Analytical techniques for reaction monitoring, mechanistic investigations, and metal complex discovery




Thomas, Gilian T.

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A variety of analytical techniques are showcased for their ability to provide insights into reaction mechanisms as well as active intermediate speciation. Pressurized Sample Infusion-Mass Spectrometry (PSI-ESI-MS), ion mobility-mass spectrometry (IMS-MS), and Nuclear Magnetic Resonance (NMR) spectroscopy are powerful analytical techniques capable of reaction monitoring. Contamination from vulcanized rubber was an issue with the PSI-ESI-MS technique as ions unrelated to the reaction were convoluting the mass spectrum. This was resolved by re-designing the PSI flask such that the septum was positioned above a condenser, preventing heat degradation of the septum and subsequent leaching of contam- inants into the reaction solution. The technique was then used to analyze the Buchwald-Hartwig amination reaction in real-time. The innovative use of Multiple Reaction Monitoring (MRM) scans facilitated observation of all catalytic intermediates, and elucidation of relative reaction rates for each step of the catalytic cycle. PSI-ESI-MS and NMR are complementary methods whereby catalytic intermediates are monitored via PSI-ESI-MS, and the rate of product formation is monitored via NMR spectroscopy. This combination of analytical methods was employed in the investigation of the Barluenga cross-coupling reaction between N-tosylhydrazones and aryl halides. A reaction screen revealed optimized homogeneous conditions, and the turnover limiting step was found to be off-cycle. IMS separates gaseous ions based on their size and shape immediately prior to MS analysis. Upon investigation of [PtCl3(C2H4)], and [PtCl3(CO)], it was found that residual [PtCl3] was forming [PtCl3(N2)] in the source of the instrument. Ion mobility was able to separate these isobaric ions, and DFT calculations and collision-induced dissociation experiments confirmed the existence of the gaseous [PtCl3(N2)] complex. NMR spectroscopy may also be employed as a strong reaction monitoring technique. The mechanism of C–H silylation by trimethyl(trifluoromethyl)silane and tetrabutylammonium difluorotriphenylsilicate was investigated using 19F-NMR. All intermediates and reaction byproducts were quantitatively observed, and the reaction conditions were optimized. A stopped-flow NMR system was used to gather data points in the first 0.2 seconds of the reaction.



Mass Spectrometry, Reaction Monitoring, Catalysis, Pressurized Sample Infusion (PSI), Ion Mobility Spectrometry