Modeling Metal-Catalyzed Polyethylene Depolymerization: [(Phen)Pd(X)]+ (X = H and CH3) Catalyze the Decomposition of Hexane into a Mixture of Alkenes via a Complex Reaction Network

Conformational Isomerization as a Process to Determine Selectivity over Reaction Pathways: Effect of Alkene Rotation in Chain Walking via Cis Alkene Intermediates

In organic reactions, bond-forming and bond-cleaving processes are generally considered to be more important than other processes such as conformational isomerization. We report herein an example where a conformational isomerization process, propeller-like alkene rotation, is considered to determine the selectivity over the reaction pathways. The transition state with the highest energy barrier in some alkylpalladium isomerization (chain walking) events was theoretically indicated to correspond to alkene rotation, while transition states for bond-cleaving β-hydride elimination and bond-forming migratory insertion were not even observed. It was also suggested both theoretically and experimentally that the palladium chain walking over internal carbons in alkyl chains proceeds via cis alkene intermediates rather than thermodynamically more stable trans alkene intermediates, due to their relative difficulty to undergo alkene rotation.

Ion-molecule reactions of mass-selected ions

Gas-phase reactions of mass-selected ions with neutrals covers a very broad area of fundamental and applied mass spectrometry (MS). Oftentimes, ion-molecule reactions (IMR) can serve as a viable alternative to collision-induced dissociation and other ion dissociation techniques when using tandem MS. This review focuses on the literature pertaining applications of IMR since 2013. During the past decade considerable efforts have been made in analytical applications of IMR, including advances in one of the major techniques for characterization of unsaturated fatty acids and lipids, ozone-induced dissociation, and the development of a new technique for sequencing of large ions, hydrogen atom attachment/abstraction dissociation. Many advances have also been made in identifying gas-phase chemistry specific to a functional group in organic and biological compounds, which are useful in structure elucidation of analytes and differentiation of isomers/isobars. With "soft" ionization techniques like electrospray ionization having become mainstream for quite some time now, the efforts in the area of metal ion catalysis have firmly moved into exploring chemistry of ligated metal complexes in their "natural" oxidation states allowing to model individual steps of mechanisms in homogeneous catalysis, especially in combination with high-level DFT calculations. Finally, IMR continue to contribute to the body of knowledge in the area of chemistry of interstellar processes.

Mechanism of Deoxygenation and Cracking of Fatty Acids by Gas-Phase Cationic Complexes of Ni, Pd, and Pt

Deoxygenation and subsequent cracking of fatty acids are key steps in production of biodiesel fuels from renewable plant sources. Despite the fact that multiple catalysts, including those containing group 10 metals (Ni, Pd, and Pt), are employed for these purposes, little is known about the mechanisms by which they operate. In this work, we utilized tandem mass spectrometry experiments (MSn) to show that multiple types of fatty acids (saturated, mono-, and poly-unsaturated) can be catalytically deoxygenated and converted to smaller hydrocarbons using the ternary metal complexes [(phen)M(O2CR)]+], where phen = 1,10-phenanthroline and M = Ni, Pd, and Pt. The mechanistic description of deoxygenation/cracking processes builds on our recent works describing simple model systems for deoxygenation and cracking, where the latter comes from the ability of group 10 metal ions to undergo chain-walking with very low activation barriers. This article extends our previous work to a number of fatty acids commonly found in renewable plant sources. We found that in many unsaturated acids cracking can occur prior to deoxygenation and show that mechanisms involving group 10 metals differ from long-known charge-remote fragmentation reactions.