Search

Developing new enantioselective reactions is an important part of chemical discovery but requires time and resources to test large arrays of potential reaction conditions. New techniques are required to analyse many different reactions quickly and efficiently. Mass spectrometry is a high‐throughput method; when combined with ion‐mobility spectrometry, this technique can monitor diastereomeric reaction intermediates and thus be a handle to study enantioselective reactions. Through this technique and others, it was noted before that in the organocatalytic 1,4‐addition to α,β‐unsaturated aldehydes, the abundance of initial diastereomeric intermediates correlates strongly to that of the final enantiomeric products. This work determines isomeric abundance for various catalysts and aldehydes and uses it to predict the enantiomeric excess of two control reactions. The prediction matches well for one reaction but does not predict the obtained results for the second. This finding confirms that the E/Z ratio of the iminium intermediates can be used as a predictor for some reactions, but the kinetics of the following steps can dramatically change the true enantioselectivity. [ABSTRACT FROM AUTHOR]

Electrochemical reactions are better understood by gaining insight into the molecular transformations at the electrodes. This paper introduces a straightforward experimental arrangement in which the voltammetry experiment in a standard electrochemical cell integrates with online electrospray‐ionization mass‐spectrometry. The procedure allows for monitoring of the ions produced at the working electrode. The ionic species sampling is facilitated by a silica capillary embedded in the working electrode and connected directly to the electrospray ionization source. This integration configures real‐time monitoring of the ions produced at the working electrode, enhancing our knowledge of the electrochemical process. The methodology is thoroughly described and exemplified by monitoring the reactive intermediates and products that arise during the electrochemical oxidation of carbazole. This design provides an opportunity to study intermediates in electrochemical reactions with advanced mass spectrometry methods such as ion mobility separation or ion spectroscopy. [ABSTRACT FROM AUTHOR]

Photocatalytic reactions involving a reductive radical‐polar crossover (RRPCO) generate intermediates with carbanionic reactivity. Many of these proposed intermediates resemble highly reactive organometallic compounds. However, conditions of their formation are generally not tolerated by their isolated organometallic versions and often a different reactivity is observed. Our investigations on their nature and reactivity under commonly used photocatalytic conditions demonstrate that these intermediates are indeed best described as free, superbasic carbanions capable of deprotonating common polar solvents usually assumed to be inert such as acetonitrile, dimethylformamide, and dimethylsulfoxide. Their basicity not only towards solvents but also towards electrophiles, such as aldehydes, ketones, and esters, is comparable to the reactivity of isolated carbanions in the gas‐phase. Previously unsuccessful transformations thought to result from a lack of reactivity are explained by their high reactivity towards the solvent and weakly acidic protons of reaction partners. An intuitive explanation for the mode of action of photocatalytically generated carbanions is provided, which enables methods to verify reaction mechanisms proposed to involve an RRPCO step and to identify the reasons for the limitations of current methods. [ABSTRACT FROM AUTHOR]

Copyright of Angewandte Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copper nitrenoids are key intermediates in copper‐catalyzed direct C−H amination reactions. Further development of this important reaction relies on knowing the properties and reactivity of the nitrenoid intermediates. This work utilizes antenna ligands to form copper nitrenoid complexes and monitor the consecutive C−H amination reactions under well‐defined single‐molecule conditions in the gas phase. The [Cu(Lphoto)(Lazide)]+ precursors (Lphoto is a bidentate antenna ligand, and Lazide is an organic azide) were stored in an ion trap at 3.5 K and irradiated by visible light, which resulted in denitrogenation of the complex. Further irradiation of the copper nitrenoid led to the consecutive C−H amination of the antenna ligand. The nitrenoid complexes, as well as the products of the C−H amination, were characterized by helium tagging IRPD spectroscopy, and the mechanism was described by DFT calculations. This research demonstrates that the antenna ligands can be used to promote the denitrogenation of metal azides in the gas phase and also channel the internal energy to promote further reactivity, which opens a new way to study the reactivity of highly reactive species under well‐defined conditions. [ABSTRACT FROM AUTHOR]

