Experimental Gas-Phase and in Silico Investigation of β-Methyl Elimination from Cationic Palladium Alkyl Species

Robustness of Threshold Collision-Induced Dissociation Simulations for Bond Dissociation Energies

The threshold collision-induced dissociation (T-CID) method is the workhorse for gas-phase bond dissociation energy (BDE) measurements. However, T-CID does not measure BDEs directly; instead, BDEs are obtained by fitting simulated data to the experimental data. We previously observed several large discrepancies between the computed and experimental BDEs. To analyze the reliability of the experimental values, we previously reported a study of the dissociation rate models in the simulation. Here, we report a study of the collision simulation part, specifically in the L-CID (ligand CID) program. We show that the BDE values are robust even to intentionally introduced mistakes in the simulations, varying in most cases by less than 3 kcal mol-1. The most significant exception is the collisional energy transfer (CET) simulation, which led to deviations larger than 10 kcal mol-1. However, we found that the BDEs obtained with explicitly simulated CET distributions deviated by only 3 kcal mol-1 from those simulated with the original model. Collectively, our results suggest that the T-CID-derived BDE values are robust and are likely to be accurate.

Modeling Gas-Phase Unimolecular Dissociation for Bond Dissociation Energies: Comparison of Statistical Rate Models within RRKM Theory

The Rice-Ramsperger-Kassel-Marcus (RRKM) theory provides a simple yet powerful rate theory for calculating microcanonical rate constants. In particular, it has found widespread use in combination with gas-phase kinetic experiments of unimolecular dissociations to extract experimental bond dissociation energies (BDEs). We have previously found several discrepancies between the computed BDE values and the respective experimental ones, obtained with our empirical rate model, named L-CID. To investigate the reliability of our rate model, we conducted a theoretical analysis and comparison of the performance of conventional rate models and L-CID within the RRKM framework. Using the previously published microcanonical rate data as well as reaction cross-section data, we show that the BDE values obtained with the L-CID model agree with the ones from the other rate models within the expected uncertainty bounds. Based on this agreement, we discuss the possible rationalization of the good performance of the L-CID model.

Gas-Phase Synthesis and Reactivity of Ligated Group 10 Ions in the Formal +1 Oxidation State

Electrospray ionization of the group 10 complexes [(phen)M(O₂CCH₃)₂] (phen=1,10-phenanthroline, M = Ni, Pd, Pt) generates the cations [(phen)M(O₂CCH₃)]⁺, whose gas-phase chemistry was studied using multistage mass spectrometry experiments in an ion trap mass spectrometer with the combination of collision-induced dissociation (CID) and ion-molecule reactions (IMR). Decarboxylation of [(phen)M(O₂CCH₃)]⁺ under CID conditions generates the organometallic cations [(phen)M(CH₃)]⁺, which undergo bond homolysis upon a further stage of CID to generate the cations [(phen)M]^+· in which the metal center is formally in the +1 oxidation state. In the case of [(phen)Pt(CH₃)]⁺, the major product ion [(phen)H]⁺ was formed via loss of the metal carbene Pt=CH₂. DFT calculated energetics for the competition between bond homolysis and M=CH₂ loss are consistent with their experimentally observed branching ratios of 2% and 98% respectively. The IMR of [(phen)M]^+· with O₂, N₂, H₂O, acetone, and allyl iodide were examined. Adduct formation occurs for O₂, N₂, H₂O, and acetone. Upon CID, all adducts fragment to regenerate [(phen)M]^+·, except for [(phen)Pt(OC(CH₃)₂)]^+·, which loses a methyl radical to form [(phen)Pt(OCCH₃)]⁺ which upon a further stage of CID regenerates [(phen)Pt(CH₃)]⁺ via CO loss. This closes a formal catalytic cycle for the decomposition of acetone into CO and two methyl radicals with [(phen)Pt]^+· as catalyst. In the IMR of [(phen)M]^+· with allyl iodide, formation of [(phen)M(CH₂CHCH₂)]⁺ was observed for all three metals, whereas for M = Pt also [(phen)Pt(I)]⁺ and [(phen)Pt(I)₂(CH₂CHCH₂)]⁺ were observed. Finally, DFT calculated reaction energetics for all IMR reaction channels are consistent with the experimental observations.

A Diverted Aerobic Heck Reaction Enables Selective 1,3-Diene and 1,3,5-Triene Synthesis through C-C Bond Scission

Substituted 1,3-dienes are valuable synthetic intermediates used in myriad catalytic transformations, yet modern catalytic methods for their preparation in a highly modular fashion using simple precursors are relatively few. We report here an aerobic boron Heck reaction with cyclobutene that forms exclusively linear 1-aryl-1,3-dienes using (hetero)arylboronic acids, or 1,3,5-trienes using alkenylboronic acids, rather than typical Heck products (i.e., substituted cyclobutenes). Experimental and computational mechanistic data support a pericyclic mechanism for C-C bond cleavage that enables the cycloalkene to circumvent established limitations associated with diene reagents in Heck-type reactions.

β-Arylation of oxime ethers using diaryliodonium salts through activation of inert C(sp)-H bonds using a palladium catalyst

An efficient method of selective β-arylation of oxime ethers was realized by using a palladium catalyst with diaryliodonium salts as the key arylation reagents. The reaction proceeded smoothly through the activation of inert C(sp3)-H bonds to give corresponding ketones and aldehydes. This convenient procedure can be successfully applied to construct new C(sp3)-C(sp2) bonds on a number of complex molecules derived from natural products and thus serves as a practical synthetic tool for direct late-stage C(sp3)-H functionalization.