PD Dr. Thomas Braun

Diplom, Chemistry, University of Würzburg, 1993
Stay at the University of Rennes, 1995
Dissertation, University of Würzburg, 1997
Research Fellow, University of York, 1997-2000
Habilitation, University of Bielefeld, 2003

Research Interests:

1. New Routes for the Metal Mediated Synthesis of Fluorinated Molecules by C-F Activation

One of our strategies for the synthesis of highly fluorinated aromatics, olefins or alkanes is initiated by the selective replacement of a fluorine atom by a transition metal (Chem. Commun. 2002, 2749). After these C-F activation reactions, the fluorinated organic ligands can be derivatised to yield fluoro-organic molecules, which are often not accessible by any other means. A particular striking example is the metal induced activation of a C-F bond in the presence of a C-Cl bond in 5-chloro-2,4,6-trifluoropyrimidine (Scheme 1; Dalton Trans. 2002, 297). On the basis of stoichiometric studies, Suzuki-Miyaura cross coupling reactions have been elaborated, which involve the transformation of the heterocycle into bisaryl derivatives using a nickel fluoro complex as catalyst (Scheme 1, Organometallics 2005, 4057). Notably, the analogous chloro complex is catalytically not active. In a different approach the direct conversion of hexafluoropropene into 1,1,1-trifluoropropane has been demonstrated. One crucial step is the selective C-F activation of the alkene on treatment with [RhH(PEt₃)₃] giving [Rh{CF=CF(CF₃)}(PEt₃)₃] (Scheme 1, Angew. Chem. Int. Ed. 2002, 41, 2745).

2. Synthesis and Reactivity of Metal Fluoro Complexes

Our studies focus on the synthesis and reactivity of new Pd, Pt and Rh complexes, which definitely hold promise for future applications in selective fluorination reactions (Angew. Chem. Int. Ed. 2002, 41, 2745, J. Fluorine Chem. 2004 , 125, 959, Dalton Trans 2005, 3331). Thus, treatment of [RhH(PEt₃)₃] with Et₃N · 3HF, which is a mild source of HF, leads to a metal fluoride, which can be converted into a bifluoride (Scheme 2).

3. Rhodium Complexes for Oxygenations

We are interested in the development of new routes for the oxygenation of inorganic and organic substrates. The project involves studies on the synthesis and reactivity of unusual rhodium peroxo and hydroperoxo complexes as well as related compounds with a RhOOSiMe₃ unit. Thus, treatment of [Rh(4-C₅NF₄)(CNtBu)(PEt₃)₃] with O₂ yields the peroxo compound trans-[Rh(O₂) (4-C₅NF ₄) (CNtBu)(PEt₃)₂] (Scheme 3). Further studies revealed, that reactions with RCl (R = Me₃Si, H) lead to silylperoxo and hydroperoxo complexes (Angew. Chem. Int. Ed. . 2005, 42, 6947). The latter give with more RCl the peroxides ROOR. The tetrafluoropyridyl group as p-acceptor ligand plays an important role to control these transformations. Starting from trans-[Rh(O₂)(4-C₅NF₄) (CNtBu)(PEt₃)₂] and MeOTf a neutral as well as cationic methylperoxo complexes are accessible. The molecular structure of [Rh(OOH)(4-C₅NF₄) (CNtBu)(PEt₃)₂] and of the cation in [Rh(OOMe) (4-C₅NF₄)(H₂O)(CNtBu)(PEt₃)₂] OTf are depicted in Scheme 3.

