Organische Chemie II - Physikalisch-Organische Chemie

Reactions of Cyclopentadienylidenes with CF₃I: Electron Bond Donation versus Halogen Bond Donation of the Iodine Atom
S. Henkel, I. Trosien, J. Mieres-Perez, T. Lohmiller, A. Savitsky, E. Sanchez-Garcia, W. Sander, J. Org. Chem. 83 (2018), 7586-7592

The interaction of cyclopentadienylidene and tetrachlorocyclopentadienyli-dene with the halogen bond donor CF₃I has been studied by matrix isolation spectroscopy. The carbenes were produced by photolysis of the corresponding diazo compounds, matrix-isolated in argon doped with 1% CF₃I at 3 K. Bimolecular reactions between the carbenes and CF₃I were induced by annealing these matrices to 25–30 K to allow for the diffusion of trapped species. Instead of classical halogen-bonded complexes, these carbenes form complexes in which the iodine atom is shared between the carbene center and the CF₃ group. Photolysis of the complexes at 3 K yields radical pairs, which reversibly react back to the complexes when the matrices are warmed to 25–30 K.

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NEW JACS PAPER

Isolation of an Antiaromatic Singlet Cyclopentadienyl Zwitterion
P. Costa, I. Trosien, J. Mieres Perez, W. Sander, J. Am. Chem. Soc. 139 (2017), 13024-13030.

The reaction of triplet tetrachloro-cyclopentadienylidene with BF3 in rare gas matrices yields a zwitterion consisting of a cyclopentadienyl cation bearing the positive charge and a negatively charged BF3 unit. IR and UV-vis spectra as well as the absence of EPR signals demonstrate a singlet ground state of the zwitterion, and its calculated geometry and magnetic properties clearly reveal a strong antiaromatic character. The zwitterion is highly labile, and by visible or IR irradiation rearranges via a 1,2-fluorine migration from boron to carbon. Interaction with a second molecule of BF3 stabilizes the zwitterion and suppresses the fluorine migration, thus providing a convenient and efficient synthesis of an antiaromatic molecule under very mild conditions.

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NEW JACS PAPER

Is Magnetic Bistability of Carbenes a General Phenomenon? Isolation of Simple Aryl(trifluoromethyl)carbenes in Both their Singlet and Triplet States
Y. A. Tsegaw, P. E. Kadam, N. Tötsch, E. Sanchez-Garcia, W. Sander, J. Am. Chem. Soc. 139 (2017), 12310-12316.

p-Tolyl(trifluoromethyl) carbene and the related fluorenyl(trifluoromethyl)carbene were synthesized in solid argon and characterized by IR, UV-vis, and EPR spectroscopy as well as by quantum mechanical calculations. The carbenes can be generated in both their triplet and singlet states, and both states coexist under the conditions of matrix isolation. According to our calculations, the singlet and triplet states of these car-benes are energetically nearly degenerate in the gas phase. Warming of matrices containing pure triplet p-tolyl(trifluoromethyl) carbene from 3 to 25 K leads to an interconversion of up to 20% of the triplet into the singlet state. This interconversion is thermally irreversible, and cooling back to 3 K does not change the ratio of singlet to triplet. Irradiation at 365 nm results in a complete interconversion of singlet to triplet, whereas 450 nm irradiation produces again up to 20% of the singlet state. An alternative way to generate the singlet carbene is the reaction of the triplet with water molecules by annealing water-doped matrices at 25 K. This results in the irreversible formation of a hydrogen-bonded complex be-tween the singlet carbene and water. For fluorenyl(trifluoromethyl)carbene very similar results were obtained, only the yield of the singlet state is even higher. Magnetic bistability of carbenes seems to be a general phenomenon that only depends on the singlet-triplet gap rather than on the nature of the carbene.

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RUB Press Release

When a tiger escapes from an impenetrable cage

Quantum effects are rarely observed for heavy atoms. Now it’s been successful. RUB chemists have shown that carbon atoms can behave like particles and like waves. This double life has already been sufficiently described for light particles and electrons. However, researchers have only rarely been able to observe the wave-particle phenomenon for heavy atoms such as carbon.

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New Paper in Angewandte Chemie

The Cope Rearrangement of 1,5-Dimethylsemibullvalene-2(4)-₁: Experimental Evidence for Heavy-Atom Tunneling
T. Schleif, J. Mieres-Perez, S. Henkel, M. Ertelt, W. T. Borden, W. Sander, Angewandte Chemie, Int. Ed. 56, (2017), 10746-10749.

