Is Mn(I) More Promising Than Fe(II)-A Comparison of Mn vs Fe Complexes for Olefin Metathesis.

Olefin metathesis is one of the most significant transformations in organic chemistry and is an excellent example for efficient homogeneous catalysis. Although most currently used catalysts are primarily based on 4d and 5d metals, cycloaddition and cycloreversion reactions can also be attributed to first-row transition metals, such as Fe. Surprisingly, the potential of Mn(I)-based catalysts for olefin metathesis has been unexplored despite their prominence in homogeneous catalysis and their diagonal relationship to Ru(II). In the present study, we have investigated the prospective capabilities of Mn complexes for cycloaddition and reversion reactions using density functional theory. Therefore, we have initially compared the literature known iron model systems and their isoelectronic Mn counterparts regarding their reactivity and electronic structure. Next, we constructed potential Mn complexes derived from synthetically accessible species, including carbonyl ligands and obeying octahedral geometry. Based on thermodynamic parameters and the calculation of electronic descriptors, we were able to validate the isodiagonal relationship. Our study serves as guidance for the experimental chemist.

No Transition Metals Required - Oxygen Promoted Synthesis of Imines from Primary Alcohols and Amines under Ambient Conditions.

The synthesis of imines denotes a cornerstone in organic chemistry. The use of alcohols as renewable substituents for carbonyl-functionality represents an attractive opportunity. Consequently, carbonyl moieties can be in situ generated from alcohols upon transition-metal catalysis under inert atmosphere. Alternatively, bases can be utilized under aerobic conditions. In this context, we report the synthesis of imines from benzyl alcohols and anilines, promoted by KOt Bu under aerobic conditions at room temperature, in the absence of any transition-metal catalyst. A detailed investigation of the radical mechanism of the underlying reaction is presented. This reveals a complex reaction network fully supporting the experimental findings.

Curious Case of Cobaltocenium Carbaldehyde.

Cobaltocenium carbaldehyde (formylcobaltocenium) hexafluoridophosphate, a long sought-after functionalized cobaltocenium salt, is accessible from cobaltocenium carboxylic acid by a three-step synthetic sequence involving (i) chlorination to the acid chloride, (ii) copper-borohydride reduction to the hydroxymethyl derivative, and (iii) Dess-Martin oxidation to the title compound. Due to the strongly electron-withdrawing cationic cobaltocenium moiety, no standard aldehyde reactivity is observed. Instead, nucleophilic addition followed by haloform-type cleavage prevails, thereby ruling out common useful aldehyde derivatization. One-electron reduction of cobaltocenium carbaldehyde hexafluoridophosphate affords the deep-blue, isolable cobaltocene carbaldehyde 19-valence-electron radical whose spin density is located fully at cobalt and not at the formyl carbon atom. 1H/13C NMR, IR, EPR spectroscopy, high-resolution mass spectrometry, cyclic voltammetry, single crystal structure analysis (XRD), and density functional theory are applied to characterize these unusual formyl-cobaltocenium/cobaltocene compounds.

A Fujiwara-Moritani-Type Alkenylation Using a Traceless Directing Group Strategy: A Rare Example of C-C Bond Formation towards the C2-Carbon of Terminal Alkenes.

Herein we report, a rhodium-catalyzed Fujiwara-Moritani-type reaction of unactivated terminal alkenes and benzoic acid derivatives bearing electron donating residues under mild conditions. The acid functionality acts as a traceless directing group delivering products alkenylated in meta-position to the electron donating substituent in contrast to the usually obtained ortho- and para-substitution in Friedel-Crafts-type reactions. Remarkably, the new C-C bond is formed to the C2 of the terminal olefin, in contrast to similar reported transformations. Initially formed mixtures of exo- and endo-double bond isomers can be efficiently isomerized to the more stable endo-products.

Oxidative Fluorination of Selenium and Tellurium Compounds using a Thermally Stable Phosphonium SF5 - Salt Accessible from SF6.

Fluorinated group 16 moieties are attractive building blocks in synthetic chemistry but only few synthetic methods are available to prepare them. Herein, we report a new oxidative fluorination reagent capable of stabilizing reactive fluorinated anions. It consists of an SF5 - anion and a chemically inert phosphonium cation and is exceptionally thermally stable. Accordingly, it was used to generate the SeF5 - and TeF5 - anions from the elemental chalcogens and to prepare the unknown tetrafluoro(phenyl)-λ5 -selenate PhSeF4 - and -tellurate PhTeF4 - from the corresponding diphenyl dichalcogenides. In addition, we show that further derivatization of [PhTeF4 ]- by oxidation to trans-PhTeF4 O- and subsequent alkylation gives access to a new class of trans-(alkoxy)(phenyl)tetrafluoro-λ6 -tellanes (trans-PhTeF4 OR), thus providing an approach to introduce the functional group into organic molecules.