The Eschenmoser coupling reaction (ECR) of thioamides with electrophiles is believed to proceed via thiirane intermediates. However, little is known about converting the intermediates into ECR products. Previous mechanistic studies involved external thiophiles to remove the sulfur atom from the intermediates. In this work, an ECR proceeding without any thiophilic agent or base is studied by electrospray ionization‐mass spectrometry. ESI‐MS enables the detection of the so‐far elusive polysulfide species Sn, with n ranging from 2 to 16 sulfur atoms, proposed to be the key species leading to product formation. Integrating observations from ion mobility spectrometry, ion spectroscopy, and reaction monitoring via flow chemistry coupled with mass spectrometry provides a comprehensive understanding of the reaction mechanism and uncovers the autocatalytic nature of the ECR reaction. [ABSTRACT FROM AUTHOR]

Complex reaction mixtures, like those postulated on early Earth, present an analytical challenge because of the number of components, their similarity, and vastly different concentrations. Interpreting the reaction networks is typically based on simplified or partial data, limiting our insight. We present a new approach based on online monitoring of reaction mixtures formed by the formose reaction by ion‐mobility‐separation mass‐spectrometry. Monitoring the reaction mixtures led to large data sets that we analyzed by non‐negative matrix factorization, thereby identifying ion‐signal groups capturing the time evolution of the network. The groups comprised ≈300 major ion signals corresponding to sugar‐calcium complexes formed during the formose reaction. Multivariate analysis of the kinetic profiles of these complexes provided an overview of the interconnected kinetic processes in the solution, highlighting different pathways for sugar growth and the effects of different initiators on the initial kinetics. Reconstructing the network's topology further, we revealed so far unnoticed fast retro‐aldol reaction of ketoses, which significantly affects the initial reaction dynamics. We also detected the onset of sugar‐backbone branching for C6 sugars and cyclization reactions starting for C5 sugars. This top‐down analytical approach opens a new way to analyze complex dynamic mixtures online with unprecedented coverage and time resolution. [ABSTRACT FROM AUTHOR]

Helium tagging photodissociation action spectroscopy in the gas phase together with laser-induced fluorescence (LIF) spectroscopy in neon matrix has been used to probe the photophysics of electrosprayed rhodamine B cation cooled to < 5 K and studied in two different minimally perturbing environments ([RhB•He]+ as well as [RhB]+ in solid neon). Helium tagging experiments were performed using a newly developed apparatus He-TAG. LIF measurements were carried out using an improved matrix-isolation spectroscopy machine Depo-II. The resulting, partially vibrationally resolved electronic spectra of the S0→S1, resp. S1→S0 transition provide benchmarks for gauging the theoretical description of this prototypical organic dye. As a first approach, we have compared our spectra to TD-DFT-based predictions of vibronic structure at the Franck-Condon (FC) and Franck-Condon Herzberg-Teller (FCHT) levels. Depletion action and fluorescence spectroscopy of rhodamine B cation at < 5 K [ABSTRACT FROM AUTHOR]

The search for the chemical structures that account for diffuse interstellar bands remains the main challenge in astrochemistry. It relies on experimental verification of structures obtained from stable isomers in laboratory conditions. The actual interstellar medium, however, is exposed to much harsher conditions. Herein we demonstrate the isomerisation of small, doubly charged polyaromatic hydrocarbon phenanthrene obtained by electron ionisation at relatively high electron energies. We performed electronic and vibrational tagging photodissociation spectroscopy measurements. Our results show that phenanthrene dication has a broad visible spectrum, making it unsuitable for DIB search. We also detected another C14H102+ isomer, originating from the isomerisation of phenanthrene dication during the ionisation process. Hole-burning spectroscopy experiments provided the IR spectrum of an isomer which could be assigned based on DFT calculations as a fully conjugated system representing the global minimum on the C14H102+ potential energy surface. [ABSTRACT FROM AUTHOR]

The kinetic enantio-recognition of chiral viologen guests by planar-chiral porphyrin cage compounds, measured in terms of ΔΔG ‡on , is determined by the planar-chirality of the host and influenced by the size, as measured by ion mobility-mass spectrometry, but not the chirality of its substituents. [ABSTRACT FROM AUTHOR]

