Oscar Verho

Artificial Photosynthesis: From Nanosecond Electron Transfer to Catalytic Water Oxidation

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 Ce(IV), 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 Ce(IV), 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.

Chemoenzymatic dynamic kinetic resolution: a powerful tool for the preparation of enantiomerically pure alcohols and amines.

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.

Co-immobilization of an Enzyme and a Metal into the Compartments of Mesoporous Silica for Cooperative Tandem Catalysis : An Artificial Metalloenzyme

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 nanoparti ...

Highly dispersed palladium nanoparticles on mesocellular foam : an efficient and recyclable heterogeneous catalyst for alcohol oxidation

Highly dispersed palladium nanoparticles on mesocellular foam : an efficient and recyclable heterogeneous catalyst for alcohol oxidation

Nanopalladium on Amino-Functionalized Mesocellular Foam : An Efficient Catalyst for Suzuki Reactions and Transfer Hydrogenations

The applications of a heterogeneous Pd0‐AmP‐MCF nanoparticle catalyst in Suzuki cross‐coupling reactions and transfer hydrogenations of alkenes are described. The catalyst was highly efficient for both transformations, resulting in 1) coupling of a wide range of aryl halides with various boronic acids in high yields and 2) chemoselective reduction of a variety of alkenes with the use of 1‐methyl‐1,4‐cyclohexadiene as hydrogen donor. Moreover, the catalyst can be recycled several times without any significant decrease in activity or leaching of metal into solution, making the protocol economical and environmentally friendly. In the case of the Suzuki cross‐coupling, a 15‐fold increase in reaction rate was observed if the reaction was performed under microwave irradiation compared to conventional heating in an oil bath.

Highly Enantioselective Cascade Transformations by Merging Heterogeneous Transition Metal Catalysis with Asymmetric Aminocatalysis

The concept of combining heterogeneous transition metal and amine catalysis for enantioselective cascade reactions has not yet been realized. This is of great advantage since it would allow for the recycling of expensive and non-environmentally friendly transition metals. We disclose that the use of a heterogeneous Pd-catalyst in combination with a simple chiral amine co-catalyst allows for highly enantioselective cascade transformations. The preparative power of this process has been demonstrated in the context of asymmetric cascade Michael/carbocyclization transformations that delivers cyclopentenes bearing an all carbon quaternary stereocenters in high yields with up to 30:1 dr and 99% ee. Moreover, a variety of highly enantioselective cascade hetero-Michael/carbocyclizations were developed for the one-pot synthesis of valuable dihydrofurans and pyrrolidines (up to 98% ee) by using bench-stable heterogeneous Pd and chiral amines as co-catalysts.

Nanopalladium on Amino-Functionalized Mesocellular Foam as an Efficient and Recyclable Catalyst for the Selective Transfer Hydrogenation of Nitroarenes to Anilines

Herein, we report on the use of nanopalladium on amino‐functionalized siliceous mesocellular foam as an efficient heterogeneous catalyst for the transfer hydrogenation of nitroarenes to anilines. In all cases, the protocol proved to be highly selective and favored the formation of the desired aniline as the single product in high yields with short reaction times if naturally occurring and renewable γ‐terpinene was employed as the hydrogen donor. Furthermore, the catalyst displayed excellent recyclability over five cycles and negligible leaching of metal into solution, which makes it an eco‐friendly and economic catalyst to perform this transformation. The scalability of the protocol was demonstrated with the reduction of 4‐nitroanisole on a 2 g scale, in which p‐anisidine was isolated in 98 % yield.

Mild and Selective Hydrogenation of Nitro Compounds using Palladium Nanoparticles Supported on Amino‐Functionalized Mesocellular Foam

We present the utilization of a heterogeneous catalyst comprised of Pd nanoparticles supported on aminopropyl‐functionalized siliceous mesocellular foam (Pd0–AmP–MCF) for the selective hydrogenation of aromatic, aliphatic, and heterocyclic nitro compounds to the corresponding amines. In general, the catalytic protocol exclusively affords the desired amine products in excellent yields within short reaction times with the reactions performed at room temperature under ambient pressure of H2. Moreover, the reported Pd nanocatalyst displayed excellent structural integrity for this transformation as it could be recycled multiple times without any observable loss of activity or leaching of metal. In addition, the Pd nanocatalyst could be easily integrated into a continuous‐flow device and used for the hydrogenation of 4‐nitroanisole on a 2.5 g scale, where the product p‐anisidine was obtained in 95 % yield within 2 h with a Pd content of less than 1 ppm.

Chemoenzymatic Dynamic Kinetic Resolution of Primary Amines Using a Recyclable Palladium Nanoparticle Catalyst Together with Lipases

A catalyst consisting of palladium nanoparticles supported on amino-functionalized siliceous mesocellular foam (Pd-AmP-MCF) was used in chemoenzymatic dynamic kinetic resolution (DKR) to convert primary amines to amides in high yields and excellent ees. The efficiency of the nanocatalyst at temperatures below 70 °C enables reaction conditions that are more suitable for enzymes. In the present study, this is exemplified by subjecting 1-phenylethylamine (1a) and analogous benzylic amines to DKR reactions using two commercially available lipases, Novozyme-435 (Candida antartica Lipase B) and Amano Lipase PS-C1 (lipase from Burkholderia cepacia) as biocatalysts. The latter enzyme has not previously been used in the DKR of amines because of its low stability at temperatures over 60 °C. The viability of the heterogeneous Pd-AmP-MCF was further demonstrated in a recycling study, which shows that the catalyst can be reused up to five times.

CO Dissociation Mechanism in Racemization of Alcohols by a Cyclopentadienyl Ruthenium Dicarbonyl Catalyst

(13)CO exchange studies of racemization catalyst (η(5)-Ph(5)C(5))Ru(CO)(2)Cl and (η(5)-Ph(5)C(5))Ru(CO)(2)(Ot-Bu) by (13)C NMR spectroscopy are reported. CO exchange for the active catalyst form, (η(5)-Ph(5)C(5))Ru(CO)(2)(Ot-Bu) is approximately 20 times faster than that for the precatalyst (η(5)-Ph(5)C(5))Ru(CO)(2)Cl. An inhibition on the rate of racemization of (S)-1-phenylethanol was observed on addition of CO. These results support the hypothesis that CO dissociation is a key step in the racemization of sec-alcohols by (η(5)-Ph(5)C(5))Ru(CO)(2)Cl, as also predicted by DFT calculations.