Increase of Radiative Forcing through Midinfrared Absorption by Stable CO2 Dimers?

We performed matrix-isolation infrared (MI-IR) spectroscopy of carbon dioxide monomers, CO2, and dimers, (CO2)2, trapped in neon and in air. On the basis of vibration configuration interaction (VCI) calculations accounting for mode coupling and anharmonicity, we identify additional infrared-active bands in the MI-IR spectra due to the (CO2)2 dimer. These bands are satellite bands next to the established CO2 monomer bands, which appear in the infrared window of Earth's atmosphere at around 4 and 15 µm. In a systematic carbon dioxide mixing ratio study using neon matrixes, we observe a significant fraction of the dimer at mixing ratios above 300 ppm, with a steep increase up to 1000 ppm. In neon matrix, the dimer increases the IR absorbance by about 15% at 400 ppm compared to the monomer absorbance alone. This suggests a high fraction of the (CO2)2 dimer in our matrix experiments. In atmospheric conditions, such increased absorbance would significantly amplify radiative forcings and, thus, the greenhouse warming. To enable a comparison of our laboratory experiment with various atmospheric conditions (Earth, Mars, Venus), we compute the thermodynamics of the dimerization accordingly. The dimerization is favored at low temperatures and/or high carbon dioxide partial pressures. Thus, we argue that matrix isolation does not trap the gas composition "as is". Instead, the gas is precooled to 40 K, where CO2 dimerizes before being trapped in the matrix, already at very low carbon dioxide partial pressures. In the context of planetary atmospheres, our results improve understanding of the greenhouse effect for planets of rather thick CO2 atmospheres such as Venus, where a significant fraction of the (CO2)2 dimer can be expected. There, the necessity of including the mid-IR absorption by stable (CO2)2 dimers in databases used for modeling radiative forcing, such as HITRAN, arises.

Encapsulation Enhances the Catalytic Activity of C-N Coupling: Reaction Mechanism of a Cu(I)/Calix[8]arene Supramolecular Catalyst.

Development of C-N coupling methodologies based on Earth-abundant metals is a promising strategy in homogeneous catalysis for sustainable processes. However, such systems suffer from deactivation and low catalytic activity. We here report that encapsulation of Cu(I) within the phenanthroyl-containing calix[8]arene derivative 1,5-(2,9-dimethyl-1,10-phenanthroyl)-2,3,4,6,7,8-hexamethyl-p-tert-butylcalix[8]arene (C8PhenMe6 ) significantly enhances C-N coupling activity up to 92 % yield in the reaction of aryl halides and aryl amines, with low catalyst loadings (2.5 % mol). A tailored multiscale computational protocol based on Molecular Dynamics simulations and DFT investigations revealed an oxidative addition/reductive elimination process of the supramolecular catalyst [Cu(C8PhenMe6)I]. The computational investigations uncovered the origins of the enhanced catalytic activity over its molecular analogues: Catalyst deactivation through dimerization is prevented, and product release facilitated. Capturing the dynamic profile of the macrocycle and the impact of non-covalent interactions on reactivity allows for the rationalization of the behavior of the flexible supramolecular catalysts employed.

Manganese-Catalyzed Dehydrogenative Silylation of Alkenes Following Two Parallel Inner-Sphere Pathways.

We report on an additive-free Mn(I)-catalyzed dehydrogenative silylation of terminal alkenes. The most active precatalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)3(CH2CH2CH3)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate which undergoes rapid Si-H bond cleavage of the silane HSiR3 forming the active 16e- Mn(I) silyl catalyst [Mn(dippe)(CO)2(SiR3)] together with liberated butanal. A broad variety of aromatic and aliphatic alkenes was efficiently and selectively converted into E-vinylsilanes and allylsilanes, respectively, at room temperature. Mechanistic insights are provided based on experimental data and DFT calculations revealing that two parallel reaction pathways are operative: an acceptorless reaction pathway involving dihydrogen release and a pathway requiring an alkene as sacrificial hydrogen acceptor.

On the Origin of E-Selectivity in the Ring-Opening Metathesis Polymerization with Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes.