Copper(II) complexes with N‐oxyl reactants such as TEMPO can selectively oxidize alcohols to aldehydes or ketones. The proposed copper intermediates of the oxidation reaction were extensively theoretically studied, but they were never experimentally detected. Here, we present an analysis of "frozen" intermediates that contain alcohols without α‐hydrogen atoms, thus preventing oxidation. The copper(II)‐TEMPO complexes with a bipyridine‐type ancillary ligand were isolated by electrospray ionization mass spectrometry and investigated spectroscopically by cryogenic photodissociation spectroscopy. The vibrational characteristics of the complexes suggest that TEMPO retains its unpaired electron even upon coordination with copper(II). In agreement, the electronic photodissociation spectra of the TEMPO‐copper(II) complexes show a characteristic band for gaseous copper(II) complexes. These results contradict interpretations of some previous density functional theory (DFT) analyses of possible reaction intermediates. The experimental data were confronted with theoretical results obtained by pure DFT (OPBE, TPSS) and hybrid DFT (TPSSH, B3LYP) calculations. The methods favor different ground states and capture various aspects of the experimental results demonstrating a multiconfigurational character of the copper(II)‐TEMPO complexes. [ABSTRACT FROM AUTHOR]

Iron‐nitrenoid complexes can activate C−H bonds and, thus, potentially directly transform C−H into C−N bonds. Low‐coordination complexes are the main reported systems; however, octahedral iron complexes can be air‐stable alternatives forming the iron‐nitrenoid species. Here, a novel octahedral non‐heme iron‐acyl‐nitrenoid complex is presented. Its reactivity was studied using a flow chemistry setup with online mass spectrometry detection, which allows qualitative reaction kinetics evaluation and the detection of the intermediates. The iron‐nitrenoid complex reacts with thiophenol by H‐Atom transfer, followed by a rebound to form a new N−S bond. The rebound intermediate could be detected and distinguished from the product complex. The reaction of the iron‐acyl‐nitrenoid complex with hydrocarbons results in the activation of the C−H bond via hydrogen‐atom transfer with no subsequent rebound process. [ABSTRACT FROM AUTHOR]

Gold(I) centers can form moderately strong (Au⋅⋅⋅H) hydrogen bonds with tertiary ammonium groups, as has been demonstrated in the 3AuCl+ (3+=1‐(tert‐butyl)‐3‐phenyl‐4‐(2‐((dimethylammonio)methyl)phenyl)‐1,2,4‐triazol‐5‐ylidene) complex. However, similar hydrogen bonding interactions with isoelectronic silver(I) or copper(I) centers are unknown. Herein, we first explored whether the Au⋅⋅⋅H bond originally observed in 3AuCl+ can be strengthened by replacing Cl with Br or I. Experimental gas‐phase IR spectra in the ∼3000 cm−1 region showed only a small effect of the halogen on the Au⋅⋅⋅H bond. Next, we measured the spectra of 3AgCl+, which exhibited significant differences compared to its 3AuX+ counterparts. The difference has been explained by DFT calculations which indicated that the Ag⋅⋅⋅H interaction is only marginal in this complex, and a Cl⋅⋅⋅H hydrogen bond is formed instead. Calculations predicted the same for the 3CuCl+ complex. However, we noticed that for Ag and Cu complexes with less flexible ligands, such as dimethyl(2‐(dimethylammonio)phenyl)phosphine (L7H+), the computations predict the presence of the respective Ag⋅⋅⋅H and Cu⋅⋅⋅H hydrogen bonds, with a strength similar to the Au⋅⋅⋅H bond in 3AuCl+. We, therefore, propose possible complexes where the presence of (Ag/Cu)⋅⋅⋅H bonds could be experimentally verified to broaden our understanding of these unusual interactions. [ABSTRACT FROM AUTHOR]

Gold(II) complexes are rare, and their application to the catalysis of chemical transformations is underexplored. The reason is their easy oxidation or reduction to more stable gold(III) or gold(I) complexes, respectively. We explored the thermodynamics of the formation of [AuII(L)(X)]+ complexes (L=ligand, X=halogen) from the corresponding gold(III) precursors and investigated their stability and spectral properties in the IR and visible range in the gas phase. The results show that the best ancillary ligands L for stabilizing gaseous [AuII(L)(X)]+ complexes are bidentate and tridentate ligands with nitrogen donor atoms. The electronic structure and spectral properties of the investigated gold(II) complexes were correlated with quantum chemical calculations. The results show that the molecular and electronic structure of the gold(II) complexes as well as their spectroscopic properties are very similar to those of analogous stable copper(II) complexes. [ABSTRACT FROM AUTHOR]

