Organische Chemie II - Physikalisch-Organische Chemie

Cover Picture in Chem. Phys. A.

Thermal Decomposition of 2- and 4-Iodobenzyl Iodide Yields Fulvenallene and Ethynylcyclopentadienes: A Joint Threshold Photoelectron and Matrix Isolation Spectroscopic Study
M. Saraswat, A. Portela-Gonzalez, G. Karir, E. Mendez-Vega, W. Sander*, P. Hemberger*, J. Phys. Chem. A. 127 (2023), 8574-8583

The potential energy surface of C7H6 has received a lot of attention from both the organic and astrochemistry communities, and it is very rich containing open-shell reactive species as well as stable compounds such as fulvenallene and the three isomeric ethynylcyclopentadienes, as depicted on the lake. The thermal interconversion between these species can alternatively be explored via gas-phase synthesis from the higher-energy isomer, a,2-didehydrotoluene diradical. In this work, we produced a,2- and a,4-didehydrotoluene diradical via pyrolysis of the corresponding diiodo precursors, but a,4-didehydrotoluene diradical readily isomerizes to a,2-didehydrotoluene diradical through two consecutive H-shifts, which subsequently undergoes ring contraction and lands on the lake, containing the lowest-energy closed-shell isomers. The cover art was created by Marta Portela González following a sketch by Adrián Portela González.

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Cover Picture in Acc. Chem. Res.

Solvation Effects on Quantum Tunneling Reactions
T. Schleif, M. Prado Merini, S. Henkel, W. Sander, Acc. Chem. Res. 55 (2022), 2180-2190

A decisive factor for obtaining high yields and selectivities in organic synthesis is the choice of the proper solvent. Solvent selection is often guided by the intuitive understanding of transition state-solvent interactions. However, quantum mechanical tunneling can significantly contribute to chemical reactions, circumventing the transition state and thus depriving chemists of their intuitive handle on the reaction kinetics. In this Account, we aim to provide rationales for the effects of solvation on tunneling reactions derived from experiments performed in cryogenic matrices. The tunneling reactions analyzed here cover a broad range of prototypical organic transformations that are subject to strong solvation effects. Examples are the hydrogen tunneling probability for the cis-trans isomerization of formic acid which is strongly reduced upon formation of hydrogen-bonded complexes and the [1,2]H-shift in methylhydroxycarbene where a change in product selectivity is predicted upon interaction with hydrogen bond acceptors.

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JOC Special Issue

Solvation Effects in Organic Chemistry
C. J. Burrows, J. B. Harper, W. Sander, D. J. Tantillo, J. Org. Chem. 87 (2021), 1599-1601

The importance of solvents and solvation processes has been recognized from the early days of organic chemistry, and often the selection of the “right” solvent determines the success or failure of a synthesis. And yet, in many cases solvent selection is based on empirical observations rather than on a rational physical basis. Small amounts of cosolvents or traces of water or other additives can dramatically affect reaction rates, yields, and selectivity, but predicting these effects even qualitatively is difficult. Many solvent mixtures show nonlinear behavior, and their properties cannot be directly deduced from those of the pure solvents. This statement is even more true for unconventional solvents, such as ionic liquids, which are frequently used as mixtures with other solvents..

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News article in Chemistry world

Heavy-atom quantum tunnelling catalysed with Lewis acids

Scientists in Germany have catalysed a heavy-atom quantum tunnelling reaction by doping the reaction matrix with Lewis acids. Boron trifluoride increased the rate to such an extent that only the products could be detected in the reaction mix. ‘very deep property of matter related to the Heisenberg uncertainty principle,’ explains Wolfram Sander from Ruhr University Bochum in Germany, who led the study.

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ACS Spotlight

Controlling Reactivity-Real-Space Imaging of a Surface Metal Carbene
J. Mieres-Perez, K. Lucht, I. Trosien, W. Sander, E. Sanchez-Garcia, K. Morgenstern, J. Am. Chem. Soc. 143 (2021), 4653-4660

Metal carbenes are key intermediates in a plethora of homogeneous and heterogeneous catalytic processes. However, despite their importance to heterogeneous catalysis, the influence of surface attachment on carbene reactivity has not yet been explored. Here, we reveal the interactions of fluorenylidene (FY), an archetypical arom. carbene of extreme reactivity, with a Ag(111) surface. For the first time, the interaction of a highly reactive carbene with a metal surface could be studied by scanning tunneling microscopy (STM). FY chemisorbs on Ag(111) with an estd. desorption energy of 3 eV, forming a surface bound silver-carbene complex. The surface interaction leads to a switching of the electronic ground state of FY from triplet to singlet, and to controlled chem. reactivity. This atomistic understanding of the interplay between carbenes and metal surfaces opens the way for the development of novel classes of catalytic systems based on surface metal carbenes.