The understanding and control of stereoselectivity is a central aspect in ring-opening metathesis polymerization (ROMP). Herein, we report detailed quantum chemical studies on the reaction mechanism of E-selective ROMP of norborn-2-ene (NBE) with Mo(N-2,6-Me2-C6H3)(CHCMe3)(IMes)(OTf)2 (1, IMes = 1,3-dimesitylimidazol-2-ylidene) as a first step to stereoselective polymerization. Four different reaction pathways based on an ene syn or ene anti approach of NBE to either the syn- or anti-isomer of the neutral precatalyst have been studied. In contrast to the recently established associative mechanism with a terminal alkene, where a neutral olefin adduct is formed, NBE reacts directly with the catalyst via [2 + 2] cycloaddition to form molybdacyclobutane with a reaction barrier about 30 kJ mol-1 lower in free energy than via the formation of a catalyst-monomer adduct. However, the direct cycloaddition of NBE was only found for one out of four stereoisomers. Our findings strongly suggest that this stereoselective approach is responsible for E-selectivity and point toward a substrate-specific reaction mechanism in olefin metathesis with neutral Mo imido alkylidene N-heterocyclic carbene bistriflate complexes.

Cationic Cycloheptatrienyl Cyclopentadienyl Manganese Sandwich Complexes: Tromancenium Explored with High-Power LED Photosynthesis.

In this contribution, we revisit the neglected and forgotten cationic, air-stable, 18-valence electron, heteroleptic sandwich complex (cycloheptatrienyl)(cyclopentadienyl)manganese, which was reported independently by Fischer and by Pauson about 50 years ago. Using advanced high-power LED photochemical synthesis, an expedient rapid access to the parent complex and to functionalized derivatives with alkyl, carboxymethyl, bromo, and amino substituents was developed. A thorough study of these "tromancenium" salts by a range of spectroscopic techniques (1H/13C/55Mn-NMR, IR, UV-vis, HRMS, XRD, XPS, EPR), cyclic voltammetry (CV), and quantum chemical calculations (DFT) shows that these manganese sandwich complexes are unique metallocenes with quite different chemical and physical properties in comparison to those of isoelectronic cobaltocenium salts or (cycloheptatrienyl)(cyclopentadienyl) sandwich complexes of the early transition metals. Electrochemically, all tromancenium ions undergo a chemically partially reversible oxidation and a chemically irreversible reduction at half-wave or peak potentials that respond to the substituents at the Cp deck. As exemplarily shown for the parent tromancenium ion, the product generated during the irreversible reduction process reverts at least partially to the starting material upon reoxidation. Quantum-chemical calculations of the parent tromancenium salt indicate that metal-ligand bonding is distinctly weaker for the cycloheptatrienyl ligand in comparison to that of the cyclopentadienyl ligand. Both the HOMO and the LUMO are metal and cycloheptatrienyl-ligand centered, indicating that chemical reactions will occur either metal-based or at the seven-membered ring, but not on the cyclopentadienyl ligand.

Synthesis of Pyrroles via Consecutive 6π-Electrocyclization/Ring-Contraction of Sulfilimines.

We present a modular, synthetic entry to polysubstituted pyrroles employing readily available 2,5-dihydrothiophenes. Ring-opening of the heterocycle provides access to a panel of 1,3-dienes which undergo pyrrole formation in the presence of inexpensive chloramine-T trihydrate. The transformation is conducted in an open flask and proceeds at ambient temperatures (23 °C) in nondry solvents. A careful adjustment of the electronics and sterics of the 1,3-diene precursor allows for the isolation of key intermediates. DFT studies identified a reaction mechanism that features a 6π-electrocyclization of a sulfilimine intermediate followed by spontaneous ring-contraction to reveal the pyrrole skeleton.

Implementation of the Freely Jointed Chain Model to Assess Kinetics and Thermodynamics of Thermosensitive Coil-Globule Transition by Markov States.