Macrocyclic metal porphyrin complexes can act as shape‐selective catalysts mimicking the action of enzymes. To achieve enzyme‐like reactivity, a mechanistic understanding of the reaction at the molecular level is needed. We report a mechanistic study of alkene epoxidation by the oxidant iodosylbenzene, mediated by an achiral and a chiral manganese(V)oxo porphyrin cage complex. Both complexes convert a great variety of alkenes into epoxides in yields varying between 20–88 %. We monitored the process of the formation of the manganese(V)oxo complexes by oxygen transfer from iodosylbenzene to manganese(III) complexes and their reactivity by ion mobility mass spectrometry. The results show that in the case of the achiral cage complex the initial iodosylbenzene adduct is formed on the inside of the cage and in the case of the chiral one on the outside of the cage. Its decomposition leads to a manganese complex with the oxo ligand on either the inside or outside of the cage. These experimental results are confirmed by DFT calculations. The oxo ligand on the outside of the cage reacts faster with a substrate molecule than the oxo ligand on the inside. The results indicate how the catalytic activity of the macrocyclic porphyrin complex can be tuned and explain why the chiral porphyrin complex does not catalyze the enantioselective epoxidation of alkenes. [ABSTRACT FROM AUTHOR]

ArIO (ArI = 2-(tBuSO2)-C6H4I) is an oxidant used to oxidize FeII species to their FeIV-oxo state, enabling hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions at low energy barriers. ArIO, as a ligand, generates masked FenO species of the type Fe(n‑2)-OIAr. Herein, we used gas-phase ion–molecule reactions and DFT calculations to explore the properties of masked iron–oxo species and to understand their unmasking mechanisms. The theory shows that the I–O bond cleavage in [(TPA)-FeIVO-(ArIO)]2+ (12+, TPA = tris-(2-pyridylmethyl)-amine)) is highly endothermic; therefore, it can be achieved only in collision-induced dissociation of 12+ leading to the unmasked iron-(VI) dioxo complex. The reduction of 12+ by HAT leads to [(TPA)-FeIIIOH-(ArIO)]2+ with a reduced energy demand for the I–O bond cleavage but is, however, still endothermic. The exothermic unmasking of the FeO bond is predicted after one-electron reduction of 12+ or after OAT reactivity. The latter leads to the 4e– oxidation of unsaturated hydrocarbons: The initial OAT from [(TPA)-FeIVO-(ArIO)]2+ leads to the epoxidation of an alkene and triggers the unmasking of the second FeO bond still within one collisional complex. The second oxidation step starts with HAT from a C–H bond and follows with the rebound of the C-radical and the OH group. The process starting with the one-electron reduction could be studied with [(TQA)-FeIVO-(ArIO)]2+ (22+, TQA = tris-(2-quinolylmethyl)-amine)) because it has a sufficient electron affinity for electron transfer with alkenes. Accordingly, the reaction of 22+ with 2-carene leads to [(TQA)-FeIIIO-(ArIO)]2+ that exothermically eliminates ArI and unmasks the reactive FeV–dioxo species. [ABSTRACT FROM AUTHOR]

Enantioselective reactions are at the core of chemical synthesis. Their development mostly relies on prior knowledge, laborious product analysis and post‐rationalization by theoretical methods. Here, we introduce a simple and fast method to determine enantioselectivities based on mass spectrometry. The method is based on ion mobility separation of diastereomeric intermediates, formed from a chiral catalyst and prochiral reactants, and delayed reactant labeling experiments to link the mass spectra with the reaction kinetics in solution. The data provide rate constants along the reaction paths for the individual diastereomeric intermediates, revealing the origins of enantioselectivity. Using the derived kinetics, the enantioselectivity of the overall reaction can be predicted. Hence, this method can offer a rapid discovery and optimization of enantioselective reactions in the future. We illustrate the method for the addition of cyclopentadiene (CP) to an α,β‐unsaturated aldehyde catalyzed by a diarylprolinol silyl ether. [ABSTRACT FROM AUTHOR]