Quantitative Determination of Active Species Transforming the R-NHC Coupling Process under Catalytic Conditions.

Palladium complexes with N-heterocyclic carbenes (Pd/NHC) serve as prominent precatalysts in numerous Pd-catalyzed organic reactions. While the evolution of Pd/NHC complexes, which involves the cleavage of the Pd-C(NHC) bond via reductive elimination and dissociation, is acknowledged to influence the catalysis mechanism and the performance of the catalytic systems, conventional analytic techniques [such as NMR, IR, UV-vis, gas chromatography-mass spectrometry (GC-MS), and high-performance liquid chromatography (HPLC)] frequently fail to quantitatively monitor the transformations of Pd/NHC complexes at catalyst concentrations typical of real-world conditions (below approximately 1 mol %). In this study, for the first time, we show the viability of using electrospray ionization mass spectrometry (ESI-MS). This approach was combined with the use of selectively deuterated H-NHC, Ph-NHC, and O-NHC coupling products as internal standards, allowing for an in-depth quantitative analysis of the evolution of Pd/NHC catalysts within actual catalytic systems. The reliability of this approach was affirmed by aligning the ESI-MS results with the NMR spectroscopy data obtained at greater Pd/NHC precatalyst concentrations (2-5 mol %) in the Mizoroki-Heck, Sonogashira, and alkyne transfer hydrogenation reactions. The efficacy of the ESI-MS methodology was further demonstrated through its application in the Mizoroki-Heck reaction at Pd/NHC loadings of 5, 0.5, 0.05, and 0.005 mol %. In this work, for the first time, we present a methodology for the quantitative characterization of pivotal catalyst transformation processes commonly observed in M/NHC systems.

Pseudo Four-Component Synthesis of 5,6-Dihydroindolo[2,1-a]isoquinolines.

An easy synthesis of novel highly functionalized 5,6-dihydroindolo[2,1-a]isoquinolines was developed via a pseudo four-component domino reaction of 1-aroyl-3,4-dihydroisoquinolines, terminal α,ß-ynones, and malononitrile. The selective formation of this biologically relevant heterocyclic core was achieved using a one-pot approach under microwave irradiation. The formation of the same skeleton through the reaction of 5,6-dihydropyrrolo[2,1-a]isoquinolines with malonic acid dinitrile supports the proposed mechanism, involving the intermediate product of the three-component reaction. Furthermore, the disproval of an alternative reaction pathway, which involved the dimerization of malononitrile followed by three-component transformation, was demonstrated. Introducing the malononitrile dimer as a CH acid resulted in the formation of a different pyrido[3',4':4,5]pyrrolo[2,1-a]isoquinoline core. Additionally, the synthesized 5,6-dihydroindolo[2,1-a]isoquinolines were examined for their photophysical properties, revealing their attractive luminescent characteristics.

Derivatization to increase the detectability of small peptides in blood serum in the analysis by ESI and MALDI high resolution mass spectrometric methods.

This work highlights the efficient approach to highly sensitive determination of dipeptides that can present in biological liquids at very low and trace quantities. The approach involves preliminary derivatization of peptides with tris(2,4,6-trimethoxyphenyl)-methyl carbenium hexafluoroborate followed by ESI and MALDI high-resolution mass spectrometry. Using model dipeptides with various amino acid compositions and sequences, it was shown that the derivatization reaction proceeded smoothly in mild conditions and gave rise to pink-red colored salt derivatives. Ready cations of interest for the analysis are easily desorbed from the salt-derivatives providing strong signals in ESI and MALDI mass spectra and this ensures high sensitivity of the analysis. Another positive aspect is the removal of the target signal from the region of a matrix noise, since the introduced fragment possesses a large mass increment (359 Da). High resolution mass spectrometry, which provides the determination of accurate weights and elemental compositions of ions, was used to reliably detect model dipeptides added to artificial urine and blood serum. A number of these dipeptides was shown to be present in real blood serum collected from volunteers. Collision induced dissociation of precursor cations composed of derivatizing reagent and dipeptide moieties gives rise to characteristic and simple fragmentation mass spectra. A comparison of limits of detection (LOD) measured for non-modified and derivatized dipeptides showed that the latter derivatives provide the highest sensitivity when LOD is determined by using multiple reaction monitoring (MRM) transitions. The suggested derivatization approach was shown to be useful for unambiguous identification of special dipeptides in artificial media and dietary supplements.

Fully Automated Unconstrained Analysis of High-Resolution Mass Spectrometry Data with Machine Learning.

Mass spectrometry (MS) is a convenient, highly sensitive, and reliable method for the analysis of complex mixtures, which is vital for materials science, life sciences fields such as metabolomics and proteomics, and mechanistic research in chemistry. Although it is one of the most powerful methods for individual compound detection, complete signal assignment in complex mixtures is still a great challenge. The unconstrained formula-generating algorithm, covering the entire spectra and revealing components, is a "dream tool" for researchers. We present the framework for efficient MS data interpretation, describing a novel approach for detailed analysis based on deisotoping performed by gradient-boosted decision trees and a neural network that generates molecular formulas from the fine isotopic structure, approaching the long-standing inverse spectral problem. The methods were successfully tested on three examples: fragment ion analysis in protein sequencing for proteomics, analysis of the natural samples for life sciences, and study of the cross-coupling catalytic system for chemistry.