infected mice, providing a cure after four oral doses.
26 February 2015 in Angewandte Chemie International Edition
Herein, a practical and mild method for the deoxygenation of a wide range of benzylic aldehydes and ketones is described, which utilizes heterogeneous Pd/C as the catalyst together with the green hydride source, polymethylhydrosiloxane. The developed catalytic protocol is scalable and robust, as exemplified by the deoxygenation of ethyl vanillin, which was performed on a 30 mmol scale in an open‐to‐air setup using only 0.085 mol % Pd/C catalyst to furnish the corresponding deoxygenated product in 93 % yield within 3 hours at room temperature. Furthermore, the Pd/C catalyst was shown to be recyclable up to 6 times without any observable decrease in efficiency and it exhibited low metal leaching under the reaction conditions.