Studies of gas-phase fragmentation reactions of [Pd(PPh3)2(OCOR)]+ by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

ORGANOMETALLIC GAS-PHASE ION CHEMISTRY AND CATALYSIS: INSIGHTS INTO THE USE OF METAL CATALYSTS TO PROMOTE SELECTIVITY IN THE REACTIONS OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES

Carboxylic acids are valuable organic substrates as they are widely available, easy to handle, and exhibit structural and functional variety. While they are used in many standard synthetic protocols, over the past two decades numerous studies have explored new modes of metal-mediated reactivity of carboxylic acids and their derivatives. Mass spectrometry-based studies can provide fundamental mechanistic insights into these new modes of reactivity. Here gas-phase models for the following catalytic transformations of carboxylic acids and their derivatives are reviewed: protodecarboxylation; dehydration; decarbonylation; reaction as coordinated bases in C-H bond activation; remote functionalization and decarboxylative C-C bond coupling. In each case the catalytic problem is defined, insights from gas-phase studies are highlighted, comparisons with condensed-phase systems are made and perspectives are reached. Finally, the potential role for mechanistic studies that integrate both gas- and condensed-phase studies is highlighted by recent studies on the discovery of new catalysts for the selective decomposition of formic acid and the invention of the new extrusion-insertion class of reactions for the synthesis of amides, thioamides, and amidines. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Formation of the exceptional [M - H]+ cation in atmospheric pressure ionization mass spectrometry analysis of 2-(diphenylsilyl) cyclopropanecarboxylate esters

RATIONALE

In general, ionization of analytes in atmospheric pressure ionization mass spectrometry (API-MS) in positive ion mode results in the formation of protonated molecules ([M + H]⁺ ) and/or cationized molecules (e.g., [M + Na]⁺ ). The formation of specific [M - H]⁺ cations in the API process is of significant interest for further investigation.

METHODS

The ionization processes of 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate esters were investigated using electrospray ionization (ESI)-MS and atmospheric pressure chemical ionization-MS in positive ion mode. Theoretical calculations were carried out with the Gaussian 03 program using the density functional theory (DFT) method at the B3LYP/6-311 + G(2d,p) level.

RESULTS

The anomalous [M - H]⁺ ion and the regular [M + Na]⁺ ion were both observed using ESI-MS. Interestingly, no [M + H]⁺ ion was obtained in the ESI-MS analysis, and acidification of the ESI solvent accelerated the formation of [M - H]⁺ rather than [M + H]⁺ ion. DFT calculations for the typical methyl 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate (1) indicated that the [1 + H]⁺ ion can thermodynamically and kinetically undergo facile H₂ elimination to generate [1 - H]⁺ .

CONCLUSIONS

The favorable formation of [M - H]⁺ ions in these compounds is attributed to the unique diphenylhydrosilyl group in their structure. The [M + H]⁺ ion formed easily underwent H₂ elimination to produce the [1 - H]⁺ ion in the API source, and thus, acidification of the ESI solvent apparently accelerates the formation of the [1 - H]⁺ ion.

Charged tags as probes for analyzing organometallic intermediates and monitoring cross-coupling reactions by electrospray-ionization mass spectrometry

Organic iodides bearing a cationic quaternary ammonium group at a remote position react with zerovalent Pd complexes, Zn, or In leading to a C-I bond insertion. The resulting charge-tagged organometallics can be detected by electrospray-ionization mass spectrometry, which provides detailed information on their stoichiometry, oxidation state, and coordination sphere. The properties of the observed organopalladium and -zinc intermediates largely agree with previous findings, whereas the organoindium species show a surprisingly high tendency to form ate complexes. Magnesium also undergoes insertion into the C-I bond of the charge-tagged organic iodides, but instead of the expected organomagnesium intermediates only the corresponding hydrolysis products could be detected in the diluted solutions. Electrospray-ionization mass spectrometry can also be used to study the reactivity of the charge-tagged species, as was demonstrated for a Pd-catalyzed Negishi cross-coupling reaction. The presented approach permits a straightforward identification of the rate-limiting step and the determination of the corresponding second-order rate constant.

Intramolecular scrambling of aryl groups in organopalladium complexes [ArPd(PPh3)2]+: from solution to the gas phase, back again, and in-between

Gas-phase experiments are used to probe the intramolecular scrambling of aryl groups between palladium and phosphorus in organopalladium complexes, [ArPd(PPh(3))(2)](+), generated by means of electrospray ionization (ESI). To this aim, ESI mass spectrometry, including tandem mass spectrometric experiments, were carried out on deuterated, non-deuterated, and substituted [ArPd(PPh(3))(2)](+) complexes. The fragment ions obtained from the deuterated parent ions clearly show the occurrence of intramolecular scrambling between the aryl group bound to palladium and the phenyl groups of the phosphine in the gas phase. Fragmentation pathways, supported by a statistical model, are proposed to explain these migrations and the implications for the condensed-phase chemistry are probed experimentally by using ESI mass spectrometry.

ESI-FTICR-MS studies on gas phase fragmentation reactions of ArPd(PPh3)2I complexes

Gas-phase fragmentation reactions of [ArPd(PPh3)2]+ were studied by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). The results of sustained off-resonance irradiation collision-activated dissociation (SORI-CAD) experiments provide detailed insights into mechanisms for the gas-phase fragmentation reactions of these complex ions. The PC bond cleavage mediated by palladium is investigated in the gas phase. There are two competitive fragmentation pathways for the complex ions [ArPd(PPh3)2]+ (Ar = p-OCH3-C6H4, p-CH3-C6H4, p-tBu-C6H4, p-NH2-C6H4, p-COCH3-C6H4, and p-F-C6H4) of electron-donating and electron-withdrawing aromatic iodides. Path A proceeds through reductive elimination of [ArPd(PPh3)2]+ to produce the product ion [PPh3Ar]+. Path B mostly proceeds via phenyl migration from the triphenylphosphine ligand to the palladium center by cleavage of the phosphorus-phenyl bond to give a palladium-phenyl intermediate, and subsequent reductive elimination of the intermediate to yield a product ion [PPh4]+. The result of deuterium-labeling experiments provides evidence for the phenyl shift between the palladium center and the coordinated ligand through cleavage of the PC bond. The complex ions [(o-CH3-C6H4)Pd(PPh3)2]+, [(o-2,6-Me2-C6H3)Pd(PPh3)2]+, and [(C10H7)Pd(PPh3)2]+ display more fragmentation pathways, two of which are similar to those of the ions [ArPd(PPh3)2]+ (Ar = p-OCH3-C6H4, p-CH3-C6H4, p-tBu-C6H4, p-NH2-C6H4, p-COCH3-C6H4, p-F-C6H4), and the third pathway involves loss of one molecule of benzene and one PPh3 ligand. The electronic effect and steric effect of the aryl groups also exhibit different influences on the fragmentation pathways.