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Infrared Ion Spectroscopy for the Gas-Phase Characterization of Organo- and Biocatalysis Intermediates

Subject Area Organic Molecular Chemistry - Synthesis and Characterisation
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Term from 2018 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 413232606
 
Final Report Year 2024

Final Report Abstract

According to the project objectives detailed in our funding application, we investigated the generation, structure and chemical reactivity of important reactive intermediates in organo- and biocatalysis in the gas-phase: (A) the so-called Breslow intermediates of carbene-based Umpolung, and (B) hydroxycarbenes. In both parts of our projects we successfully applied electrospray ionization (ESI) for phase transfer from the condensed phase to the gas phase and generated and probed selected ionic species with Tandem Mass Spectrometry and Infrared Ion Spectroscopy to gain fundamental insights on chemical reactivity of labile and delicate reactive intermediates. Firstly, the structure and reactivity of isolated Breslow intermediates (BIs, chemically: aminoenols) were studied without any influence of counterions or stabilization of a solvate shell in the gas phase on the molecular level. We accomplished a fundamental study of a comprehensive set of tailor-made BIs in the gas phase. In doing so, we complemented condensed phase NMR spectroscopy and x-ray crystallography data of the solid state, with IR ion spectroscopy of isolated BIs and their keto-tautomers investigated in the gas phase [see M. Paul et al., Breslow Intermediates (Aminoenols) and their Keto Tautomers: First Gas-Phase Characterization by IR Ion Spectroscopy, Chemistry a European Journal 27 (2021) 2662-2669.] The respective report was highly ranked as a "very important paper" by the publisher Wiley, and received further recognition and attention by a Cover in Chem. Eur. J.. Related to that important study we extended our analytical arsenal to ion mobility spectrometry (IMS) to probe the three-dimensional shape of N- heterocyclic carbenes (NHC). This work was done in collaboration with Professor Helmut Schwarz at the TU Berlin and yielded another set of important results and a well-received contribution [M. Paul, et al., Intermediates of N-Heterocyclic Carbene (NHC) Dimerization Probed in the Gas Phase by Ion Mobility Mass Spectrometry: C-H···꞉C Hydrogen Bonding vs. Covalent Dimer Formation, Chemistry A European Journal 25 (2019) 2511-2518.]. With this study we were able to detect and analyze the non-covalent intramolecular C-H···꞉C hydrogen bonding interactions in dimeric NHCs. This paper marks a trailblazing IMS analysis of NHC organocatalysts in the gas phase. Secondly, we turned our attention to gas-phase studies of tunneling phenomena, in particular to quantum mechanical H-tunneling (QMHT) in hydroxycarbenes. To do so, we first selected a charge-tagged phenyl pyruvic acid derivative for our gas-phase study that delivers clean CO2-loss upon collision activation similarly to phenylhydroxycarbene, our first well characterized analyte [see M. Schäfer et al., Journal of the American Chemical Society 139 (2017) 5779−5786]. In this case however, we did not find any spectroscopic evidence for formation of the carbene intermediate, but identified exclusively the enol tautomer [see M. Paul, et al., Hydrogen Tunneling Avoided: Enol-Formation from a Charge-tagged Phenyl Pyruvic Acid Derivative Evidenced by Tandem-MS, IR Ion Spectroscopy and Theory, Physical Chemistry Chemical Physics, 21 (2019) 16591-16600]. Subsequently, we paid close attention to the effects of intramolecular hydrogen bonding, specifically of C-H···꞉C hydrogen bonding interactions on QMHT kinetics of flexible aliphatic hydroxycarbenes. Due to its outspoken novelty, importance and scientific outcome, this study developed to the cornerstone of the projects conducted in this funding scheme, which culminated in a publication that certainly stands out in terms of thoroughness and impact on the field [M. Paul, et al., Hydrogen Bonding Shuts Down Tunneling in Hydroxycarbenes: A Gas-Phase Study by Tandem-Mass Spectrometry, Infrared Ion Spectroscopy, and Theory, Journal of the American Chemical Society, 145 (2023) 12124-12135]. In this outstanding paper, we report our findings on QMHT in a set of purposely designed linear and bicyclic aliphatic hydroxycarbenes. A focus lies on the effective inhibition of QMHT reactivity by intramolecular C:…H-C interactions. The exhaustive collection of tandem mass spectrometric, IR ion spectroscopic, computational and synthetic results presented in this article (and in the very large supporting information) represents and summarizes a large part of the work conducted in this project jointly pursued by A. Berkessel and M. Schäfer et al.. In the course of the above joint research efforts, we accomplished substantial progress in the indicated fields and beyond. We note that a substantial number of the proposed projects turned out to be very successful and fruitful, while the execution of a few others was hampered by technical difficulties, unexpected synthetic complications and unfortunate obstacles related to the COVID pandemic. Especially the integration of an electrospray ion source in a cryogenic ion trap proved to be more complicated than expected and led to the postponement of the fundamental investigations on quantum mechanical tunneling (QMT) at various low temperatures and at reduced back ground pressures. These pending parts of the research program will be addressed in a sequel project embedded in a MARIE Skłodowska-CURIE ACTIONS MSCA Doctoral Network (DN) proposed for funding to the European Union. Nevertheless, we collected numerous important results in the actual project period from 7-2018 to 12-2023 funded by the DFG (BE 998/16-1 & SCHA 871/10-1) which led to a remarkable number of important contributions published in highly ranked journals as listed below. Besides the substantial scientific progress, the DFG funding allowed us to provide interesting research projects and scientific training to a number of graduate students which enabled them to present hot topic results at national and international conferences, and thereby educate them on the highest possible level (AK Schäfer: Marcel Bast (PhD in Physics in the AK Schlemmer in 2024), Dennis Zeh (PhD in 2023), Leon Huenemeyer, PhD in Chemistry in the AK Schäfer in 2025; AK Berkessel: Graduate Student MSc Thomas Thomulka PhD 2021). In the following report we give a short overview on the results yielded in the DFG projects BE 998/16-1 & SCHA 871/10-1.

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