Thomas Braun

Name:

Organization: Humboldt-Universit?t zu Berlin , Germany

Department: Department of Chemistry

Title: (PhD)

Chemicals
References

Et3GeH versus Et3SiH: controlling reaction pathways in catalytic C–F bond activations at a nanoscopic aluminum chlorofluoride

Co-reporter:G. Meißner;D. Dirican;C. Jäger;T. Braun;E. Kemnitz

Catalysis Science & Technology (2011-Present) 2017 vol. 7(Issue 15) pp:3348-3354

Publication Date(Web):2017/07/31

DOI:10.1039/C7CY00845G

Catalytic C–F bond activation reactions of mono- and polyfluoroalkanes at Lewis acidic amorphous aluminum chlorofluoride (ACF) are presented. The hydrogen sources Et3GeH or Et3SiH control the selectivity of the conversions. The immobilization of Et3GeH at ACF resulted in catalytic dehydrohalogenation reactions to yield olefins under very mild conditions. In contrast, if Et3SiH is immobilized at ACF, C–C coupling occured and the formation of Friedel–Crafts products was observed. MAS NMR spectroscopic studies revealed information about the surface binding of the substrates.

Synthesis and reactivity of a cationic palladium complex as possible intermediate in a Suzuki-Miyaura cross-coupling reaction

Co-reporter:Johannes Kohlmann, Thomas Braun

Journal of Fluorine Chemistry 2017 Volume 203(Volume 203) pp:

Publication Date(Web):1 November 2017

DOI:10.1016/j.jfluchem.2017.08.003

•A fluorinated 4-aryl phenylalanine derivate containing an SF5 group was prepared as cross-coupling product.•The high reactivity of an intermediary Pd-F complex towards a boronic ester is demonstrated.•A cationic palladium species is obtained as conceivable intermediate in the transmetallation step.A stoichiometric reaction of a palladium fluorido complex with an aromatic boronic ester yielded a fluorinated 4-aryl phenylalanine derivative (aryl = 4-C6H4SF5) as Suzuki-Miyaura cross-coupling product. Due to the high reactivity of the metal complex, the reaction proceeded smoothly at ambient temperature. Low-temperature NMR investigations revealed a possible role of trans‐[Pd{BF(4-C6H4SF5)(pin)}(PheEt)(PiPr3)2] (5, PheEt = bound phenylalanine derivative = 4-C6H4CH2C{NHC(O)CH3}(CO2Et)2, pin = pinacolato = O2C2Me4) as a potential intermediate of the transmetallation step. Compound 5 resulted from fluoride transfer from palladium to boron. The similar complex trans-[Pd(BF4)(PheEt)(PiPr3)2] (7) was generated on treatment of the fluorido complex with NaBF4. Complex 7 is not stable at room temperature. Degradation gave the phosphonium salt [PiPr3PheEt][BF4] (4). Interestingly, the same compound was also found in the initially mentioned cross-coupling reaction as minor product.Download high-res image (105KB)Download full-size image

Selective reduction of a CCl bond in halomethanes with Et3GeH at nanoscopic Lewis acidic Aluminium fluoride

Co-reporter:Gisa Meißner, Michael Feist, Thomas Braun, Erhard Kemnitz

Journal of Organometallic Chemistry 2017 Volume 847(Volume 847) pp:

