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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - FunCatDesign (Fundamental Studies in Catalysis â€“ Reactivity Design with Experimental and Computational Tools)

Teaser

Summary

Catalysis is ubiquitous in modern academic and industrial chemistry as well as an integral and indispensable discipline that may contribute to solutions of current global challenges. While the field has grown significantly over the past few decades, with numerous transformations that were previously unthinkable now being possible, progress has frequently relied on serendipitous discoveries or elaborate screening efforts. Although it is indisputable that high-throughput screening is an extremely lucrative approach to generate a wealth of chemical information, the next frontier in the development of innovative approaches to meet the high demand for predictable, selective and sustainable processes will likely arise from fundamental insight. However, owing to the complexity of catalytic processes, the fleeting and frequently highly sensitive nature of intermediates and the associated challenges in gaining fundamental mechanistic understanding, insight-driven developments and especially reactivity designs have so far been extremely rare. The objective of this project is to capitalize on the tools of experimental and computational chemistry as a powerful means to gain access to the fundamental mechanistic details of key catalytic steps that are required to allow reactivity design. The specific subject for study will focus on the most significant challenges in nickel-catalysis - a highly promising area in the context of sustainability and synthetic diversity owing to nickelâ€™s relatively large abundance and also high reactivities towards relatively inert bonds. The proposed studies will address challenges in relation to Ni-catalyzed C-H activation, cross-coupling and trifluoromethylation reactions, as well as the exploration of novel avenues in catalysis at multinuclear sites.

Work performed

The project FunCatDesign is focused on the combined experimental and computational study of fundamental aspects of some of the most challenging aspects in homogeneous organometallic catalysis, specifically in relation to nickel and multinuclear catalysis. Already in the first reporting period we were fortunate to accumulate very promising results that led to well recognized publications in internationally high ranked, peer-reviewed journals. We were able to establish a dinuclear Pd(I) complex as a robust, air-stable catalyst with great promise in catalysis. For example, we succeeded in the rapid room temperature C-C-coupling of poly(pseudo)halogenated arenes under Pd(I)-catalysis in air. The chemoselective functionalization of poly(pseudo)halogenated arenes had been a challenge for decades, being substrate, conditions and ligand-dependant. Our report is not only highly practical, but presents the first general solution to this long-standing challenge and builds on several years of mechanistic studies within our group. Following up chemoselective cross-coupling reactions enabled by an air-stable dinuclear Pd(I) catalyst, we were able to broaden its scope further towards the chemoselective formation of C-S and C-Se bonds. Moreover, we developed a highly practical protocol for sequential C-C bond formations that allow for a triply selective, a priori predictable coupling at C-Br, C-OTf and C-Cl sites. Additionally, we were able to utilize the air-stable Pd(I) dimer catalyst in very rapid and air-tolerant polymerization reactions to form a variety of conjugated polymers, such as polyfluorenes and polycarbazoles, which are of relevance in optical applications. The unique reactivity of the dinuclear Pd(I) catalyst also allowed us to couple phosphorothioates with aryl iodides, whereas classical Pd(0)/Pd(II) catalysis was ineffective. This allowed us to develop a general and convenient synthesis to axially chiral S-aryl phosphorothioates, a class of compounds that is a reoccurring motif in important agrochemicals, pharmaceuticals, and chiral catalysts.
Another example of orthogonal selectivity is the chemoselective coupling of aryl iodides and bromides in the presence of arylgermanes using Pd(0)/Pd(II)-catalysis and the orthogonal coupling of the organogermanes using Pd-nanoparticle catalysis. This was developed as part of our continuing studies on transmetalation, where we explored the reactivity of arylgermanes. This also led to the development of an orthogonal coupling of arylgermanes with arenes under gold-catalysis, which is complementary to Pd-catalysis. In the context of transmetalation, we were able to uncover a fundamentally new mechanism for the transmetalation of trifluoromethyl trimethylsilane at Pd(II). In contrast to the corresponding stannane, which selectively transfers one of its alkyl moieties, the silane displays unique reactivity by releasing a difluorocarbene, which then reacts with Pd(II)-F to form Pd(II)-CF3. This showed that the widely proposed cyclic mechanism for transmetalation is not operative for R3SiCF3 and highlighted novel possibilities for transmetalating agents via the potential utilization of in situ formed difluorocarbene.
As part of our investigations in Ni-catalysis, specifically the challenge of introducing trifluoromethyl groups, we encountered a highly promising side-reactions, which ultimately allowed for the first efficient synthesis of trifluomethyl amines through a formal Umpolung strategy form the bench-stable precursor (Me4N)SFC3. The mildness and high functional group tolerance render this method highly attractive, allowing applications in the synthesis of N-CF3 analogues of various important pharmaceuticals.
In the context of our ongoing interest in the reactivity of dinuclear Ni(I) complexes, we uncovered a unique reactivity of Ni(I) metaloradical towards terminal olefins that allows for selective olefin isomerization. Our mechanistic studies indicated that the Ni(I) metaloradical can

Final results

The Schoenebeck group is interested in understanding reactivity and mechanisms, ultimately seeking the design of new catalysts and the development of novel applications in organic, bioorganic and materials chemistry. The current research emphasis of the project primarily involves studies of organometallic catalysis, applying experimental organic chemistry and state-of-the-art computational methods. This provides an impact for the use in further research and industrial investigations and for the understanding of catalytical processes worldwide.