Selected publications:

• T. Braun, R.N. Perutz, M. I. Sladek, Chem. Commun. 2001, 2254-2255. "Catalytic C-F Activation of Polyfluorinated Pyridines by Nickel-Mediated Cross-Coupling Reactions "

• M. I. Sladek, T. Braun, B. Neumann, H.-G-Stammler, J. Chem. Soc., Dalton Trans. 2002, 297-299. "Aromatic C-F Activation at Ni in the Presence of a Carbon Chlorine Bond: The Nickel Mediated Synthesis of new Pyrimidines"

• T. Braun, B. Blöcker, V. Schorlemer, B. Neumann, A. Stammler, H.-G. Stammler, J. Chem. Soc., Dalton Trans. 2002, 2213-2218. "Synthesis, Structure and Reactivity of Fluorovinyl Nickel Complexes: Formation of a Phosphonioethenyl Complex"

• T. Braun, D. Noveski, B. Neumann, H.-G. Stammler, Angew. Chem. Int. Ed. 2002, 41, 2745-2747. "Conversion of Hexafluoropropene into 1,1,1-Trifluoropropane by C-F Activation at Rhodium"

• T. Braun, R. N. Perutz, Chem. Commun. 2002, 2749-2757. "Routes to Fluorinated Organic Derivatives by Nickel Mediated C-F Activation of Heteroaromatics"

• T. Braun, M. I. Sladek, B. Neumann, H.-G. Stammler, New. J. Chem. 2003, 27, 313-318. "3-Fluoropyridyl Nickel Complexes as Useful Tools for the Selective Synthesis of New 2,4,5,6-Tetrafluoropyridines: A Route Complementing the Established Methods to Access Fluorinated Pyridines"

• T. Braun, S. Rothfeld, V. Schorlemer, A. Stammler, H.-G. Stammler, Inorg. Chem. Commun. 2003, 6, 752-755. "Palladium Mediated Activation of a C-F Bond in Pentafluoropyridine: Synthesis, Structure and Reactivity of a Pyridyloxy Complex"

• D. Noveski, T. Braun, M. Schulte, B. Neumann, H.-G. Stammler, Dalton Trans. 2003, 4075-4083. "C-F activation and hydrodefluorination of fluorinated alkenes at rhodium"

• D. Noveski, T. Braun, S. Krückemeier, J. Fluorine Chem. 2004, 125, 959-966. "Synthesis and Reactivity of Rhodium Fluoro Complexes"

• N. A. Jasim, R. N. Perutz, A. C. Whitwood, T. Braun*, J. Izundu, B. Neumann, S. Rothfeld, H.-G. Stammler, Organometallics 2004, 23, 6140-6149. "Contrasting reactivity of fluoropyridines at palladium and platinum: C-F oxidative addition at palladium, alkyl transfer from phosphorus to platinum"

• D. Noveski, T. Braun, B. Neumann, A. Stammler, H.-G. Stammler, Dalton Trans. 2004, 4106-4119. "C-F or C-H bond activation and C-C coupling reactions of fluorinated pyridines at rhodium: synthesis, structure and reactivity of a variety of tetrafluoropyridyl complexes"

• T. Braun, A. Steffen, M. I. Sladek, B. Neumann, H.-G. Stammler, Organometallics 2005, 23, in the press. "Catalytic C-C Coupling Reactions at Ni by C-F Activation of a Pyrimidine in the Presence of a C-Cl Bond: The Crucial Role of Highly Reactive Fluoro Complexes"

• T. Braun, Angew. Chem. Int. Ed. 2005, 44, 5012-5014. "Oxidative Addition of NH₃ to a Transition Metal: a Key-Step for the Metal-Mediated Derivatization of Ammonia?"

• T. Braun, A. Steffen, V. Schorlemer, B. Neumann, H.-G. Stammler, Dalton Trans. 2005, 3331-3336. "Routes to Unique Palladium A-Frame Complexes with a Bridging Fluoro-Ligand"

• M. Ahijado, T. Braun, D. Noveski, N. Kocher, B. Neumann, D. Stalke, H.-G. Stammler, Angew. Chem. Int. Ed. 2005, 42, 6789. "Rhodium- mediated Formation of Peroxides from Dioxygen: Isolation of Hydroperoxo, Silylperoxo and Methylperoxo Intermediates"