Publication Date(Web):1 October 2017

DOI:10.1016/j.jorganchem.2017.04.030

•Selective monohydrodechlorination reactions of polychlorinated alkanes were achieved.•Hydrodechlorination of CHCl2F and CHClF2 occurs with selectivity towards C−Cl activation.•Heterogeneous catalytic reactions were developed.•PTA studies give information about the interaction between the substrates at ACF · Et3GeH.The selective activation of CCl bonds of hydrochlorofluoromethanes and chloromethanes at moderate reaction conditions using ACF in a combination with Et3GeH is presented. The reactions of the chloromethanes (CH3Cl, CH2Cl2, CHCl3 and CCl4) in the presence of Et3GeH and ACF as catalyst led to the activation of only one CCl bond resulting in the hydrodechlorination. Friedel-Crafts reactions with benzene as solvent are suppressed by Et3GeH. A selective hydrodechlorination of hydrochlorofluoromethanes was achieved, because a transformation of a CF bond into a CH bond by the combination of ACF with Et3GeH did not occur. Supporting PulseTA® experiments illustrated the interaction between the solid catalyst and Et3GeH, the solvent benzene or CH2Cl2.Download high-res image (122KB)Download full-size image

Completing the Heterocubane Family [Cp*AlE]4 (E = O, S, Se, and Te) by Selective Oxygenation and Sulfuration of [Cp*Al]4: Density Functional Theory Calculations of [Cp*AlE]4 and Reactivity of [Cp*AlO]4 toward Hydrolysis

Co-reporter:Adrian C. Stelzer, Peter Hrobárik, Thomas Braun, Martin Kaupp, and Beatrice Braun-Cula

Inorganic Chemistry 2016 Volume 55(Issue 10) pp:4915

Publication Date(Web):April 29, 2016

DOI:10.1021/acs.inorgchem.6b00462

The subvalent aluminum compound [Cp*Al]4 (1) reacts with dioxygen, N2O, or sulfur to yield the heterocubane complexes [Cp*AlX]4 [X = O (2) and S (3)]. Treatment of [Cp*AlO]4 (2) with (tBuO)3SiOH gave [(tBuO)3SiOAlO]4 (6) and Cp*H. The structures and spectroscopic data of the Al clusters are supported by density functional theory (DFT) calculations, which also demonstrate the importance of noncovalent interactions (NCI) in oligomeric Al(I) complexes as well as in [Cp*AlS]4 and the heavier homologues of Se and Te. The computed 27Al NMR shifts indicate a deshielding at the Al centers with increasing electronegativity of the chalcogen atom as well as significant spin–orbit shielding effects within the heavier heterocubane [Al4E4] cores. Further hydrolysis of 6 with an additional amount of silanol in the presence of water resulted in the formation of [Al4(OH)6(OH2)2(OSiOtBu3)6] (7), which shows a structural motif found in boehmite and diaspore.

Functionalization of Fluorinated Molecules by Transition-Metal-Mediated C–F Bond Activation To Access Fluorinated Building Blocks

Co-reporter:Theresia Ahrens, Johannes Kohlmann, Mike Ahrens, and Thomas Braun

Chemical Reviews 2015 Volume 115(Issue 2) pp:931

Publication Date(Web):October 27, 2014

DOI:10.1021/cr500257c

Synthesis of an Iridium Peroxido Complex and Its Reactivity Towards Brnsted Acids

Co-reporter:Hanna Baumgarth;Beatrice Braun;Reik Laubenstein ;Roy Herrmann

European Journal of Inorganic Chemistry 2015 Volume 2015( Issue 19) pp:3157-3168

Publication Date(Web):

DOI:10.1002/ejic.201500384

Abstract

Treatment of the iridium(I) complex trans-[Ir(4-C₅F₄N)(CNtBu)(PiPr₃)₂] (2) with O₂ or H₂O₂ gave the iridium(III) peroxido complex trans-[Ir(4-C₅F₄N)(O₂)(CNtBu)(PiPr₃)₂] (3a). The peroxido complex 3a reacts with Brønsted acids to give the iridium(III) complexes cis-[Ir(4-C₅F₄N)(X)₂(CNtBu)(L)(PiPr₃)] (4: X = Cl, L = PiPr₃, 5: X = CF₃COO, L = H₂O) and hydrogen peroxide. In the presence of formic acid, 3a yields the carbonato complex trans-[Ir{κ²-OC(O)O}(4-C₅F₄N)(CNtBu)(PiPr₃)₂] (6) as the main product. Reactions between formic acid or dihydrogen and the iridium(I) complex 2 give the isomeric dihydrido complexes trans,trans-[Ir(4-C₅F₄N)(H)₂(CNtBu)(PiPr₃)₂] (7a) and cis,trans-[Ir(4-C₅F₄N)(H)₂(CNtBu)(PiPr₃)₂] (9a), respectively.