As an experimental test of the theoretical prediction that heavy-atom tunneling is involved in the degenerate Cope rearrangement of semibullvalenes at cryogenic temperatures, monodeuterated 1,5-dimethylsemibullvalene isotopomers were prepared and investigated by IR spectroscopy using the matrix isolation technique. As predicted, the less thermodynamically stable isotopomer rearranges at cryogenic temperatures in the dark to the more stable one, while broadband IR irradiation above 2000 cm^-1 results in an equilibration of the isotopomeric ratio. Since this reaction proceeds with a rate constant in the order of 10^-4 s^-1 despite an experimental barrier of Ea=4.8 kcal mol^-1 and with only a shallow temperature dependence, the results are interpreted in terms of heavy-atom tunneling.

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RUB Press Release

Astrochemistry: Did life come from space?

Researchers have now verified around 190 chemical compounds in space. But it is still not clear in what way and under what conditions they are created. The team led by Prof Dr Wolfram Sander at the RUB Institute of Organic Chemistry II is interested in these questions – although the core topic of the group is actually different.

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New Paper in Angewandte Chemie

Activation of the B-F Bond by Diphenylcarbene: A Reversible 1,2-Fluorine Migration between Boron and Carbon
P. Costa, J. Mieres-Perez, N. Özkan, W. Sander, Angewandte Chemie, Int. Ed. 7 , 56 (2017), 1760-1764.

Experiments in low-temperature matrices reveal that triplet diphenylcarbene inserts into the very strong B-F bond of BF₃ in a two-step reaction. The first step is the formation of a strongly bound Lewis acid–base complex between the singlet state of diphenylcarbene and BF₃. This step involves an inversion of the spin state of the carbene from triplet to singlet. The second step requires visible-light photochemical activation to induce a 1,2-F migration from boron to the adjacent carbon atom under formation of the formal insertion product of the carbene center into BF₃. The 1,2-F migration is reversible under short-wavelength UV irradiation, thus leading back to the Lewis acid–base adduct.

Read our paper in Angew. Chem. Int. Ed.

New Paper in Nature Communications

Competitive solvent-molecule interactions govern primary processes of diphenylcarbene in solvent mixtures
J. Knorr, P. Sokkar, S. Schott, P. Costa, W. Thiel, W. Sander, E. Sanchez-Garcia, P. Nuernberger, Nature Communications 7 , Article number: 12968 (2016)

Photochemical reactions in solution often proceed via competing reaction pathways comprising intermediates that capture a solvent molecule. A disclosure of the underlying reaction mechanisms is challenging due to the rapid nature of these processes and the intricate identification of how many solvent molecules are involved. Here combining broadband femtosecond transient absorption and quantum mechanics/molecular mechanics simulations, we show for one of the most reactive species, diphenylcarbene, that the decision-maker is not the nearest solvent molecule but its neighbour. The hydrogen bonding dynamics determine which reaction channels are accessible in binary solvent mixtures at room temperature. In-depth analysis of the amount of nascent intermediates corroborates the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking analogy to complexes found at cryogenic temperatures. Our results show that adjacent solvent molecules take the role of key abettors rather than bystanders for the fate of the reactive intermediate.

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Hot Paper and Cover Picture

Azulenylcarbenes: Rearrangements on the C11H8 Potential Energy Surface
S. Henkel, C. H. Pollok, T. Schleif, W. Sander, Chem. Eur. J. 22 (2016), 12479-12486

Four isomeric azulenylcarbenes were synthesized in argon matrices by photolysis of the corresponding diazo precursors, and the photochemistry of these carbenes was studied. The carbenes and their rearranged products were characterized by IR, UV/Vis, and EPR spectroscopy, and the experimental data were compared to results from DFT calculations. While 2-, 5- and 6-azulenylcarbene show triplet ground states, 1-azulenylcarbene exhibits a singlet ground state, in accord with theoretical predictions. The rearrangements of the azulenylcarbenes give access to a number of unusual C11H8 isomers, such as other carbenes and strained allenes.

Read our paper in Chem. Eur. J.

Paolo won a prize

Paolo Costa won the Dr.-Heinrich-Kost-Preis of the Ruhr University for his PhD thesis

Paolo studied the interplay between solvents and reactive intermediates on the molecular scale. He observed that a diphenyl carbene, matrix isolated in solid argon at cryogenic temperatures changes its spin state from triplet to singlet because of the interaction between a single methanol or water molecule by forming hydrogen bonds. He found the same behaviour for Fluorenyliden and water in argon matrix. Using water matrices the corresponding carbo cations are formed via protonation. Paolo's PhD thesis was supervised by Prof. W. Sander.

Pressemeldung der RUB.

New JACS paper

Light and Temperature Control of the Spin State of
Bis(p-methoxyphenyl)carbene:
A Magnetically Bistable Carbene
P. Costa, T. Lohmiller, I. Trosien, A. Savitsky, W. Lubitz, M. Fernandez-Oliva, E. Sanchez-Garcia, and W. Sander, J. Am. Chem. Soc. 138 (2016), 1622-1629.

