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Fundamental Studies in Catalysis – Reactivity Design with Experimental and Computational Tools

Periodic Reporting for period 4 - FunCatDesign (Fundamental Studies in Catalysis – Reactivity Design with Experimental and Computational Tools)

Reporting period: 2019-11-01 to 2020-04-30

The project FunCatDesign involved the combined experimental and computational studies of fundamental aspects of some of the most challenging aspects in homogeneous organometallic catalysis as well as the application of the gained insights to overcome synthetic challenges.
The group demonstrated a metalloradical induced olefin isomerization, establishing a fundamentally novel mechanism (1,3 H-atom shift) for olefin migration that is triggered by a non-precious nickel radical and gives rise to exclusive E-selectivity. The mechanism was elucidated and the synthetic scope demonstrated. (Science 2019, 363, 391). The work followed fundamental studies that questioned the role of Ni(I) in cross-coupling reactions, both as monomer (ACS Catalysis 2017, 7, 2126) and dimer (Angew. Chem. Int. Ed. 2017, 56, 13431). The group presented unambiguous mechanistic support of the feasibility of dinuclear metal catalysis, and also demonstrated unique reactivity features associated with such multinuclear metal catalysts. For example, the first direct catalytic phosphorothioation of aryl iodides under dinuclear Pd(I) catalysis was developed (Angew. Chem. Int. Ed. 2019, 58, 11395). This reaction does not occur under conventional Pd(0)/Pd(II) catalysis owing to a lack of driving force to exchange iodide for the weakly nucleophilic phosphorothioate at the Pd(II) intermediate and a prohibitively high reductive elimination barrier. In stark contrast, the nucleophile exchange under Pd(I) dimer catalysis happens at oxidation state (I) which is associated with favorable driving force. Moreover, using the same air-stable iodide-bridged Pd(I) dimer, arylations as well as alkylations can be done in a substrate-independent and fully selective manner within seconds even in the presence of oxygen. Historically in Pd(0) catalysis, site selectivities of poly(pseudo)halogenated arenes have been unpredictable, as they were dependent on the catalyst, medium, additive as well as the steric and electronic effects of the substrate. We demonstrated that the Pd(I) dimer enables fully selective and sequential functionalizations of C-Br, C-Cl and C-OTf (or alternatively also C-OFs) in a substrate-independent manner [see: (i) Angew. Chem. Int. Ed. 2020, 59, 2115; (ii) Angew. Chem. Int. Ed. 2018, 57, 12573; (iii) Angew. Chem. Int. Ed. 2017, 56, 7078; (iv) Angew. Chem. Int. Ed. 2017, 56,1581]. This exquisite site-selectivity and speed is even retained when the C-Br site is challenged with an adamantyl group in ortho position (Angew. Chem. Int. Ed. 2020, 59, 7721) or C-S couplings conducted (Angew. Chem. Int. Ed. 2018, 57, 12425). The concept was also demonstrated to be effective to make polymers in less than a minute at room tempearture, which usually require hours to days (Angew. Chem. Int. Ed. 2019, 58, 10179). The first decarbonylation of acid fluorides was also developed, which was used to access aryl trifluoromethyl arenes (Angew. Chem. Int. Ed. 2018, 57, 4073). A key challenge in this context is that reactive CF3 anions usually deactivate Pd(II) intermediates. Our approach was to harness acid fluorides which would give a Pd(II)-F intermediate, which is the only Pd(II)-X intermediate that can be transmetalated directly with TMSCF3 without the need for additives. We also unraveled the mechanism of transmetalation of Pd(II)-F complexes with R3Si-CF3. We identified that a difluorocarbene is liberated in the process which subsequently inserts into the Pd(II)-F bond (Angew. Chem. Int. Ed. 2018, 57, 15081). These discoveries were made through state of the art Born Oppenheimer molecular dynamics studies as well as experimental verifications. Within the realm of organofluorine chemistry and organic methodology, we developed the first synthetic access to N-trifluoromethyl hydrazines, N-CF3 indoles and their derivatives (S. Bouayad-Gervais, T. Scattolin, F. Schoenebeck, Angew. Chem. Int. Ed 2020, DOI: 10.1002/anie.202004321) as well as the first efficient access to trifluoromethyl amines (N-CF3) via a formal umpolung strategy from the bench-stable precursor (Me4N)SCF3 (T. Scattolin, K. Deckers, F. Schoenebeck, Angew. Chem. Int. Ed. 2017, 56, 221).
Following various insight on the fundamental aspects of the transmetalation step in metal-catalyzed cross coupling reactions, we rationalized why organogermanes are rather sluggish in standard Pd(0)/Pd(II) homogeneous catalysis. We could show that by contrast, with nanoparticles as catalyst, the organogermanes are the most reactive coupling partners and outcompete all established alternatives. (Angew. Chem. Int. Ed. 2019, 58, 7788). Moreover, we showed that organogermanes are also highly reactive in gold-catalyzed C-H functionalizations (ACS Catalysis 2019, 9231). We also showcased the possibility for modular & selective arylation of aryl germanes (C-GeEt3) over C-Bpin, C-SiR3 and halogens enabled by light-activated gold-catalysis (Angew. Chem. Int. Ed 2020, DOI: 10.1002/anie.202005066).
Please see above section under “Work performed” for a concise summary. These studies and findings were all beyond state of the art. Patents were filed on the basis of these results, and a new and innovative research program was stimulated. The co-workers were trained in synthetic organic chemistry, organofluorine chemistry, homogeneous metal catalysis as well as state-of-the-art computational and mechanistic chemistry.