Activation of Si–Si and Si–H bonds at Pt: a catalytic hydrogenolysis of silicon–silicon bonds

Co-reporter:Jan Voigt, Maren A. Chilleck and Thomas Braun

Dalton Transactions 2013 vol. 42(Issue 11) pp:4052-4058

Publication Date(Web):23 Jan 2013

DOI:10.1039/C3DT32530J

The activation of Ph2HSiSiHPh2 and Me3SiSiMe3 at [Pt(PEt3)3] (1) yielded the products of oxidative addition. The formation of [Pt(SiHPh2)2(PEt3)2] (2) as a mixture of the cis and trans isomers appears to proceed quantitatively, whereas a conversion to give cis-[Pt(SiMe3)2(PEt3)2] (3) was not complete. Treatment of 1 with one equivalent of H2SiPh2 led to cis-and trans-[Pt(H)(SiHPh2)(PEt3)2] (cis-4, trans-4) together with the dinuclear complex [(Et3P)2(H)Pt(μ-SiPh2)(μ–η2-HSiPh2)Pt(PEt3)] (5). In contrast, HSiMe3 reacts with [Pt(PEt3)3] to yield cis-[Pt(H)(SiMe3)(PEt3)2] (7) exclusively. Catalytic reactions of dihydrogen with the disilanes Ph2HSiSiHPh2 or Me3SiSiMe3 in the presence of catalytic amounts of [Pt(PEt3)3] (1) led to the products of hydrogenolysis, H2SiPh2 and HSiMe3. The conversion of Me3SiSiMe3 is much slower and needs higher temperature to proceed.

Synthesis of Fluorinated Building Blocks by Transition-Metal-Mediated Hydrodefluorination Reactions

Co-reporter:Dr. Moritz F. Kuehnel;Dr. Dieter Lentz;Dr. Thomas Braun

Angewandte Chemie International Edition 2013 Volume 52( Issue 12) pp:3328-3348

Publication Date(Web):

DOI:10.1002/anie.201205260

Abstract

The activation and functionalization of carbon–fluorine bonds can be considered as a major challenge in organometallic chemistry. The growing demand for means to introduce fluorine into new materials or into biologically active molecules has inspired the development of diverse synthetic strategies. Hydrodefluorination is regarded as a promising approach to access partially fluorinated building blocks from readily available perfluorinated bulk chemicals. We provide an overview of transition-metal-based complexes and catalysts that were developed to mediate hydrodefluorination reactions. Special emphasis will be placed on discussing the underlying mechanistic patterns and their impact on scope and selectivity. In addition, future requirements for further developing this field will be highlighted.

Et3GeH versus Et3SiH: controlling reaction pathways in catalytic C–F bond activations at a nanoscopic aluminum chlorofluoride

Co-reporter:G. Meißner, D. Dirican, C. Jäger, T. Braun and E. Kemnitz

Catalysis Science & Technology (2011-Present) 2017 - vol. 7(Issue 15) pp:NaN3354-3354

Publication Date(Web):2017/07/18

DOI:10.1039/C7CY00845G

Activation of Si–Si and Si–H bonds at Pt: a catalytic hydrogenolysis of silicon–silicon bonds

Co-reporter:Jan Voigt, Maren A. Chilleck and Thomas Braun

Dalton Transactions 2013 - vol. 42(Issue 11) pp:NaN4058-4058

Publication Date(Web):2013/01/23

DOI:10.1039/C3DT32530J