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Molecular Magnets

Molecular Magnets: The Synthesis and Characterization of High-Spin Nitrenes
D. V. Chapyshev, E. Mendez-Vega, W. Sander, Chem. Eur. J. 27 (2021), 1258-1269.

Among all C-, N-, and O-centered polyradicals, high-spin nitrenes possess the largest magnetic anisotropy and are of considerable interest as multi-level molecular spin systems for exploration of organic molecular magnetism and quantum information processing. Although the first representatives of quintet and septet nitrenes were obtained almost 50 years ago, the experimental and theoretical studies of these highly reactive species became possible only recently, owing to new achievements in molecular spectroscopy and computational chemistry. Meanwhile, dozens of various quintet dinitrenes and septet trinitrenes were successfully characterized by IR, UV/Vis, and EPR spectroscopy, thus providing important information about the electronic structure, magnetic properties and reactivity of these compounds.

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HOT PAPER in Angewandte

The Mystery of the Benzene-Oxide/Oxepin Equilibrium-Heavy-Atom Tunneling Reversed by Solvent Interactions
T. Schleif, M. Prado Merini, W. Sander, Angew. Chem. Int. Ed. 59 (2020), 20318-20322.
(Cover picture by Carla Prado Merini)

The equilibrium between benzene oxide (1) and oxepin (2) is of large importance for understanding the degradation of benzene in biological systems and in the troposphere. Our studies reveal that at cryogenic temperatures, this equilibration is governed by rare heavy-atom tunneling. In solid argon at 3 K, 1 rearranges to 2 via tunneling with a rate constant of approximately 5.3×10-5 s-1. Thus, in a nonpolar environment, 2 is slightly more stable than 1, in agreement with calculations at the CCSD(T) level of theory. However, if the argon is doped with 1 % of H₂O or CF₃I as typical hydrogen or halogen bond donors, respectively, weak complexes of 1 and 2 are formed, and now 2 is tunneling back to form 1. Thus, by forming non-covalent complexes, 1 becomes slightly more stable than 2 and the direction of the heavy-atom tunneling is reversed.

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New Cover Picture in Chem. Eur. J.

Heavy-Atom Tunneling in Semibullvalenes: How Driving Force, Substituents, and Environment Influence the Tunneling Rates
T. Schleif, J. Tatchen, J. f. Rowen, F. Beyer, E. Sanchez-Garcia, W. Sander, Chem. Eur. J. 26 (2020), 10366.

Invited for the cover of this issue are the groups of Elsa Sanchez-Garcia and Wolfram Sander at the Universität Duisburg-Essen and the Ruhr-Universität Bochum. The image depicts the ideas skillfully visualized by Markus Henkel on the shift in equilibrium induced by isotopic labelling.

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New JOC Paper

Conformer-Specific Heavy-Atom Tunneling in the Rearrangement of Benzazirines to Ketenimines
T. Schleif, J. Mieres-Perez, St. Henkel, E. Mendez-Vega, H. Inui, R. J. McMahon, W. Sander, J. Org. Chem. 84 (2019), 16013-16018.

5-Methoxy-2H-benzazirine was prepared via irradiation of the corresponding phenyl azide, isolated in an argon matrix at cryogenic temperatures. It undergoes ring expansion to the corresponding ketenimine in the dark at T < 30 K despite a calculated activation barrier of 4.9 kcal mol–1 [B3LYP/6-311++G(d,p)]. Since this rearrangement proceeds with a rate constant in the order of 10–4 s–1, exhibiting only a shallow temperature dependence, the results are interpreted in terms of heavy-atom tunneling. Of the four isomeric benzazirines resulting from the initial photolysis, only one can be observed to rearrange; this conformer specificity is explained by the other potentially observable rearrangements being either too fast or too slow to be detected due to the differences in heights and widths of their respective activation barriers.

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New JOC Paper

How Protic Solvents Determine the Reaction Mechanisms of Diphenylcarbene in Solution
J. Knorr, P. Sokkar, P. Costa, W. Sander, E. Sanchez-Garcia, P. Nuernberger, J. Org. Chem. 84 (2019), 11450-11457.

We investigate the effects of small admixts. of protic solvent mols., such as water and alcs., on the ultrafast dynamics of diphenylcarbene in acetonitrile at room temp. Broadband transient absorption measurements and quantum mechanics?/mol. mechanics mol. dynamics simulations allow elucidating the dominant reaction mechanism of an intermediate hydrogen-bonded complex between singlet diphenylcarbene and a protic solvent mol., thus competing with intersystem crossing. Anal. of the data indicates that complex formation is a diffusion-?controlled process with orientational requirements.

