Palladium Intermediates in Solution

Charged Tags as Probes for Analyzing Organometallic Intermediates and Monitoring Cross-Coupling Reactions by Electrospray-Ionization Mass Spectrometry

“…This method permits the transfer of ions from solution into the gas phase, thus allowing the sampling of dissolved charged organometallics in situ . It is therefore not surprising that ESI mass spectrometry has been applied to the analysis of numerous different organometallic systems . The successful detection of various charged organometallics, including rather labile ones, , is consistent with the commonly accepted view that ESI constitutes a relatively “soft” ionization technique, which transfers only limited amounts of energy into the probed ions and does not significantly change their nature .…”

“…It is therefore not surprising that ESI mass spectrometry has been applied to the analysis of numerous different organometallic systems . The successful detection of various charged organometallics, including rather labile ones, , is consistent with the commonly accepted view that ESI constitutes a relatively “soft” ionization technique, which transfers only limited amounts of energy into the probed ions and does not significantly change their nature . This assumption forms the basis on which properties of the solution-phase system are deduced from gas-phase measurements.…”

“…Provided that the charged tags have only low tendencies to form ion pairs with the counterions in the chosen solvent, almost the complete population of neutral organometallics can thus be ionized. Commonly employed tags are quaternary ammonium cations ,,− and sulfonate anions. , In these ions, the charge is spread over several atoms, which not only reduces their propensity to ion pairing but also minimizes possible interactions with the metal center and unwanted changes in reactivity. Most of the examples reported so far bear a charged tag linked to coordinating ligands, such as phosphines ,,,, or carbenes .…”

Mechanism of Palladium(II)-Mediated Uncaging Reactions of Propargylic Substrates

“…Continuing with the aim to unveil the nature of the catalytic species, high-resolution electrospray ionization mass spectrometry (ESI–HRMS) was used to monitor the reaction and possible detection of key reaction intermediates. − ESI is a soft ionization technique that can be used to analyze both cations and anions, displays high sensitivity, and allows the immediate transfer to the gas phase of most ionic species present in the reaction solution. Working in negative ion mode, the in situ ESI(−)–HRMS monitoring of the reaction, with 20 mol % of Na 2 PdCl 4 in phosphate-buffered aqueous solution pH 7.4 (5% DMSO) provided 10 min after the start of the reaction, identified the following proposed species: the product DNP ( m / z = 183) as the major species (Figure a) and three other Pd species interacting with the substrate at m / z 434.8, 416.9, and 638.9 (Figure b–d).…”

Mass Spectrometric Studies of Reductive Elimination from Pd(IV) Complexes

“…It is necessary to design a model compound which would lead in the course of the reaction to the potential high-oxidation-state palladium intermediates such that they would be stable enough to be detected and characterized . An alternative solution could be electrospray ionization mass spectrometry (ESI-MS) which has been successfully utilized in the elucidation of other mechanisms of palladium-catalyzed reactions . The great advantage of this approach is the ability to identify possible intermediates which are in very low concentrations and hence below the detection limits of other methods that work with the whole reaction mixture .…”