We revived and implemented a method developed by Kuhn in 1934, originally only published in German, that is, the so-called "freely jointed chain" model. This approach turned out to be surprisingly useful for analyzing state-of-the-art computer simulations of the thermosensitive coil-globule transition of N-Isopropylacrylamide 20-mer. Our atomistic computer simulations are orders of magnitude longer than those of previous studies and lead to a reliable description of thermodynamics and kinetics at many different temperatures. The freely jointed chain model provides a coordinate system, which allows us to construct a Markov state model of the conformational transitions. Furthermore, this guarantees a reliable reconstruction of the kinetics in back-and-forth directions. In addition, we obtain a description of the high diversity and variability of both conformational states. Thus, we gain a detailed understanding of the coil-globule transition. Surprisingly, conformational entropy turns out to play only a minor role in the thermodynamic balance of the process. Moreover, we show that the radius of gyration is an unexpectedly unsuitable coordinate to comprehend the transition kinetics because it does not capture the high conformational diversity within the different states. Consequently, the approach presented here allows for an exhaustive description and resolution of the conformational ensembles of arbitrary linear polymer chains.

Quantum Chemical Microsolvation by Automated Water Placement.

We developed a quantitative approach to quantum chemical microsolvation. Key in our methodology is the automatic placement of individual solvent molecules based on the free energy solvation thermodynamics derived from molecular dynamics (MD) simulations and grid inhomogeneous solvation theory (GIST). This protocol enabled us to rigorously define the number, position, and orientation of individual solvent molecules and to determine their interaction with the solute based on physical quantities. The generated solute-solvent clusters served as an input for subsequent quantum chemical investigations. We showcased the applicability, scope, and limitations of this computational approach for a number of small molecules, including urea, 2-aminobenzothiazole, (+)-syn-benzotriborneol, benzoic acid, and helicene. Our results show excellent agreement with the available ab initio molecular dynamics data and experimental results.

Rapid Assembly of Tetrasubstituted Furans via Pummerer-Type Rearrangement.

Despite the many methods available for the synthesis of furans, few methods remain that allow for the custom-made assembly of fully substituted furans. Here we report a powerful protocol to rapidly construct tetrasubstituted, orthogonally functionalized furans under mild reaction conditions. The developed method involves the regioselective ring-opening of readily available 2,5-dihydrothiophenes followed by an oxidative cyclization to provide the heterocycle. The selective oxidation at sulfur is promoted by N-chlorosuccinimide as an inexpensive reagent and proceeds at ambient temperature in high yield within 30 min. The obtained furans serve as exceptionally versatile intermediates and were shown to participate in a series of valuable postmodifications. The fate of the initial sulfonium intermediate was investigated by mechanistic experiments, and computational studies revealed the existence of an unprecedented Pummerer-type rearrangement. The potential for organic synthesis is highlighted by the total synthesis of bisabolene sesquiterpenoids (pleurotins A, B, and D).

On the synergy of matrix-isolation infrared spectroscopy and vibrational configuration interaction computations.

The key feature of matrix-isolation infrared (MI-IR) spectroscopy is the isolation of single guest molecules in a host system at cryogenic conditions. The matrix mostly hinders rotation of the guest molecule, providing access to pure vibrational features. Vibrational self-consistent field (VSCF) and configuration interaction computations (VCI) on ab initio multimode potential energy surfaces (PES) give rise to anharmonic vibrational spectra. In a single-sourced combination of these experimental and computational approaches, we have established an iterative spectroscopic characterization procedure. The present article reviews the scope of this procedure by highlighting the strengths and limitations based on the examples of water, carbon dioxide, methane, methanol, and fluoroethane. An assessment of setups for the construction of the multimode PES on the example of methanol demonstrates that CCSD(T)-F12 level of theory is preferable to compute (a) accurate vibrational frequencies and (b) equilibrium or vibrationally averaged structural parameters. Our procedure has allowed us to uniquely assign unknown or disputed bands and enabled us to clarify problematic spectral regions that are crowded with combination bands and overtones. Besides spectroscopic assignment, the excellent agreement between theory and experiment paves the way to tackle questions of rather fundamental nature as to whether or not matrix effects are systematic, and it shows the limits of conventional notations used by spectroscopists.

N-Heterocyclic Carbene Gold(I) Complexes: Mechanism of the Ligand Scrambling Reaction and Their Oxidation to Gold(III) in Aqueous Solutions.

