New methodology for probing catalytic reactions by ESI-MS




Vikse, Krista Lynn

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Bis(dimethylamino)-2-(4-methoxyphenyl)naphthalene (3) and 1,8-bis(dimethylamino)-4-diphenylphosphonaphthalene (5b) were synthesized as ESI-active analogues of the common organometallic ligands η6-anisole and triphenylphosphine. The water-soluble phosphine, sodium triphenylphosphine monosulfonate, was re-purposed as an ESI-active ligand. Its solubility in organic solvents and amenability to electrospray ionization was improved by replacing Na+ with the non-coordinating bis(triphenylphosphine)iminium cation. A new sample introduction method named PSI (pressurized sample infusion) was developed for the continuous infusion of air/moisture-sensitive samples into the mass spectrometer. The flow rate can be determined using a modified version of the Hagen-Poiseuille equation, and the ability of PSI (coupled with an ESI tag) to give quantitative kinetic data is demonstrated. A method for maintaining a dry, air-free ESI source is described for the analysis of highly reactive samples. The above developments were applied to the study of the copper-free Sonogashira (Heck alkynylation) reaction. The proposed active catalyst (Pd(0)L2, where L = PPh3 or 7) was observed, and its reactivity with iodomethane in the gas phase was determined to be less than that of Pd(0)L. Nevertheless, Pd(0)L2 is extremely reactive and even oxidatively adds dichloromethane (t1/2 = 10.7 min at 40 °C). Under standard reaction conditions intermediates corresponding to oxidative addition and transmetallation were detected, and coordination of base to palladium was observed for secondary amines but not triethylamine. Reductive elimination was achieved in the gas phase for a series of para-substituted aryl iodides with phenylacetylene, and the slope of the resulting Hammett plot (ρ) was -0.5. No evidence for the previously hypothesized anionic mechanism was observed. Simultaneous kinetic analysis of charged substrate, products and intermediates in the copper-free Sonogashira reaction was conducted using PSI-ESI-MS and high quality, information rich data for each species over time was obtained. In the absence of protons, reductive elimination is rate-limiting and the rate of reaction is relatively high. In the presence of protons (a byproduct of the reaction), transmetallation is rate-limiting and the rate of reaction is much slower. The use of a strong base was shown to improve the efficiency of the reaction, and an experimentally-derived catalytic cycle for the copper-free Sonogashira reaction is proposed.



Electrospray, Mass Spectrometry, Catalysis, Organometallic, ESI-MS, Pressurized Sample Infusion, Online Reaction Monitoring