Bis(p-methoxyphenyl)carbene is the first carbene that at cryogenic temperatures can be isolated in both its lowest energy singlet and triplet states. At 3 K, both states coexist indefinitely under these conditions. The carbene is investigated in argon matrices by IR, UV–vis, and X-band EPR spectroscopy and in MTHF glasses by W-band EPR and Q-band ENDOR spectroscopy. UV (365 nm) irradiation of the system results in formation of predominantly the triplet carbene, whereas visible (450 nm) light shifts the photostationary equilibrium toward the singlet state. Upon annealing at higher temperatures (>10 K), the triplet is converted to the singlet; however, cooling back to 3 K does not restore the triplet. Therefore, depending on matrix temperature and irradiation conditions, matrices containing predominantly the triplet or singlet carbene can be generated. Controlling the magnetic and chemical properties of carbenes by using light of different wavelengths might be of general interest for applications such as information storage and radical-initiated polymerization processes.

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New JACS paper

Switching the Spin State of Diphenylcarbene via Halogen Bonding
S. Henkel, P. Costa, L. Klute, P. Sokkar, M. Fernandez-Oliva, W. Thiel, E. Sanchez-Garcia, and W. Sander, J. Am. Chem. Soc. 138 (2016), 1689-1697.

The interactions between diphenylcarbene DPC and the halogen bond donors CF3I and CF3Br were investigated using matrix isolation spectroscopy (IR, UV–vis, and EPR) in combination with QM and QM/MM calculations. Both halogen bond donors CF3X form very strong complexes with the singlet state of DPC, but only weakly interact with triplet DPC. This results in a switching of the spin state of DPC, the singlet complexes becoming more stable than the triplet complexes. CF3I forms a second complex (type II) with DPC that is thermodynamically slightly more stable. Calculations predict that in this second complex the DPC···I distance is shorter than the F3C···I distance, whereas in the first (type I) complex the DPC···I distance is, as expected, longer. CF3Br only forms the type I complex. Upon irradiation I or Br, respectively, are transferred to the DPC carbene center and radical pairs are formed. Finally, on annealing, the formal C–X insertion product of DPC is observed. Thus, halogen bonding is a powerful new principle to control the spin state of reactive carbenes.

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Reactions with molecular oxygen

Reaction of Triplet Phenylnitrene with Molecular Oxygen
J. Mieres-Perez, E. Mendez-Vega, K. Velappan, W. Sander, J. Org. Chem. 80 (2015), 11926-11931.

Triplet carbenes react with molecular oxygen with rates that approach diffusion control to carbonyl O-oxides, whereas triplet nitrenes react much slower. For investigating the reaction of phenylnitrene with O2, the nitrene was generated by flash vacuum thermolysis (FVT) of phenylazide and subsequently isolated in O2-doped matrices. FVT of the azide produces the nitrene in high yield and with only minor contaminations of the rearranged products that are frequently observed if the nitrene is produced by photolysis. The phenylnitrene was isolated in solid Ar, Xe, mixtures of these rare gases with O2, and even in pure solid O2. At temperatures between 30 and 35 K an extremely slow thermal reaction between the nitrene and O2 was observed, whereas at higher temperatures, solid Ar and O2 rapidly evaporate. Only O2-doped Xe matrices allowed us to anneal at temperatures above 40 K, and at these temperatures, the nitrene reacts with O2 to produce nitroso O-oxide mainly in its syn conformation. Upon visible light irradiation (450 nm), the nitroso oxide rapidly rearranges to nitrobenzene.

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Poster Award

Two Poster Awards for Soumya

Soumya has won two Poster Awards for her poster "Matrix Isolation and Solvation studies of Diphenylmethyl Radical". Her poster was awarded at the 2015 GORDON RESEARCH CONFERENCE ON PHYSICAL ORGANIC CHEMISTRY in Holderness, New Hampshire (June 21-26) and at the EUROPEAN SYMPOSIUM ON ORGANIC REACTIVITY in Kiel, Germany (Aug 30 - Sep 04). Soumya presented the results of her research. She was able to generate the diphenylmethyl radical in argon matrix at 3 K and the benzhydryl cation upon irradiation with 308 nm. Further irradiation leads back to the formation of the precursor. She also studied the reversibility between the radical and cation species using infrared (IR) and UV-Vis spectrocopy.

Read Soumya's Poster

NEW Publication

Activation of Molecular Hydrogen by a Singlet Carbene through Quantum Mechanical Tunneling
S. Henkel, W. Sander, Angew. Chem. Int. Ed. 54 (2015), 4603-4607.