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New RUB Press Release

Molekül verändert seine magnetischen Eigenschaften durch Licht

Grünes Licht wirkt auf diese chemische Verbindung ganz anders als blaues. Weil es sich so gut steuern lässt, ist das Molekül auch für die Computerindustrie interessant.

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New VIP in Angew. Chem.

Conformational Spin Switching and Spin-Selective Hydrogenation of a Magnetically Bistable Carbene
I. Trosien, E. Medez-Vega, T. Thomanek, W. Sander, Angew. Chem. Int. Ed. 58 (2019), 15162.

The control of the spin states of molecules opens the path to tuning selectivity in chemical reactions and to developing novel magnetically switchable materials. 3-Methoxy-9-fluorenylidene is a carbene that is generated in cryogenic matrices both in its lowest energy singlet and triplet states, and the ratio of these states can be shifted by selective irradiation. The interconversion of the nearly degenerate spin states is induced by a conformational change of the methoxy group: switching the methoxy group into the “up” position results in the singlet state and switching into the “down” position in the triplet state. The spin control via a remote functional group makes this carbene unique for the study of spin-specific reactions, which is demonstrated for the hydrogenation reaction. Spin switching by switching the conformation of a remote functional group is a novel phenomenon with potential applications in the design of functional materials.

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New RUB Press Release

Next step in producing magnetic organic molecules

Unlike metallic magnets, magnets consisting of organic molecules could be lightweight, transparent, flexible or liquid. Normally, however, they are unstable.

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VIP in Angewandte Chemie

Persistent Organic High-Spin Trinitrenes
E. Medez-Vega, J. Mieres-Perez, S. V. Chapyshev, W. Sander, Angew. Chem. Int. Ed. 58 (2019), 12720.

The septet ground state trinitrenes 1,3,5-trichloro-2,4,6-trinitrenobenzene and 1,3,5-tribromo-2,4,6-trinitrenobenzene were isolated in inert (Ar, Ne, and Xe) as well as reactive matrices (H₂, O₂, and H₂O) at cryogenic temperatures. These trinitrenes were obtained in high yields by UV photolysis of the corresponding triazides and characterized by IR and UV–vis spectroscopy. The trinitrenes, despite bearing six unpaired electrons, are remarkably unreactive towards molecular oxygen and hydrogen and are persistent in water ice up to 160 K where the water matrix starts to sublime off.

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Cover Picture

Solvent-Enhanced Conformational Flexibility of Cyclic Tetrapeptides
N. Berger, L. J. B. Wollny, P. Sokkar, S. Mittal, J. Mieres-Perez, R. Stoll, W. Sander. E. Sanchez-Garcia, ChemPhysChem 20 (2019), 1663.

Solvent and temperature can affect the structural properties of cyclic peptides by controlling their flexibility. Here, we investigate two cyclic peptides, featuring beta turns. Using temperature-dependent NMR and FT-IR, we observed a pronounced temperature effect on the conformation of the cyclic peptide D-1 in CHCl₃ but a much smaller effect in CH₃CN. Almost no effect was observed for its diastereomer L-1 within a similar temperature range and using the same solvents. With the aid of Replica Exchange Molecular Dynamics simulations and Quantum Mechanics/Molecular Mechanics calculations, we were able to explain this behavior based on the increased flexibility of D-1 CHCl₃ in terms of intramolecular hydrogen bonding. The largest temperature dependence is observed for D-1 in CHCl₃, while the temperature effect is less pronounced for L-1 in CHCl₃ and for both peptides in CH₃CN. This work provides new insights into the role of the environment and temperature on the conformations of cyclic peptides.

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Feature

Our paper mentioned in 'Chemistry Views'

Reactions of Arylcarbenes with Lewis Acids
A. H. Raut, P. Costa, W. Sander, Chem. Eur. J. 24 (2018), 18043-18051

Our recent paper was mentioned in the magazine "Chemistry Views"

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NEW Paper in Chemistry

Activation of Molecular Hydrogen by Arylcarbenes
E. Mendez-Vega, M. Maehara, A. H. Raut, J. Mieres-Perez, M. Tsuge, Y-P. Lee, W. Sander, Chem. Eur. J. 24 (2018), 18801-18808.

The hydrogenation reactions of diphenylcarbene 1, fluorenylidene 2, and dibenzocycloheptadienylidene 3 were investigated in solid H₂ and D₂ matrixes and in H₂ and D₂ doped argon matrixes at cryogenic temps. The reactivity of the carbenes towards H₂ increases in the order 1<3<2. Whereas 1 is stable in solid H₂, 2 and 3 react fast under the same conditions via quantum chem. tunneling. In D₂ both 1 and 3 are stable, whereas 2 slowly reacts. The different reactivity of the three carbenes is rationalized in terms of differing carbene stabilization energies.