N-Heterocyclic carbene (NHC) gold(I) complexes offer great prospects in medicinal chemistry as antiproliferative, anticancer, and antibacterial agents. However, further development requires a thorough understanding of their reaction behavior in aqueous media. Herein, we report the conversion of the bromido[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(I) ((NHC)AuIBr, 1) complex in acetonitrile/water mixtures to the bis[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(I) ([(NHC)2AuI]+, 7), which is subsequently oxidized to the dibromidobis[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(III) ([(NHC)2AuIIIBr2]+, 9). By combining experimental data from HPLC, NMR, and (LC-)/HR-MS with computational results from DFT calculations, we outline a detailed ligand scrambling reaction mechanism. The key step is the formation of the stacked ((NHC)AuIBr)2 dimer (2) that rearranges to the T-shaped intermediate Br(NHC)2AuI-AuIBr (3). The dissociation of Br- from 3 and recombination lead to (NHC)2AuI-AuIBr2 (5) followed by the separation into [(NHC)2AuI]+ (7) and [AuIBr2]- (8). [AuIBr2]- is not stable in an aqueous environment and degrades in an internal redox reaction to Au0 and Br2. The latter in turn oxidizes 7 to the gold(III) species 9. The reported ligand rearrangement of the (NHC)AuIBr complex differs from that found for related silver(I) analogous. A detailed understanding of this scrambling mechanism is of utmost importance for the interpretation of their biological activity and will help to further optimize them for biomedical and other applications.

Thermosensitive Hydration of Four Acrylamide-Based Polymers in Coil and Globule Conformations.

To characterize the thermosensitive coil-globule transition in atomistic detail, the conformational dynamics of linear polymer chains of acrylamide-based polymers have been investigated at multiple temperatures. Therefore, molecular dynamic simulations of 30mers of polyacrylamide (AAm), poly-N-methylacrylamide (NMAAm), poly-N-ethylacrylamide (NEAAm), and poly-N-isopropylacrylamide (NIPAAm) have been performed at temperatures ranging from 250 to 360 K for 2 µs. While two of the polymers are known to exhibit thermosensitivity (NEAAm, NIPAAm), no thermosensitivity is observed for AAm and NMAAm in aqueous solution. Our computer simulations consistently reproduce these properties. To understand the thermosensitivity of the respective polymers, the conformational ensembles at different temperatures have been separated according to the coil-globule transition. The coil and globule conformational ensembles were exhaustively analyzed in terms of hydrogen bonding with the solvent, the change of the solvent accessible surface, and enthalpic contributions. Surprisingly, independent of different thermosensitive properties of the four polymers, the surface affinity to water of coil conformations is higher than for globule conformations. Therefore, polymer-solvent interactions stabilize coil conformations at all temperatures. Nevertheless, the enthalpic contributions alone cannot explain the differences in thermosensitivity. This clearly implies that entropy is the distinctive factor for thermosensitivity. With increasing side chain length, the lifetime of the hydrogen bonds between the polymer surface and water is extended. Thus, we surmise that a longer side chain induces a larger entropic penalty due to immobilization of water molecules.

Decomposing anharmonicity and mode-coupling from matrix effects in the IR spectra of matrix-isolated carbon dioxide and methane.

Gas-phase IR spectra of carbon dioxide and methane are nowadays well understood, as a consequence of their pivotal roles in atmospheric- and astrochemistry. However, once those molecules are trapped in noble gas matrices, their spectroscopic properties become difficult to conceptualize. Still, such spectra provide valuable insights into the vibrational structure. In this study, we combine new matrix-isolation infrared (MI-IR) spectra at 6 K in argon and neon with in vacuo anharmonic spectra computed by vibrational self-consistent field (VSCF) and vibrational configuration interaction (VCI). The aim is to separate anharmonicity from matrix effects in the mid-infrared spectra of 12C16O2, 12CH4, and 12CD4. The accurate description of anharmonic potential energy surfaces including mode-coupling allows to reproduce gas-phase data with deviations of below 3 cm-1. Consequently, the remaining difference between MI-IR and VSCF/VCI can be attributed to matrix effects. Frequency shifts and splitting patterns turn out to be unsystematic and dependent on the particular combination of analyte and noble gas. While in the case of neon matrices these effects are small, they are pronounced in xenon, krypton, and argon matrices. Our strategy allows us to suggest that methane rotates in neon matrices - in contrast to previous reports.

Synthesis of Vicinal Quaternary All-Carbon Centers via Acid-catalyzed Cycloisomerization of Neopentylic Epoxides.

We report our studies on the development of a catalytic cycloisomerization of 2,2-disubstituted neopentylic epoxides to produce highly substituted tetralins and chromanes. Termination of the sequence occurs via Friedel-Crafts-type alkylation of the remote (hetero)arene linker. The transformation is efficiently promoted by sulfuric acid and proceeds best in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as the solvent. Variation of the substitution pattern provided detailed insights into the migration tendencies and revealed a competing disproportionation pathway of dihydronaphthalenes.