Carbenes are among the few metal-free molecules that are able to activate molecular hydrogen. Whereas triplet carbenes have been shown to insert into H₂ through a two-step mechanism that at low temperature is assisted by quantum mechanical tunneling (QMT), singlet carbenes insert in concerted reactions with considerable activation barriers, and are thus unreactive towards H₂ at cryogenic temperatures. Here we show that 1-azulenylcarbene with a singlet ground state readily inserts into H₂, and that QMT governs the insertion into both H₂ and D₂. This is the first example that shows that QMT can also be important for singlet carbenes inserting into dihydrogen.

Read our publication in Angew. Chem. Int. Ed.

Angewandte Chemie - Cover Picture

The Fluorenyl Cation
P. Costa, I. Trosien, M. Fernandez-Oliva, E. Sanchez-Garcia, W. Sander, Angew. Chem. Int. Ed. 54 (2015), 2656-2660.

The fluorenyl cation is a textbook example for a 4p antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4p antiaromatic five-membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible-light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.

Read our publication in Angew. Chem. Int. Ed.

Carbenes

The Highly Reactive Benzhydryl Cation Isolated and Stabilized in Water Ice
P. Costa, M. Fernandez-Oliva, E. Sanchez-Garcia, W. Sander, J. Am. Chem. Soc. 136 (2014), 15625-15630.

Diphenylcarbene (DPC) shows a triplet ground-state lying approximately 3 kcal/mol below the lowest singlet state. Under the conditions of matrix isolation at 25 K, DPC reacts with single water molecules embedded in solid argon and switches its ground state from triplet to singlet by forming a strong hydrogen bond. The complex between DPC and water is only metastable, and even at 3 K the carbene center slowly inserts into the OH bond of water to form benzhydryl alcohol via quantum chemical tunneling. Surprisingly, if DPC is generated in amorphous water ice at 3 K, it is protonated instantaneously to give the benzhydryl cation. Under these conditions, the benzhydryl cation is stable, and warming to temperatures above 50 K is required to produce benzhydryl alcohol. Thus, for the first time, a highly electrophilic and extremely reactive secondary carbenium ion can be isolated in a neutral, nucleophilic environment avoiding superacidic conditions.

Read our publication in J. Am. Chem. Soc.
More about our Research on Carbenes

Carbenes

Hydrogen Bonding Switches the Spin State of Diphenylcarbene from Triplet to Singlet
P. Costa, W. Sander, Angew. Chem. Int. Ed. 53 (2014), 5122-5125.

Spin specificity is one of the most important properties of carbenes in their reactions. Alcohols are typically used to probe the reactive spin states of carbenes: O[BOND]H insertions are assumed to be characteristic of singlet states, whereas C-H insertions are typical for the triplets. Surprisingly, the experiments presented here suggest that the spin ground state of diphenylcarbene 1 switches from triplet to singlet if the carbene is allowed to interact with methanol. Carbene 1 and methanol form a strongly hydrogen-bonded singlet ground state complex that was synthesized in low-temperature matrices and characterized by IR spectroscopy. This methanol complex is only metastable, and even at 3K slowly rearranges to form the product of O-H insertion through quantum chemical tunneling. Thus, the ground state triplet (in the gas phase) carbene 1 forms exclusively the products expected from a singlet carbene. Whereas the assumption of spin specific reactions of carbenes is correct, the spin state itself can be changed by solvent interactions, and therefore widely accepted conclusions drawn from earlier experiments have to be revisited.

Read our publication in Angew. Chem. Int. Ed.

Frustrated Lewis Pairs

Carbonylation Reactions of Intramolecular Vicinal Frustrated Phosphane/Borane Lewis Pairs
M. Sajid, A. Lawzer, W. Dong, C. Rosorius, W. Sander, B. Schirmer, S. Grimme, C. G. Daniliuc, G. Kehr, G. Erker, J. Am. Chem. Soc.135 (2013), 18567-18574.

There have been interesting developments in recent years in small molecule binding and activation by frustrated Lewis pairs (FLPs). Intramolecular FLPs contain a donor center (usually phosphane or amine) and an acceptor center (mostly a borane) that are spatially separated. Here, we present first examples of the coordination behavior of a pair of intramolecular vicinal frustrated phosphane/borane Lewis pairs with carbon monoxide.

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polyradicals

Triradicals
M. Winkler, W. Sander, Acc. Chem. Res. 47 (2014), 31-44.

Consistent with the definition of diradicals, triradicals are species in which three electrons occupy three (nearly) degenerate orbitals, resulting in close-lying quartet and doublet states. The same concepts and rules that can be used to predict and rationalize the ground state multiplicity of diradicals also apply to triradicals, but the greater no. of states in triradicals generally leads to more complex electronic structures.

Read our publication in Acc. Chem. Res.
More about our Research on Polyradicals

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