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NEW JACS Paper

Switching the Spin State of Pentafluoro-phenylnitrene: Isolation of a Singlet Arylnitrene Complex
J. Mieres-Perez, P. Costa, E. Mendez-Vega, R. Crespo-Otero, W. Sander, J. Am. Chem. Soc. 140 (2018), 17271-17277.

The chemistry of arylnitrenes is dominated by their triplet ground states and excited open-shell singlet states. This results in radical-type reactions and unwanted rearrangements which diminish the use of arylnitrenes as intermediates in organic synthesis. While the closed-shell singlet states of arylnitrenes are expected to undergo useful chemical transformations (comparable to the closed shell singlet states of carbenes), these states are too high in energy to be chemically accessible. When triplet pentafluorophenylnitrene is interacting with the Lewis acid BF₃ under the conditions of matrix isolation, a Lewis acid-base complex consisting of the closed-shell singlet state of the nitrene and two molecules of BF₃ is formed. Although the closed shell singlet state of pentafluorophenylnitrene is calculated (CCSD(T)) to lie more than 25 kcal/mol above its triplet ground state, the reaction with BF₃ results in switching the spin state from triplet to singlet. The formation of the singlet complex was monitored by IR, UV-vis, and EPR spectroscopy. DFT, CCSD(T), and CASPT2 calculations confirm the experimental findings.

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NEW PAPER in Nature Communications

Light-controlled switching of the spin state of iron(III)
S. Shankar, M. Peters, K. Steinborn, B. Krahwinkel, F. D. Sönnichsen, D. Grote, W. Sander, T. Lohmiller, O. Rüdiger, R. Herges, Nature Comm. 9 (2018), Article number: 4750.

Controlled switching of the spin state of transition metal ions, particularly of Fe(II) and Fe(III), is a prerequisite to achieve selectivity, efficiency, and catalysis in a number of metalloenzymes. Here we report on an iron(III) porphyrin with a photochromic axial ligand which, upon irradiation with two different wavelengths reversibly switches its spin state between low-spin (S=1/2) and high-spin (S=5/2) in solution (DMSO-acetone, 2:598). The switching efficiency is 76% at room temperature. The system is neither oxygen nor water sensitive, and no fatigue was observed after more than 1000 switching cycles. Concomitant with the spin-flip is a change in redox potential by ~60mV. Besides serving as a simple model for the first step of the cytochrome P450 catalytic cycle, the spin switch can be used to switch the spin-lattice relaxation time T1 of the water protons by a factor of 15.

Read our paper in Nature Communications

NEW Angewandte PAPER

Imaging the Solvation of a One-Dimensional Solid on the Molecular Scale
K. Lucht, I. Trosien, W. Sander, K. Morgenstern, Angew. Chem. Int. Ed. 57 (2018), 16334-16338.

We have observed the inversion of the solvation environment of a one-dimensional solid by low-temperature scanning tunneling microscopy. Adsorption of 3-methoxy-9-diazofluorene on Ag(111) yields highly oriented supramolecular chains, which are then exposed to water molecules. The annealing of dry and water-decorated chains results in diametrically opposed outcomes. While the former simply leads to an increase in chain length and number, the latter results in a complete loss of order and produces water clusters decorated with the organic molecule.

Read our paper in Angewandte Chemie

NEW 'Hot Paper' in CHEMISTRY A european journal

Reactions of Arylcarbenes with Lewis Acids
A. H. Raut, P. Costa, W. Sander, Chem. Eur. J. 24 (2018), 18043-18051.

The reactions of the three triplet ground state arylcarbenes diphenylcarbene (1), fluorenylidene (2), and diben-zocycloheptadienylidene (3) with the Lewis acids H₂O, ICF₃,and BF₃ were studied under the conditions of matrix isolation. H₂O was selected as typical hydrogen bond donor, ICF₃ as halogen bond donor, and BF₃ as strong Lewis acid. H₂O forms hydrogen-bonded complexes of the singlet carbenes with 1 and 2, but not with 3. This is rationalized by the larger singlet–triplet gap of 3, which does not allow to stabilize the singlet state below the triplet state by hydrogen bonding.

With ICF₃, both 1 and 3 form halogen-bonded complexes of the singlet states of the carbenes. This indicates that halogen bonding stabilizes singlet carbenes more than hydrogen bonding. Carbene 2 reacts differently from 1 and 3 by forming an iodonium ylide, thus avoiding antiaromatic destabilization of the fluorenyl unit. With BF₃, all threecarbenes form zwitterionic Lewis acid/base complexes.

Read our paper in Chem. Eur. J.

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